Download TOPIC 12. THE ELEMENTS

TOPIC 12.
THE ELEMENTS - the Periodic Table.
For millennia, humans have been discovering and extracting elements from nature and
using them either in their elemental state or in combination with others as compounds.
This process has accelerated over the past few centuries and in today’s highly technical
environment we are dependent on a continued supply of many elements, the names of
some of which were rarely mentioned or even recognised by chemists just a few
decades ago. Without an assured supply, much of the technology that is now widely
taken for granted would no longer be viable and potential future developments will be
hampered or rendered impossible. As an example, note how dependent current
technology has become on the supply of the so-called rare earth elements which
underpin many of the advances made in computing, communications and the many
applications in which powerful rare earth magnets are the basis. Other less exotic
elements are even more important for today’s living standards - consider how
dependent agriculture is on a continued supply of phosphorous in the form of
phosphate fertilisers, a supply which may be fated to be fully depleted in the future. In
these notes, a selection of elements which are among those that are essential
components of technology today are discussed in the context of the Periodic Table.
This system of classification of the elements is not only a convenient summary of the
chemistry associated with various families of elements in which the Table’s origin lies,
but in its modern form the Periodic Table provides the scaffold underlying the
electronic structures of the atoms and upon which their various properties and
reactions depend.
The following are a few illustrative examples to ponder concerning the elements which
are discussed in this Topic.
Nitrogen molecules in the atmosphere contain one of the most stable bonds yet about
half the nitrogen atoms in our bodies were extracted artificially from the air.
Salt is destroying vast areas of agricultural land in Australia and has undesirable
consequences if consumed in excess in our diet yet is a valuable source of essential
products.
The air surrounding us contains a large proportion of a highly corrosive gas that
originated as the most polluting ever and which reacts with almost all other elements
yet is indispensable to all animal life - oxygen.
The rechargeable batteries which power mobile phones and computers are based on
the extremely small size of the lithium ion.
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Many nuclear diagnostic procedures rely on using artificially produced atoms that
originate from nuclear reactors such as that at Lucas Heights in Sydney.
An irreplaceable gas with vital applications in magnetic resonance imaging machines
is used and lost - filling party balloons!
An object made from some metals retains a memory of its initial shape and if distorted,
it will return to the original shape when heated.
Origin of the elements.
As discussed in Topic 1, there are 90 naturally occurring elements. In addition, there
are about 28 other elements which have been produced synthetically but some of these
exist only as very short-lived radioactive species which have been produced in
extremely small quantities using high energy particle accelerators. How did the 90
naturally occurring elements originate? The most widely accepted version of the
origin of all matter in the universe is the BIG BANG THEORY which proposes that
in an instant, all of space, energy and matter which had been confined to a volume the
size of a grain of sand at an infinitely high temperature underwent an explosive
expansion. Within 10 minutes of the big bang, nuclei of mostly hydrogen and helium
were formed from more basic particles and over the next million years these nuclei
cooled enough to capture electrons and form atoms. After about a billion years, the
gravitational attraction between atoms which were still mostly hydrogen and helium,
lead to clumps of matter which by gravitational attraction gradually increased in size.
With increasing size, the temperature of these clumps also increased and in some
regions of space the larger clumps became hot enough to initiate fusion reactions
between nuclei, forming stars which are in effect giant nuclear fusion reactors. Our
own sun, like all stars, converts hydrogen to helium with the concurrent release of
extremely large amounts of energy known as the BINDING ENERGY associated with
the strong nuclear force which was discussed briefly in Topic 2. Within a few billion
years vast numbers of new stars were formed and these in turn, through gravitational
attraction, clustered to create galaxies, each of which contains enormous numbers of
stars. By that point in time, the universe would have looked much as it does today. As
the hydrogen fuelling the fusion reactions in a star is consumed, other fusion reactions
can occur in which heavier elements form. Fusion reactions leading to new atoms of
elements as heavy as iron all release energy and can continue to fuel a star. Ultimately,
when a star has consumed most of its available fuel, it may simply cool and dim or in
some cases it may initially implode and then undergo an enormous explosion which
flings much of its constituent material and energy out into space in what is called a
SUPERNOVA EVENT. It is only during supernovae that elements heavier than iron
are formed. It is estimated that the present elemental composition of the universe is
92.7% hydrogen atoms, 7.2% helium atoms and just 0.1% atoms of all the other
elements. The shock waves of energy and material sent into space from supernovae
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may interact with existing clouds of gas, ice and dust to eventually form new stars and
planets such as our solar system. Our sun was not one of the original stars in the
universe but is probably a second or third generation star, formed in part from the
energy and residues released by previous supernovae. Although already 5 billion
years old, it still contains 71% hydrogen and 27% helium, so the sun will burn for
several billion more years.
Thus on the basis of this theory, the elements which constitute all matter on earth apart
from hydrogen were originally formed from stars that existed before our sun and which
had consumed all their available hydrogen, converted it to helium and progressed to
other nuclear fusion reactions that created heavier elements and finally underwent a
supernova explosion in which more of the heavier elements were produced.
Discovery and isolation of the elements.
Most of the non-gaseous elements on earth are chemically combined with other
elements as compounds. Few non-gaseous elements are found in the free state. For
thousands of years gold, silver, copper, sulfur and carbon had been known because
they do occur in the free form, although they were not necessarily recognised as
elements - indeed the concept of an element as we know it today was not firmly
established until the 18th century through the visionary work of Lavoisier. While
some metals such as gold and silver are so unreactive that they can be found as free
elements, most elements occur as compounds in MINERALS. Extraction of
metals such as copper and tin from their ores by the process of SMELTING was
probably accidentally discovered when minerals were used as fireplaces. The
extraction process relied upon the use of bellows made from animal hides to
increase the heat obtained from a fire to the point where decomposition could occur
of rocks containing for example copper (malachite, a copper carbonate compound)
and tin (cassiterite, an oxide of tin). Charcoal in the fire reduced the copper and tin
compounds to the free elements. Later it was discovered that mixing about 9 parts
of copper and 1 part of tin together and melting them produced an alloy called
BRONZE which is much harder than either of its constituent elements, a feature
exploited in the bronze age from about 5000 years ago. Later, iron was isolated
from its ores by similar means - probably more than 3000 years ago. Prior to 1600,
the elements gold, silver, carbon, sulfur, copper, tin, lead, iron and mercury had
been discovered by persons unknown. However arsenic, isolated by Albertus
Magnus in 1250, is the first recorded instance of an attributed method of isolation
of an element.
The prehistoric technical advance of using crude furnaces to smelt ores set the
pattern for future discoveries of elements which resulted from newly devised
processes. The rate of isolation of the elements was a process that tended to occur in
steps where development of a new method or technology gave impetus to a flurry of
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discoveries, often followed by a quieter period prior to another new method or
technology being devised. This is shown in the chart on page XII-5 which plots the
number of known elements against dates of discovery based on the data given in the
Tables on Pages XII-4 and 5. In the Table on Page XII-6, the dates of discovery of
those elements known in prehistoric times are simply listed as <1600 while in the chart
the prehistoric discoveries have been placed undated prior to 1600 along the axis.
Prior to 1600, the driver of isolation of elements such as copper, tin, lead and mercury
was augmentation of the use of fire with bellows to create a furnace. Isolation of
elements in the 18th century became increasingly rapid, boosted by several new
technologies. Methods for analysing minerals reached a high state of development,
particularly as a result of the advent of the blowpipe. This tool allows the ready
decomposition of minerals on a carbon block, a prerequisite for analysis of their
components. Elements discovered through the improved analytical techniques during
the 18th century were cobalt, bismuth, platinum, zinc, nickel, manganese, molybdenum,
tellurium, tungsten and chromium.
However, the greatest advance in new techniques during the 18th century was the
development of apparatus designed to handle gases. This not only allowed the isolation
of the elements hydrogen, nitrogen, oxygen and chlorine as well as gaseous compounds
such as carbon dioxide, but lead to our current understanding of the nature of chemical
processes and to fundamental laws such as the law of conservation of matter.
The development of the battery and its application to isolating elements via the method
of electrolysis was exploited, particularly by Humphry Davy, in the early years of the
19th century. He used large batteries made from copper and zinc to electrolyse molten
salts of elements from Groups 1 and 2, isolating potassium, sodium, calcium and
barium in rapid succession in 1807 - 1808.
Throughout the 19th century methods for analysing minerals continued to develop.
These methods included the use of the blowpipe to obtain high temperatures in
conjunction with acid digestion of minerals, selective precipitation of salts, gravimetric
analysis and fractional crystallization allowing separation of pure salts from mixtures.
Along with improved analytical techniques, the development of two new pieces of
apparatus, the Bunsen burner and the spectroscope by Bunsen and Kirchoff
respectively lead to discovery of elements initially by their atomic emission spectra.
The Bunsen burner allowed high enough temperatures to produce atomic emission
spectra from salts and the spectroscope allowed the unique pattern of spectral lines of
each element to be observed and recorded. In 1860 they discovered the element
caesium initially through its spectral lines which they found in samples extracted from
minerals containing other Group 1 elements. In 1878 holmium and in 1879 samarium
were first discovered by the presence of their spectral lines in extracts from ores. In
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1868 Lockyer observed previously unknown spectral lines in sunlight and he attributed
them to an unknown element which he called helium. Subsequently in the 1890's it
was found that helium is present in our atmosphere along with the other noble gases
neon, argon, krypton and xenon. Their discovery was in part prompted by Ramsay’s
observation that there was room for gaseous elements at the end of each Period of the
Table. Removing all the known gases from air left a small, unreactive component
which was called argon.
However, it required yet another technological advance before neon, krypton and
xenon were isolated. This time the new technology was the ability to liquefy air.
Fractional distillation of liquid air after gases such as oxygen, nitrogen, argon and
carbon dioxide had been removed, left a small fraction which showed an unknown
spectrum - this element was krypton. Careful fractional distillation of this remaining
material revealed the presence of yet another gaseous element with an unknown
spectrum - neon. Xenon was finally isolated by repeated fractionation of liquid
krypton, again it was determined that they had obtained a new gaseous element by
observing its spectrum. Thus a completely new family of hitherto unsuspected
elements, the noble gases had been discovered as a result of combining spectroscopy
and fractional distillation of liquid air.
The development of sensitive apparatus for measuring radioactivity by the Curies was
central to their isolation of the elements polonium (1898) and radium (detected in 1898
but not isolated until 1903).
The chart shows a long gap until another rise in the rate of isolation of elements around
1940 when nuclear reactors were first invented and elements such as plutonium and
americium were isolated for the first time. More recently, the development of high
energy particle accelerators has been applied to production of synthetically produced
elements, generally in microscopic quantities.
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Isolation of the elements - chronological listing.
Prehistory
Au, Ag, C, S, Cu, Sn, Pb, Fe, Sb
1250
1669
1735
1746
1751
1753
1755
1766
1772
1774
1778
1781
1782
1789
1790
1791
1794
1797
1801
1802
1803
1807
1808
1811
1817
1823
1826
1828
1830
1839
1843
1844
1860
1861
1863
As (but used earlier)
P
Co, Pt
Zn (but used earlier)
Ni
Bi (but used earlier)
Mg
H
O, N
Mn, Cl
Mo
W
Te
U, Zr, Be
Sr
Ti
Y
Cr
Nb
Ta
Os, Pd, Ce, Rh, Ir
Na, K
Ba, B, Ca
I
Li, Se, Cd
Al, Si
Br
Th
V
La
Tb, Nd, Er
Ru
Cs
Tl, Rb
In
1866
1875
1878
1879
1880
1885
1886
1894
1895
1896
1898
1899
1900
1907
1917
1923
1925
1937
1939
1940
1944
1945
1947
1950
1952
1953
1955
1958
1961
1965
1970
1974
1976
1982
1984
F
Ga
Yb, Ho
Sm, Sc, Tm
Gd
Pr
Dy, Ge
Ar
He
Eu
Po, Ra, Kr, Ne, Xe,
Ac
Rn
Lu
Pa
Hf
Re
Tc
Fr
Np, Pu, At
Cm
Am
Pm
Bk, Cf
Es
Fm
Md
No
Lr
Rf
Db
Sg
Bh
Mt
Hs
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DATES OF DISCOVERY OF THE ELEMENTS
2
1
H
He
1766
5
B
6
7
8
9
1895
10
Li
4
Be
C
N
O
F
Ne
1817
1798
1808
<1600
1772
1772
1866
1898
11
Na
12
Mg
13
Al
14
Si
15
P
16
S
17
Cl
18
Ar
1807
19
1755
1823
1669
<1600
1774
1894
33
As
34
Se
35
Br
36
Kr
3
21
Sc
22
Ti
23
V
24
Cr
25
26
27
28
29
30
1823
31
K
20
Ca
Mn
Fe
Co
Ni
Cu
Zn
Ga
32
Ge
1807
1808
1879
1791
1830
1797
1774
<1600
1735
1751
<1600
1746
1875
1886
1250
1817
1826
1898
37
Rb
38
Sr
39
Y
40
Zr
41
Nb
42
Mo
43
Tc
44
Ru
45
Rh
46
Pd
47
Ag
48
Cd
49
In
50
Sn
51
Sb
52
Te
53
I
54
Xe
1861
1790
1794
1789
1801
1778
1937
1803
<1600
<1600
<1600
1782
1811
1898
57-71
72
Hf
73
Ta
74
W
75
Re
Os
77
Ir
78
Pt
79
Au
1817
80
1863
56
Ba
1844
76
1803
55
Cs
81
Tl
82
Pb
83
Bi
84
Po
85
At
86
Rn
87
Fr
88
Ra
89-103
104
Rf
105
Db
106
Sg
107
Bh
108
Hs
109
Mt
1939
1898
1965
1970
1974
1976
1984
1982
LANTHANIDES
ACTINIDES
Hg
57
La
58
Ce
59
Pr
60
Nd
61
Pm
62
Sm
63
64
Eu
Gd
65
Tb
66
Dy
67
Ho
68
Er
69
Tm
70
Yb
89
Ac
1899
90
Th
1828
91
Pa
1917
92
U
1789
93
Np
1940
94
Pu
1940
95
Am
1945
96
Cm
1944
97
Bk
1950
98
Cf
1950
99
Es
1952
100
Fm
1953
101
Md
1955
102
No
1958
71
Lu
103
Lr
1961
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The Periodic Table
As each element has its own characteristic properties, this implies that one would need
to be familiar with more than 100 different sets of chemical properties in order to
understand the chemistry of all the elements. However the elements actually consist of
families or groups, each of which contains a number of elements that all share many
similar properties. Thus by knowing the general properties of each group of elements,
the task is made much easier. Further, recognition of the existence of these groups has
led to an understanding of why various properties are associated with each of them.
The arrangement of the elements as chemical groups constitutes the PERIODIC
TABLE, one of the fundamental cornerstones of chemistry which not only embodies
the outward properties of elements, but also incorporates the inner atomic structure of
their atoms.
Development of the Periodic Table.
The earliest suggested grouping of elements was simply on the basis of very obvious
properties such as being shiny or malleable (classed as METALS) or not (classed as
NON-METALS). Metals were further grouped as COINAGE METALS (silver,
gold, copper) or as REACTIVE METALS. From this simple beginning, the modern
Periodic Table evolved.
One of the first classifications into families was by Dobereiner (1829) who noted that
there were often groups of three elements which shared similar properties, e.g.
Ca, Sr, Ba
Li, Na, K
S, Se, Te
Cl, Br, I
Fe, Co, Mn
reactive metals
very reactive soft metals
foul smelling hydrides
highly corrosive non-metals
hard metals, coloured salts
In each case, the atomic weight of the middle member was approximately the
arithmetic mean of the other two. As chemical knowledge increased along with the
number of elements isolated, other bases for classification became possible. One basis
tried was to arrange the elements in order of increasing atomic weight. Newlands
(1864) observed that the chemical properties seemed to be repeated every 8 elements
when this order was used, leading to his law of octaves: "the span between repetitions
in chemically similar species is an octave". While Newlands's classification appeared
valid for the lower atomic weight elements, at higher atomic weights there were many
obvious absurdities with elements of very disparate properties being classed as a
family. This problem arose because many elements had not yet been isolated in 1864,
and no spaces had been left for them in his classification. Dobereiner’s suggestion of
chemical families being related to atomic weight apparently inspired Mendeleev in
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1868 to assemble a small fragment of a Periodic Table, based on more accurate atomic
weights and chemical properties.
element
atomic
weight
element
atomic
weight
element
atomic
weight
Cl
35.5
K
39
Ca
40
Br
80
Rb
85
Sr
88
I
127
Cs
133
Ba
137
In 1869 Mendeleev proposed a classification of the then known 65 elements which
placed priority on allocating elements to each family on the basis of similar properties
with special emphasis on valence rather than atomic weight alone. He left blanks in
families where discrepancies would otherwise appear and repositioned some elements,
disregarding their accepted atomic weights and/or valencies in recognition that these
could be in error. In 1871 he produced an updated version of the Table which included
then unknown elements to which he gave names such as eka-aluminium (following
aluminium: now gallium) and eka-silicon (following silicon: now germanium). Using
this Table he predicted the properties of the missing elements based on the
corresponding properties of the preceding and following element in the Group. His
predictions were remarkably accurate when the missing elements were ultimately
isolated.
The Periodic Law proposed by Mendeleev states: “When arranged by atomic mass, the
elements exhibit a periodic recurrence of similar properties”.
The modern Periodic Table.
With the discovery of the sub-atomic particles and the subsequent knowledge of the
structure of atoms, the fundamental basis for the periodic classification was realised to
be arrangement in order of increasing atomic number (the number of protons in the
nucleus), rather than atomic weight (Moseley, 1913). The difference from atomic
weight order is due to the various isotopes that contribute to the atomic weight (the
weighted average of all naturally occurring isotopes of that element) so that an element
which exists as an abundant heavy isotope but a lower atomic number would be out of
order and appear in the wrong Group. There are only three instances where atomic
weight order departs from atomic number order arising from the proportions of the
different isotopes of each element. Atomic number is the number of protons in the
atom’s nucleus and as atoms are electrically neutral, is numerically the same as the
number of electrons in the atom. As the arrangement of electrons around the nucleus
depends on how many electrons are present, then it is the electronic structure of the
atom which determines the properties of each element. Thus what began as a
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classification solely on the basis of properties of elements was found to be a
classification based on the atomic structure of the elements. A copy of the modern
Periodic Table is given on the last page of this book.
The Periodic Table in Review.
(i) The periods.
Each horizontal row or PERIOD of the Table starts with a Group 1 element and ends
with a Group 18 element. [Note: There are several different Group numbering systems
in use. One uses Roman numerals and extends from Group I to VII plus 0 or VIII for
the noble gases while the current IUPAC system uses normal numbers and extends
from 1 to 18. The IUPAC system is used in this Topic.] In between, there are various
numbers of elements as follows:
1st period
H, He
2 elements
2nd period Li v Ne
8 elements
3rd period Na v Ar
8 elements
4th period K v Kr
18 elements
5th period Rb v Xe
18 elements
6th period Cs v Rn
32 elements
7th period Fr v
see “The Search Continues” on Page XII-20.
Elements in each period do not constitute a family and there is no value in specifically
committing to memory the members of each Period of the Table. Indeed, across each
Period there is a steady change in properties from those elements classed as metals (on
the left hand side) to those classed as non-metals (on the right hand side). Properties of
metals compared with non-metals have been mentioned in previous Topics, but the
following extended summary of physical and chemical properties of metals compared
with non-metals is appropriate at this point.
METALS
Good conductors of heat and electricity
Malleable, ductile
Shiny appearance when freshly cut
React with non-metals to form cations in salts
Dissolve in acids to form cations
Ionic halides
Have oxides which are ionic and dissolve
in acids to form salts (basic oxides)
NON-METALS
Poor conductors
Brittle, often powders or gases
Dull appearance
Form anions in salts
Not soluble in acids
Covalent halides
Have covalent oxides which are
insoluble in acids but may
dissolve in bases to form salts
(acidic oxides).
The basis for the above properties and why they are associated with a metal or a nonmetal are now well understood in terms of the structure of each element’s atoms.
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Variation of atomic properties across Periods
The sizes of the atoms (as measured by the atomic radius) decreases from left to right
across any Period of the table. [See diagram on page XII-10] This may seem strange at
first, given that additional electrons are being added to the structure of the atom as the
Group number increases. The explanation lies in the fact that as the atomic number
increases for each extra electron added to the structure, there is also an extra proton
present in the nucleus. For a given Period, the added electrons are all in the same orbit
and so all outer electrons experience the additional attraction of the increased number
of protons, leading to reduced atomic radius. At the end of each Period, the last
element is a noble gas after which there is no more room in the electron orbit that is
being filled to accommodate further electrons. The next element (a Group 1 element)
is the first in the following Period and its added electron must occupy a higher orbit,
further out from the nucleus. Each time a new orbit is occupied, the electron allocated
to it is partially SCREENED from the attraction of the nucleus and so its atomic
radius increases to become the largest for that next Period. Moving across this next
Period, the same process is repeated with atomic radius decreasing from left to right,
becoming smallest for the next noble gas, then again increasing for the subsequent
Group 1 atom as electrons occupy the next highest orbit, and so on throughout the
Table.
Consequently if one examines the attraction felt by outer electrons of the atoms from
left to right across any Period, it increases until reaching a maximum for the noble gas
element at the end of that Period. This can be expressed as the charge actually felt by
the outer electrons called the EFFECTIVE NUCLEAR CHARGE Sometimes this is
called the CORE CHARGE. Noble gases are unreactive because their outer electrons
experience a very large effective nuclear charge and too much energy is required for
them to be removed in a chemical reaction. Thesingle outer electron of atoms of Group
1 elements experiences a much smaller effective nuclear charge due to the screening of
that electron by all the other electrons and so only a relatively small amount of energy
is required to convert Group 1 atoms to 1+ ions. Having lost an electron to form the
1+ cation and thus become isoelectronic with a noble gas, a very large amount of
energy would then be needed to remove a second electron - the high effective nuclear
charge of the noble gas structure is reinforced by the excess 1+ charge on the ion - and
so a 2+ ion does not form for Group 1 elements.
For Group 2 elements, it is not until after two electrons have been removed that the
noble gas structure is attained with its large effective nuclear charge. While it does
require more energy to remove the second electron from a Group 2 atom than was
required to remove its first electron, stable 2+ ions still result for Group 2 elements
(apart from Be2+ due to the small radius of the Be atom).
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Similarly stable 3+ ions are the norm for most elements in the Group starting with
boron, but not boron itself due to its extremely small atomic radius. From the Group
starting with carbon onwards across a Period, the large excess positive charge that
would result on the nucleus prevents stable cations forming.
The atoms of the Group just prior to the noble gas at the end of each Period (the
halogens) have an effective nuclear charge which is only slightly less than that of the
adjacent noble gas and they have room for one more electron in that outer energy level.
Consequently these atoms are readily able to form 1! anions by gaining an electron
and actually release some energy in the process. Hence all the halogen atoms are
readily converted to the corresponding halide ions carrying a 1! charge. However, in
order to gain a second electron and form a 2! ion, that second electron would have to
occupy the next outer atomic orbit and thus be screened from the nuclear charge by all
the other electrons, so reducing the effective nuclear charge it would experience. To
achieve this would require the input of too much energy to form a stable anion and so
no 2! ions of halogens exist.
Similarly, outer electrons of the atoms of the Group starting with oxygen which are all
just 2 electrons short of the structure of the noble gas at the end of the Period are
subject to a high effective nuclear charge and can form anions with a 2! charge, but
formation of a 3! ion is energetically too difficult. Thus it can be seen that the
transition from metal to non-metal from left to right across a period can in part be
attributed to the accompanying increase in effective nuclear charge of the atoms.
(ii) The Groups.
Each of the vertical columns of the Table headed Groups 1 through to 18 constitutes a
chemical family of elements. Each family has many chemical properties in common.
(You will recognise eight of these as the groups that you committed to memory at the
beginning of this course.) The chemical similarities within each Group are attributable
to there being the same number of electrons in the outer level of the atoms of elements
in that Group. For example, atoms of all Group 1 elements have 1 electron in their
outer level; all Group 2 atoms have 2 electrons in the outer level and so on. This is the
reason for there being common valencies within any given Group, a property to which
Mendeleev gave priority in devising his version of the Table.
While the elements of each Group have many properties in common, there are also
differences in properties within the Group. These differences typically are exhibited as
a trend from the first to the last element within each Group. The most notable is that
metallic properties (which are associated with the ease of removal of outer electrons)
increase down the Group. This is because the outer electrons are partially screened
from the nuclear charge by the inner electrons and thus attracted less strongly to it and
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so are located at an increasing distance from the nucleus. The larger the atomic radius,
the weaker the outer electrons are held.
Probably the most striking instance of this transition is to be seen in the elements of
Group 15. At the top of this Group, the element nitrogen is totally non-metallic - it is a
gas, exists as covalently bonded N2 molecules and forms an anion with a 3! charge in
salts. The element bismuth at the bottom of Group 15 is a solid with metallic bonding
between atoms and forms 3+ cations in salts.
Another good example of how increasing atomic radius leads to greater reactivity as a
consequence of the outer electrons being further from the attraction of the nucleus is to
be found in the reactions of Group 1 elements with water to form hydrogen gas and
hydroxide ions. The element at the top of this Group, lithium, only reacts mildly with
water. The element at the bottom, caesium, reacts violently. The elements in between
become increasingly reactive to water down the Group.
The d-block and f-block elements.
In addition, there is a large block of elements, called the d-block, located towards the
middle of the table containing Groups 3 to 12. These elements are often called the
transition elements. You will also recognise some of these elements as those in the last
section of Table 2 from Topic 1.
Finally, there is another block of elements called the f-block located near the bottom of
the Periodic Table.
Properties of the Groups
The following notes provide an outline of the more important chemical properties of
each of the main Groups and also very briefly discusses the d-block and f-block
elements. Also a range of the elements are discussed in more detail with emphasis on
those properties which have significant useful applications.
GROUP 1. Li, Na, K, Rb, Cs, (Fr)
Group Overview.
Group 1 elements are known as the alkali metals because they react with water to form
hydroxides which are all soluble in water. They are all soft, very reactive metals
which can be cut with a knife and tarnish rapidly so must be kept under oil to
protect them from reacting with air. They all react with water to form hydrogen gas
and hydroxide ions, the reaction becoming increasingly violent down the Group.
This is a redox reaction in which the metal is oxidized and hydrogen atoms in water are
reduced.
2Na + 2H2O v 2Na+ + 2OH! + H2(g)
XII - 16
They form ionic compounds called "salts" with non-metals. For example, sodium and
chlorine combine in another redox reaction to form sodium chloride, a compound
known as common salt or table salt. The elements in such compounds are no longer
made up of electrically neutral atoms of free element, but instead are present as species
called ions - atoms which have gained or lost electrons so as to have an electrical
charge. When a salt forms, the Group 1 metal atoms have each had one electron
removed, leaving them as +1 charged cations. As all Group 1 elements have a single
outer electron, they all form only the M+ ion in their compounds because this
arrangement leaves the ions with the noble gas electron structure. In all compounds of
Group 1 elements, each atom shows a combining power or valence of 1 only. To form
cations with a 2+ or higher charge would require too much energy. The physical and
chemical properties of Group 1 elements can be clearly related to this aspect of their
atomic structure. The lone outer electron of Group 1 elements in the solid state can
move from atom to atom easily when an electrical voltage is applied, causing these
elements to be good conductors. As there is only the one outer electron available to
bind each atom of the metal to its neighbours in the solid state, the metallic bond
between the atoms is easily broken and causes the softness of Group 1 elements. In
later groups where there are more outer electrons available to participate in metallic
bonding, the elements become harder. The increased reactivity down the Group is a
consequence of the outer electron being further from the nucleus and therefore more
easily removed.
The atoms of a non-metal reacting with Group 1 elements gain the electrons that were
removed from the metal atoms when their cations formed and so the non-metal atoms
also become ions but with a negative charge (anions). The two oppositely charged ions
are then held together by electrostatic attraction to form an ionic compound. These are
characteristic reactions of all Group 1 elements.
For example, in the reaction of sodium with the non-metal chlorine, the process could
be shown as
2Na + Cl2 v 2Na+Cl–
However, the charges on the Na+ and Cl– in such compounds are not usually shown in
their formulas. It is a typical property of metals that they form ionic halides whereas
non-metals form covalent halides.
When a salt is melted or when it is dissolved in water, it shows electrical conduction.
This is because the ions are no longer held together in a solid crystal but instead, under
the influence of an electrical voltage, move to the electrode of opposite charge and
there undergo a reaction in which cations gain electrons and anions lose electrons to
form free elements again. This process is called electrolysis and is the reverse of the
reaction by which the original salt was formed. Electrolysis of molten sodium chloride
XII - 17
will produce the elements sodium and chlorine at the electrodes according to the
following equations.
2Na+(l) + 2e– v 2Na(l) and 2Cl–(l) v Cl2(g) + 2e–
Reactions of this type, redox reactions, were examined in more detail in Topic 11.
Sodium and potassium ions are important in the conduction of nerve impulses.
Lithium ions provide a treatment for schizophrenia. Common compounds of Group 1
include sodium hydroxide (caustic soda), used in oven cleaners and as a starting
material in many industrial processes and sodium hydrogencarbonate (NaHCO3) which
is used in cooking.
FOR THOSE WHO WANT TO KNOW MORE - lithium, sodium, caesium.
LITHIUM
Lithium is the third lightest atom and has the smallest radius of any metal. The Li+ ion
is the smallest stable metal cation, a property which makes it ideal for use in the
lithium ion battery. [See the table of radii on page XII-13].
Lithium is extracted from salt beds in the deserts of some South American countries.
Unlike other Group 1 elements, it has an insoluble carbonate which provides an easy
method of extraction and purification. Until about 1990, lithium had only minor uses,
the main one being as a treatment for bipolar disorder which is readily controlled with
lithium carbonate. Prior to the introduction of this treatment, psychiatric hospitals
housed large numbers of sufferers but following the introduction of lithium treatment,
they were emptied. The mode by which it works is not known but is assumed to
replace sodium ions in some of the functions of the brain in a way that calms the
depression and suicidal thoughts that are part of bipolar disorder.
At that time, apart from treating bipolar disorder, the other main application for lithium
compounds was in greases where lithium soaps are added to normal grease to make it
flow better.
Lithium aluminium alloys are now being used in aircraft construction.
Lithium batteries.
The market for lithium increased vastly when batteries based on lithium were devised,
especially the lithium ion battery. Lithium batteries now power most portable
electronic devices where the light weight and in the case of lithium ion batteries, the
ability to recharge them are unchallenged at present. They are also being used to
power electric vehicles and to store electricity from renewable sources to smooth out
the variation in supply. These batteries have the advantages of large charge density
and being rechargeable many times before they degrade.
XII - 18
There are two types of lithium battery:
(i) Non-rechargeable. These are typically the small button-sized batteries used in
small electronic devices. This type contains elemental lithium and the oxidation of Li
atoms to Li+ ions is the source of the electron flow from the negative electrode. The
positive electrode is typically MnO2. Because lithium is so reactive, this type of
battery is not suitable for large applications.
(ii) Rechargeable (lithium ion) battery..
The lithium ion battery is so named because the lithium is not present as lithium metal
but only as Li+ ions and there is no redox reaction involving them. The Li+ ions are so
small that they can move through solid graphite which constitutes the negative
electrode and also through the compounds of various metals such as CoO2 used for the
positive electrode. In a lithium ion battery the graphite anode and cathode which
contains a ‘mixed metal’ oxide, typically LiCoO2 are divided by a separator which
allows Li+ cations, but not electrons to flow through it in the same way that H+ ions
move through the familiar lead battery. When the battery is being charged, the
charging voltage causes electrons to be removed from the metal electrode which in the
case of cobalt undergoes an oxidation number increase to +IV. To preserve electrical
neutrality in this electrode, Li+ ions move out from the mixed metal electrode into the
electrolyte and through the barrier into the other half cell containing the graphite
electrode. At that electrode, the charging voltage forces electrons into the graphite
layers and the resulting excess negative charge is neutralised by the flow of Li+ which
squeeze into the spaces between the carbon layers. When fully charged, the battery has
the lithium ions packed between the layers of carbon atoms in the graphite anode.
When discharging, the reverse process occurs with the electrons flowing from the
graphite, through the device to be powered and into the mixed metal electrode where
the cobalt accepts the electrons and undergoes reduction from +IV to a lower oxidation
state. To preserve electrical neutrality in the battery, the lithium ions move from the
graphite electrode into the sponge like structure of the mixed metal oxide.
A typical lithium ion battery produces 3.7 V compared to 1.5 V of a standard dry cell.
There is intense research in this area to discover an electrode system that can work
with the larger, and much cheaper, sodium ion. Because the Na+ ion is too large to fit
between the layers of graphite, chemical reactions are being used to expand the gap
sufficiently. Also under investigation is the use of silicon, another Group 14 element,
to replace graphite.
The fully charged and discharging states of the lithium ion battery are represented in
the following diagram.
XII - 19
SODIUM
The symbol Na for sodium is from the Latin, natrium. Elemental sodium is produced
by electrolysis of molten sodium chloride. Large deposits of sodium chloride occur in
dried-out lake beds.
Elemental sodium is used in sodium vapour lamps in which the metal is heated by an
electric current under vacuum inside a glass envelope Sodium atoms are released into
the gas phase and electrons are excited to higher energy orbits before falling back to
the ground state and releasing the yellow light which is often used for street lighting.
Sodium vapour lights are much more efficient that incandescent lamps, converting
70% of the electrical energy to light.
Sodium compounds are so ubiquitous that they are the source of the yellow flashes
from their atomic spectrum that can be seen in gas flames and fires.
Sodium chloride.
The common name “salt” used for sodium chloride is derived from the Latin and is
incorporated into numerous English words such as salacious, salsa and salary - the
latter because Roman soldiers were paid in salt. Historically salt has played a major
role in commerce and social changes over millennia. The vital need for salt to preserve
food and to keep people and animals healthy meant that controlling the supply of salt
was most advantageous both commercially and militarily. Salt was of equal value to
gold the middle ages and was the basis for the existence and prosperity of cities such as
Venice which had a monopoly of salt supply in that region.
Capitalising on monopolies of salt through salt taxes were one of the underlying causes
of the French revolution and the American war of independence. The beginning of
XII - 20
civil disobedience in India was started by Ghandi’s march across the country to fight
the British monopoly of salt there. Lack of salt was one factor for Napoleon’s defeat
in Russia and the defeat of the confederate army in the American civil war. By the 20th
century, refrigeration and modern packaging reduced the need for salt to preserve food.
Sodium chloride is still used in some cases as a food preservative and additive but
before the advent of refrigeration, food such a meat was “salted” for storage and hence
the expression “to salt something away” meaning to conserve it.
Because it is readily accessible, salt is the ideal starting material for the manufacture of
industrial chemicals such as sodium hydroxide and chlorine.
In countries where snow falls, large amounts of sodium chloride are spread on roads to
melt snow and ice.
Sodium ions are an essential component of the nervous system because conduction of
currents through nerves is achieved by movement of Na+ ions through membranes
rather than movement of electrons as in metallic conduction. Sodium chloride also
plays an important role in regulating blood pressure. This is because the sodium and
chloride ions attract water molecules - in particular, each Na+ ion can attract more than
20 H2O molecules around it. The effect of excessive salt concentrations in the blood is
to increase water uptake causing the volume and thus blood pressure to increase.
However, lack of salt for example when suffering from diarrhea can be fatal. Lack of
circulating salt in the blood can cause cramps. The normal mass of sodium chloride in
the human body is about 250 g. Salt licks are used to attract cattle to various locations
in paddocks.
Sodium hydroxide.
Sodium hydroxide in water solution provides the strong base, OH!. Such strongly
basic solutions have many industrial applications, such as:
Paper manufacture - wood consists of cellulose fibres held together with lignin. In the
Kraft method of paper production, wood chips are boiled in a mixture of sodium
hydroxide and sodium sulfite which removes the lignin. Subsequent squeezing out of
the water solution and drying results in paper sheets,
Soap - soaps are salts of fatty acids which are the components of fat. Heating with
sodium hydroxide causes the fats to break down to the sodium salts of these
component acids. Soaps dissolve dirt and grease because their anions consist of a
negatively charged end and a long hydrocarbon end. The negative end of the anion
can interact with water through ion-dipole attractions while the hydrocarbon end can
interact with non-polar entities such as grease through dispersion forces (see
Supplementary Topic 4). As a result, particles of grease and dirt or oils can be
dispersed into water as very tine droplets.
XII - 21
Sodium hydrogencarbonate.
Also commonly known as sodium bicarbonate, NaHCO3, this compound is used in
cooking to make cakes rise. In water solution it has a pH of about 8 and is used where
a mildly alkaline environment is required and it is used in antacid tablets. Soft drinks
used to contain sodium bicarbonate which released bubbles of carbon dioxide by
reacting with acid in the drink and hence the name soda pop although now the carbon
dioxide is gas forced in under pressure. Heating causes carbon dioxide to be released
from sodium hydrogencarbonate which is why it is used in cooking where it is usually
mixed with an acidic ingredient. It is also utilised in solid state fire extinguishers.
Sodium carbonate.
Sodium carbonate, Na2CO3, is soluble (as are all sodium salts) and produces a basic
solution of pH about 10 . It is used in washing powders where it functions as a water
softener. Sodium carbonate is also an important component in the manufacture of
glass. Some toothpastes include sodium carbonate and sodium hydrogencarbonate to
counteract the acids in the mouth resulting from eating carbohydrates.
CAESIUM
Caesium, symbol Cs, has a very low melting point, 28o C. A vial of solid caesium will
melt if held in the hand. It is the softest of all the solid elements. Being at the bottom
of Group 1 of the Periodic Table, its outer electron is the most readily removed of any
element. Thus caesium is extremely reactive and must be protected from the
atmosphere but its compounds are very stable.
Caesium was the first element to be initially discovered by observing its spectrum in a
flame. This discovery was made by Robert Bunsen using his newly invented gas
burner in conjunction with Gustav Kirchoff, the inventor of the spectroscope.
Salts of caesium such as caesium formate are used in drilling fluids in the mining
industry where the relatively high density of the Cs+ ion causes less dense liquids such
as oil to be displaced.
The radioactive isotope 137Cs which is derived from nuclear reactors is used for various
medical applications.
Caesium clocks.
Extremely accurate methods of determining time are of increasing importance. High
precision is needed for GPS devices, electricity grid stability and telecommunications
amongst many others. Use is made of the frequency at which a particular electron
transition occurs in the 133Cs atom which can be measured with extreme accuracy.
Using this standard, an accuracy of 1 second in 20 million years is currently available
and using a different basis, it is anticipated that an accuracy of 1 second in 13 billion
XII - 22
years will be achieved, a greater time interval than the age of the universe. Because
the rotation of the earth is not constant and is slowing, in order to keep the time as set
by the atomic clock synchronised with “normal” time, a leap second is introduced
occasionally.
GROUP 2. Be, Mg, Ca, Sr, Ba, (Ra)
Group Overview.
These are also metals, but are harder than Group 1 elements and their reactions are
slower because more energy is required to remove the valence electrons.
Beryllium is hard enough to scratch glass but at the bottom of the Group, barium is
only slightly harder than lead. Increasing hardness compared with Group 1 elements
can be attributed to the doubling of the number of outer electrons and a resulting
increase in the strength of the metallic bonds. Apart from beryllium, they generally
form salts with non-metals, always showing a valence of 2 in their compounds due to
the presence of 2 outer electrons in all their atoms. Removal of the two outer electrons
leaves all Group 2 elements with the noble gas structure. Beryllium forms covalent
compounds with non-metals, a property more like that of a non-metal than a metal.
The reason for this is that the Be atom is very small and the energy required to remove
even one of its outer electrons is relatively high. Removal of both outer electrons from
subsequent atoms in the Group becomes easier due to the increased size of the atoms
and therefore less attraction between outer electrons and the nucleus and this explains
the increased reactivity observed down the Group.
Some typical reactions of Group 2 elements (i) burn Mg in air
2Mg + O2 v 2MgO
(ii) Ca in water
Ca
+ 2H2O
v Ca(OH)2 +
H2
calcium hydroxide
Alloys containing beryllium are used in aerospace applications. Beryllium is used as
windows in X ray sources as it is transparent to X rays. It is also used in the nuclear
industry because it reflects neutrons. However beryllium dust can cause fatal lung
diseases. Mg is the central atom in chlorophyll. The compound calcium carbonate
(CaCO3) is present in shells and calcium phosphate (Ca3(PO4)2) is of particular
importance as a component of teeth and bones. Large amounts of calcium carbonate
(limestone) are used commercially in the manufacture of glass and cement. Radium
has been used in radiotherapy treatment of cancer.
XII - 23
FOR THOSE WHO WANT TO KNOW MORE - calcium.
CALCIUM
Calcium is found as various salts, mostly as calcium carbonate of which limestone is
constituted. Calcium compounds in the earth’s crust are slowly eroded and washed
into the oceans where calcium ions are combined with carbonate ions formed from
carbon dioxide which has dissolved in the water. Over long periods the insoluble
calcium carbonate settles to the ocean floor, joined by shells from microscopic
organisms. The usual geological processes result in compression and ultimately
uplifting to reveal limestone such as the huge cliffs that border the shores of much of
the Southern Ocean in Western Australia. Thus limestone represents a vast storage of
carbon dioxide, extracted from the atmosphere in previous eras. Shells of sea creatures
and exoskeletons of some marine organisms are made from calcium and carbonate ions
present in sea water. Carbon dioxide dissolved in water forms a small amount of
carbonic acid. The consequence of increasing CO2 in the atmosphere is an increase in
the acidity of the oceans. Because calcium carbonate, the main constituent of marine
invertebrates, dissolves in acids, the acidity of their environment is critical and much
marine life is threatened by the increased atmospheric CO2 concentration.
Cement.
The most widespread use of a calcium compound is for manufacturing cement which is
the vital ingredient of concrete. Limestone is heated strongly to drive the carbon
dioxide off leaving calcium oxide or lime. Adding water to lime produces slaked lime,
a strong waterproof bonding material. However, the process of driving the CO2 off the
limestone contributes significantly to the increased concentration of this greenhouse
gas in the atmosphere, not only by the heating needed but also the CO2 released from
the limestone. The production of 1000 kg of cement releases 800 kg of carbon dioxide
to the atmosphere and amounts to 5% of the world’s total annual CO2 release.
Calcium sulfate.
Calcium sulfate (gypsum) is another calcium compound found in large deposits and it
is used in the building trade for plastering and also for setting broken bones in which
application it is known as Plaster of Paris.
Teeth and bones.
Calcium compounds are a large proportion of the body’s weight as bones and teeth are
made of calcium phosphate. Bones form when special cells produce a scaffold made
from protein which gives the bone its tensile strength and then calcium and phosphate
ions form the solid bone which provides its compressive strength. Calcium ion intake
is required throughout life to maintain healthy bone.
Other physiological functions of calcium ions.
XII - 24
Apart from solid structures in the bodies of most living creatures, calcium ions are
also a vital component in numerous physiological processes. For example, calcium
ions are used in the transmission of nerve impulses, facilitate the operation of muscles
and serve as cofactors that helps various enzymes to operate in functions such as
stabilising blood pressure and blood clotting. In plants, calcium ions are vital in the
mechanism that causes the stomata to close when required as well as many other
functions.
GROUP 13. B, Al, Ga, In, Tl
Group Overview.
Boron is a non-metal, having a black powdery appearance and not having any of the
usual properties of metals. Boron is a non-conductor of electricity and has a very high
melting point (2040 o C), indicating covalent network bonding of the boron atoms in the
solid. It has an acidic oxide, typical of non-metals and it does not form ions when it
reacts to produce compounds, but instead bonds by sharing electrons with the bonded
atoms to form covalent compounds. This is the method by which non-metals bond to
each other. The other elements of Group 13 mostly form ionic compounds. The
Group 13 elements usually have a valence of 3 in their compounds, due to the presence
of 3 outer electrons in their atoms.
Aluminium, the third most abundant element in the earth’s crust, has considerable
commercial application due to its very high strength to weight ratio and also its being a
particularly good conductor of electricity. Aluminium is produced in large quantities
by electrolysis of aluminium oxide ores such as bauxite. Although aluminium is very
reactive, it forms an oxide layer (Al2O3) on its surface which protects the metal from
further corrosion, making it useful as a building material. However, at high
temperatures, aluminium burns vigorously.
This Group shows clearly what is a general trend whereby the elements display
increasing metallic properties down the Group. This trend is quite subtle in Group 1
where lithium does form a few covalent compounds and is more pronounced in Group
2 where beryllium forms compounds that are predominantly covalently bonded but
retains many other properties of a metal including metallic bonding in the solid
element. In Group 13, boron is essentially a non-metal in appearance and in physical
and chemical properties. Aluminium forms both ionic and covalent compounds as do
the other elements of this Group, but all exist as stable cations in solution. This trend
from non-metallic to metallic properties down a Group continues to be apparent in
Groups 14 - 16, attributable as discussed previously to the larger size of the lower
atoms in each Group resulting in weaker attraction between nucleus and electrons and
thus less energy being required to form cations.
XII - 25
FOR THOSE WHO WANT TO KNOW MORE - boron, aluminium, gallium,
indium.
BORON
Boron forms an oxide, B2O3, which dissolves in water to form a weak acid, boric acid.
Both compounds have widespread applications. Compounds of boron have some mild
anti-microbial properties. Boric acid is used to keep contact lenses free from bacteria
and as an eye wash. Its sodium salt is used as an insecticide which is harmless to
humans but effective against insects and it is typically the active ingredient of ant and
cockroach traps. It is also used in trace amounts as fertilizer for plants. The very small
size of the B atom allows boron atoms to fit into small spaces in metals’ crystal lattices
where they prevent sliding of planes of metal atoms and thus impart greater strength to
the crystal structure.
Boronsilicate glass.
Adding up to 15% of boron oxide to normal glass made from silicon dioxide (silica)
imparts the property of having minimal expansion on heating. This allows its use in
applications such as laboratory glassware and cooking utensils where heating would
cause cracking of normal glass. Consequently one can heat test tubes, beakers,
distillation flasks and other equipment made from boron-containing glass without
risking breakage. Varying the boron content of glass allows any coefficient of
expansion to be produced for a given task. Lenses made from normal glass suffer from
achromatic distortion whereby the lens fails to focus the various wavelengths (colours)
of light equally, leading to blurred images. Using boronsilicate glass eliminates this
difficulty and achromatic lenses can operate down to the limits imposed by the
wavelength of light.
Abrasives.
The compound boron carbide is one of the hardest materials known, third only to
diamond and another boron compound, boron nitride. It is used as an industrial
abrasive (carborundum) and also to make body armour. Because boron is also a very
light element, the weight of boron carbide armour is minimised, an important
consideration when wearing it.
Reactor control rods.
Nuclear reactors operate because of a flow of neutrons from decaying 235U atoms
impinging on other uranium atoms and continuing the fission process which produces
more neutrons in a chain reaction. If the flow of neutrons is not regulated, the reactor
would overheat and meltdown of the core would occur. The 10B atom’s nucleus has
the ability to capture neutrons. Control rods packed with boron oxide or boron nitride
are placed beside the fissile material in the core. By raising or lowering the rods, the
reaction can be turned on or off as required.
XII - 26
Boron neutron capture therapy.
This technique, still being developed, relies on the ability of the 10B nucleus to absorb a
neutron and convert to the unstable 11B atom which then decomposes to release an alfa
particle and a lithium atom - the same process that is used in nuclear reactors. In this
procedure, a suitable boron compound is injected and the tumour site is exposed to a
focussed source of neutrons of the appropriate energy. The release of alfa particles
that ensues damages the tissues that have absorbed the boron compound. To be
effective, the boron needs to be well distributed throughout the tumour.
ALUMINIUM.
Aluminium is the most abundant metal in the earth’s crust and occurs in easily
accessible deposits well distributed world-wide, so its supply is guaranteed. A lot of
electricity is required to liberate the free metal by electrolysis of its common ore,
bauxite, which is aluminium oxide or Al2O3. Consequently aluminium smelters are
usually located close to cheap sources of electricity such as hydroelectric generators.
Aluminium is very reactive but is stabilised by the formation of an oxide layer on its
surface. The low density of aluminium combined with mechanical strength and
resistance to corrosion are why it is used in so many industrial applications which
include motor vehicle, aeroplane and boat construction. Food and beverages are
ubiquitously contained in aluminium cans. Other application use the very shiny
surface attainable on aluminium metal. Thus it is used in insulation materials and
thermal blankets and also, if sprayed onto glass as a thin layer, it is used to make
mirrors,
Sapphire.
Aluminium oxide is one of the hardest substances known and is used as an abrasive
including sandpaper. Sapphires are crystals of Al2O3 which was subjected to very high
temperatures and pressures deep in the earth’s crust before ultimately being uplifted by
geological processes. Sapphires are used in various applications which take advantage
of their property of hardness. These include as bearings that resist wear such as in
mechanical clocks and watches and as components of various scientific instruments.
The colour of sapphires is derived from small amounts of impurities consisting of
atoms of elements of similar size to that of aluminium. Small amounts of iron atoms
results in the blue colour which is common. Other colours are attributed to addition of
atoms of titanium, copper, magnesium and chromium, all of which are a neat fit to
replace aluminium atoms in the crystal. Addition of chromium atoms produces the red
colour of ruby which is another gemstone of aluminium oxide. Artificial sapphires are
grown at very high temperatures around 2500o C
Recycling.
Aluminium is recycled very efficiently with typically about 50% being reused. This is
commercially viable because the amount of energy needed to reuse aluminium metal is
XII - 27
only 5% of that needed to produce it from its ore. There is no limit on how often
aluminium can be recycled. In theory, ultimately there would be no need for newly
smelted aluminium but given its expanding market, that would be a long time into the
future.
GALLIUM AND INDIUM.
These two consecutive elements in Group 13 are both soft metals with low melting
points. Indium is named not for the country but after the characteristic indigo spectral
line by which it was first discovered. The existence of gallium along with some of its
properties was predicted by Mendeleev when proposing his version of the Periodic
Table and he called the yet to be discovered element eka-aluminium for “following
aluminium”.
Gallium melts at 30o C and indium melts at 157o C . Both “wet” glass which means
that they adhere to glass which makes them ideal for soldering electrodes to glass
surfaces. They are both soft, shiny metals. Indium has a crystal structure that causes it
to emit a crying sound when bent, like tin, due to planes of their atoms moving relative
to each other. They do not occur in minerals in quantities worth mining but instead are
extracted as trace elements from ores of zinc (indium) or aluminium (gallium).
Compounds.
Alloys of gallium, indium and often tin are used for example as solders for special
applications. The compound indium tin oxide is a conductor of electricity and is used
to coat touch screens and liquid crystal displays. An alloy of gallium and indium with
tin (galinstan) is a liquid at room temperature and is used in medical thermometers.
With mercury being banned, its use in normal thermometers will provide a substitute.
Light emitting diodes.
Compounds of gallium and indium with various Group 15 elements are used in the
production of light emitting diodes (LEDs) and semi-conducting lasers which are the
type found in modern electronic devices. To manufacture LEDs, a substrate is used
onto which is deposited very thin layers of atoms of gallium, indium and various other
elements. For lighting LEDs, the substrate is a small sapphire but different materials
including silicon are also used for other applications. LEDs are much more efficient
than conventional globes as almost all the electricity used is converted to light whereas
an incandescent light globe converts 95% of the energy supplied to heat. As their cost
is reduced, LEDs are rapidly replacing other types of lighting because of their greater
efficiency and very long lifetimes. About 35% of worldwide electricity production is
used for lighting so widespread replacement with LEDs will make a significant
reduction in carbon dioxide emissions. Another rapidly growing application is the use
of LEDs in TV picture tubes.
XII - 28
The principle of LEDs can be used in reverse to convert light energy to electricity as
solar panels. Compounds based on these two elements have produced efficiencies of
40% in conversion of sunlight compared with 10% typically for the normal silicon
panels because unlike silicon, these compounds can utilise all visible frequencies. At
present the cost precludes their mass production but they are used in some special
environments. Using mirrors to focus sunlight onto a small area of such materials
overcomes their larger cost.
Solid state lasers.
The small blue-light lasers used in CD and DVD players are mostly made using
gallium(III) nitride, GaN, deposited onto saphire substrate. The process to do this
successfully was elusive for many years but finally was put on a commercial basis in
the late 1990s, a feat which earned its developers the 2014 Nobel Prize in Physics.
Blue light lasers lead to the development of high density DVD discs which allow much
more data to be stored because the blue light is of shorter wavelength than that used
previously.
Recycling.
Due to the lack of concentrated sources of ores containing gallium and indium,
significant amounts are obtained by recycling. Indium supplies are likely to be a
problem at some time in the future.
GROUP 14. C (non-metal) Si, Ge (intermediate) Sn, Pb (metals)
Group Overview.
Carbon occurs as graphite and diamond as well as an amorphous (non-crystalline) form
such as charcoal. Carbon also occurs as hollow, soccer-ball shaped arrangements of up
to 76 or more C atoms known as fullerenes. These are examples of allotropes different physical forms of the one element arising from different arrangements in the
way their atoms are bonded.
XII - 29
Diamond contains a large number of carbon atoms joined by network covalent
bonding. Each C atom is bonded to 4 other carbon atoms by covalent bonds which are
pointing to the corners of a tetrahedron. Graphite contains carbon atoms which are
bonded to 3 other carbon atoms, all in the same plane, and also has weak bonds to
carbon atoms in the plane above and below. It is these weak bonds which cause
graphite to easily peel off into flakes when it is used in "lead" pencils or in graphite
lubricants. Graphite is exceptional among non-metals in being a conductor of
electricity, also due to the mobility of the electrons that constitute the weak bonds
between the planes of carbon atoms.
Carbon has an acidic oxide, CO2, forms only covalent bonds in compounds including
its halides and apart from graphite, does not conduct electricity - typical properties of a
non-metal. Carbon is the fundamental element in living cells, and is recycled through
the carbon cycle. Many millions of carbon compounds exist as natural products or
through laboratory or commercial synthesis. Because carbon has the ability to form
molecules containing long chains of covalently bonded carbon atoms along with atoms
of many other elements, almost unlimited numbers of carbon compounds are yet to be
prepared. The chemistry of carbon compounds is called ORGANIC CHEMISTRY.
Important inorganic compounds of carbon include carbon dioxide, CO2, a product of
RESPIRATION and the main GREENHOUSE GAS from burning fossil fuels.
Carbon monoxide, CO, which results from the incomplete combustion of carbon
compounds is a powerfully toxic substance. Fortunately, carbon monoxide rapidly
converts to carbon dioxide in the atmosphere and does not accumulate to large
concentrations.
Silicon is the fundamental element of mineral chemistry, and it makes up the majority
of the earth's crust - usually in combination with oxygen as compounds called
SILICATES. For example, the white sand on beaches is substantially silicon dioxide
(silica), SiO2. About 87 % of the earth's crust is made up of SiO2 and related
compounds. Silicon does not form such extensive chains through bonding as does
carbon, and has nowhere near as many compounds. Apart from its role in mineral
chemistry, elemental silicon is used to make solid state electronic components
including photovoltaic cells for converting sunlight directly to electricity. Elemental
silicon displays network covalent bonding in the solid state. Do not confuse the
element silicon with the class of compounds called SILICONES which are polymers
containing silicon bonded to hydrocarbon groups in particular. Silicon and germanium
have some properties of both metals and non-metals so are often called SEMIMETALS or METALLOIDS.
While carbon and silicon do not form cations, germanium, tin and lead are more like
metals in that they do form cations in compounds but they can also covalently bond.
Group 14 again clearly illustrates this trend observed in most Periodic Table Groups
XII - 30
towards an increase in metallic properties of the elements down each Group and both
tin and lead in the solid state exhibit metallic bonding. Tin and lead are both relatively
low melting metals used together to form solder. Tin is used to plate the ubiquitous
steel cans because, unlike iron, tin does not corrode readily. Lead has anti-fungal
properties which make it valuable as a component in paint, usually in the form of lead
oxides. Concerns about health risks through ingestion of paint have caused lead to be
deleted from most paints at present. Lead is also used in large amounts in the lead acid
accumulator (i.e. the common rechargeable battery used in cars). There, alternate
plates of lead metal and lead(IV) oxide, PbO2, are immersed in sulfuric acid, all
contained in a plastic case. Electricity is provided by the redox reactions which take
place at the electrodes. Redox reactions were discussed in detail in Topic 11.
Group 14 elements usually have valence = 4, but lead and tin in particular exist also in
compounds with valence = 2, often as salts containing the ions Pb2+ or Sn2+.
FOR THOSE WHO WANT TO KNOW MORE - carbon, silicon, tin, lead.
CARBON
Graphite.
The most common form of carbon is graphite and it can be converted to another form,
diamond, by application of heat and high pressure. The traditional use for graphite has
been as a dry lubricant. Oils eventually dry and form thick gums and thus cease
lubricating the surfaces that require it. Because the layers in graphite can peel apart
and slide over each other, they provide lubrication without these problems. Writing
with a “lead” pencil is in fact depositing thin layers of graphite on the written surface.
However, in recent years a number of revolutionary materials based on carbon atoms
has been developed and graphite has become an important starting material for their
production. Some of these materials are discussed below.
Carbon fibre.
CARBON FIBRE was the first and it consists of C atoms bonded to each other in
long strings. On a weight basis, carbon fibres are much stronger than metals as the
bond energy of the covalent bonds between carbon atoms greatly exceeds that of
typical metallic bonds. [Recall how the bonding in metals relies on mobile outer
electrons - see Topic 3.] The carbon fibres can be woven into mats and set with resin,
the fibres providing the tensile strength. Before that stage, the carbon mat can be
shaped to simple or complex designs as required, for example in components of
aircraft, especially where stress resistance is paramount. Apart from strength and the
shaping advantages of carbon mat construction, its significantly lighter weight makes it
ideal for applications such as aircraft and motor car bodies. The carbon fibres are
made from preexisting chains of carbon atoms in polymers such as rayon from which
all the non-carbon atoms have been removed.
XII - 31
Graphene.
A very recent development among carbon materials is the discovery of graphene.
Graphene was initially made by putting sticky tape onto graphite and pulling it off.
Adhering to the tape is a single layer of C atoms bonded to each other in the same
hexagonal pattern as in the layered graphite structure which has been likened to
chicken wire in appearance. The backing material can be dissolved to leave the free
graphene. On a weight basis, it is one of the strongest materials known and has the
ability to stretch as much as 20% of it length without damage. Potential uses for
graphene are being discovered regularly. It is an excellent conductor of heat and
electricity and has promise as a component of batteries and also photovoltaic cells.
The C atoms in the chicken wire-like structure of graphene provide an impermeable
surface through which even helium atoms cannot penetrate. No other material has this
property. Its flexibility allows it to be made into curved surfaces which holds promise
for making curved solar panels. Another use mooted is for the touch screens of mobile
phones due to its transparency and electrical conductivity as well as its ruggedness, all
properties required for that application. Graphene can also be used to make filters
which admit only a specific sized entity by putting the desired size hole in it so it could
be used as membranes in desalination plants for example.
Nanotubes.
Tubes of carbon atoms called NANOTUBES is another newly discovered carbon
material with much potential. These can be made by passing a large electric current
through a graphite electrode and sheets of the chickenwire structured atoms fly off and
spontaneously roll up to form the nanotubes, so named because they typically have a
diameter of 1 nanometre. The C/C bonds in nanotubes are stronger than those in
diamond and carbon nanotubes are hundreds of times stronger than any metal.
Applications to take advantage of the properties of carbon nanotubes are still being
explored but already they are being used in bulk, added to polymers to increase
strength. Individual nanotubes potentially could be used in biomedical applications,
as transistors and in conjunction with copper to make high capacity electricity
conductors.
Diamond.
The very regular tetrahedrally arranged structure of the C atoms in diamond coupled
with the strength of the C/C bonds gives diamond the property of being the hardest
known material. Consequently it is in widespread use for cutting and grinding.
Diamond also has another unique property in being the substance having the greatest
heat conductivity. As a non-metal, this might seem surprising because most nonmetals are poor conductors of heat. The explanation lies in the highly regular
arrangement of its atoms in the crystal. As described in Topic 3, heat conduction arises
by two mechanisms. One is the increased movement of electrons which bump into
atoms and other electrons and transfer energy along the material from the heat source.
XII - 32
Metals have mobile outer electrons associated with their atoms and so this mode of
conduction is exhibited by metals. The second is transfer of vibrational energy by
atoms moving in a synchronous manner under the influence of increasing temperature.
This mechanism requires very regular arrays of atoms with no impurities or
discontinuities in the solid. There are no free electrons to move in diamond unlike in
metals, but it does meet the other requirements for transmitting heat by vibrations of
the C atoms. The C atoms in diamonds are arranged to a high degree of symmetry and
few impurities are present in the diamond crystal. Diamond is four times better at
conducting heat than copper. Consequently diamond is used in the windows of high
powered infrared lasers for cutting steel for example, allowing ten times more energy
to pass through than glass. Diamond is also used as heat sinks for solid state
components on electronic circuit boards where heat dissipation is a significant
problem. Artificial diamonds of industrial grade can be made from graphite by
applying heat and extreme pressure. By this method, a lump of graphite can be
converted into a mass of microscopic diamonds in less than an hour. High quality
jewellery grade diamonds can be made more slowly.
SILICON.
The element silicon from the Latin silax, symbol Si, is not to be confused with
silicones which are polymers containing silicon atoms. Silicon and oxygen are the two
most abundant elements in the earth’s crust, frequently combined in minerals together.
Silicon is a grey coloured solid and is a semiconductor of electricity. Silicon and
oxygen are the two most abundant elements in the earth’s crust, frequently combined in
minerals together. Combined with oxygen, silicon forms a dioxide which unlike the
molecular covalently bonded CO2, is a network covalent solid named silica of formula
SiO2 which is ubiquitous as sand and many minerals. Like carbon, it forms four
covalent bonds in its compounds such as SiH4 (silane) and SiCl4 (silicon tetrachloride).
Glass.
Common glass is made from silica with some small amounts of additional substances
to improve its properties. Glass can be drawn into fibres and used for fibre optic
cables required for communications transmission. Chopped into small lengths and set
in resin, fibreglass is a common construction material.
Alloys.
Silicon is added to many different combinations of metals to form alloys with specific
properties. The most common alloy is a mixture of iron with silicon, accounting for
about 80% of total silicon production. Bronze is an alloy of copper (ca 90%) with tin
(ca 10%) which is much harder than copper and was the basis for bronze age
implements. Its properties can be enhanced by adding small amounts of other elements
including silicon. Silicon bronze is ideal where corrosion resistance under severe
conditions is required.
XII - 33
Semiconductors.
Silicon is one of a number of elements known as semiconductors which conduct
electricity less well than metals but better than non-metals. One of their properties is
the ability to pass electric current more easily in one direction than the other which can
be used to convert alternating current (AC) to direct current (DC). When purified to an
extreme degree, silicon forms the basis of transistors and integrated circuits which
underlie modern electronics.
Solar photovoltaic (PV) panels.
Silicon can also convert visible light to direct current electricity and is the most
common material for rooftop PV panels. The conversion efficiency is currently about
20% when crystalline silicon is used but technology is rapidly improving this
performance.
Silicones.
Silicones are polymers of Si atoms bonded to O atoms as a chain with organic side
groups bonded to the two remaining tetrahedral positions on the Si atom as shown
below.
Silicone chains can be cross linked to make three dimensional polymers which serve
for example as rubbers, sealants, greases and cookware. In dissolved form, silicones
are used as lubricants and water dispersant sprays, ingredients of hair conditioners and
fabric waterproofing agents as well as many other applications. Properties which make
silicones so valuable include their inertness, non-toxicity, electrical insulating capacity
and resistance to water and UV light.
TIN.
Tin was one of the earliest elements discovered, around 3000BC, presumably as a
result of tin-bearing rocks being used in fires. When mixed with copper which was
also discovered in ancient times, an alloy of the two metals called bronze results.
Bronze (12% tin) has superior properties to either of its component elements and gave
rise to the weapons and tools of the Bronze Age.
XII - 34
Pewter is another alloy of tin (85 - 90% tin) and has properties such as being resistant
to corrosion and non-toxic, thereby making it suitable for the manufacture of eating
and drinking utensils.
Tin is a soft, low melting point metal which resists corrosion due to the formation of a
protective oxide layer. It used to be combined with lead to make solder for electronics
circuits and plumbing, but the toxicity of lead has resulted in tin now being used
alone. The lack of lead in solder allows tin to develop problems known as tin whiskers
and tin pest to develop. Tin has a highly crystalline structure and bending a bar of tin
causes layers of atoms within the crystal to slide over each other leading to an audible
sound, sometimes called the “cry of tin”. Properties of tin have resulted in it being
used in organ pipes(50% with lead) to produce their characteristic mellow sound and
in bells (20% tin, 80% copper) from which long resonances are produced when struck.
However tin solder is not without its faults because the instability of the crystal can
result in small “whiskers” of tin erupting and leading to possible failures of printed
circuit boards. Tin pest is an eruption of pustules on the surface of tin arising from the
same cause. These problems can be reduced by adding small amounts of other
elements such as bismuth and antimony to tin. Tin plating of cans provides a corrosion
resistant surface. However this application is being superceded by polymer coatings or
using aluminium cans.
Tin has an important role through a revolution in the manufacture of sheet glass. Sheet
glass manufacture used to be a very hot and dangerous labour intensive occupation
until a process developed by the Pilkington company in the mid-20th century. The
basis for the Pilkington process is to float the molten glass sheet on top of a tank
containing molten tin. The tin is hot enough to stop the glass from solidifying as it
floats out to form a flat sheet which then moves along a continuous production line.
Compounds of tin with organic molecules find uses such as biocides and as stabilisers
in plastics such as PVC without which the polymer breaks down rapidly.
LEAD
Lead was one of the earliest elements discovered and isolated. Lead ores have been
mined in England from pre-historic times and at one period lead was exported to
Rome. The symbol for lead, Pb, is derived from its Latin name, plumbum, which has
also given rise to names of activities traditionally utilising lead such as plumbing and
devices such a the plumb bob. It is a soft, malleable metal which is shiny when freshly
cut but gradually tarnishes in air. Lead metal is very resistant to corrosion even by
strong acids such as sulfuric acid a property used in the lead acid battery. Many of the
applications in which lead or lead compounds were used have been largely
discontinued because of health concerns. Lead compounds when ingested or inhaled
cause damage to the nervous system and in particular, are associated with brain
XII - 35
damage. Some psychologists even claim evidence shows that leaded fuel caused
increased incidence of violent crime due to its effect on parts of the brain. Applications
now discontinued in most parts of the world include the following:
Lead solder - lead melts at 328o C and it wets copper and tin so it is an ideal solder for
electrical and plumbing applications, usually mixed with tin. Solder now contains no
lead.
Lead as a fuel additive - for many decades lead compounds were added to petrol to
improve its combustion properties and also to act as a lubricant for the engine’s valves.
Lead compounds in paints - these provide excellent antifungal properties but when old
paint was subsequently being burnt off, the inhaled vapours are dangerous. Flaking
paint also poses an ingestion hazard. The reaction against leaded paints has been so
strong that even artists’ paints cannot be legally sold in a some countries. White lead
carbonate is acknowledged as the best primer for oil painting as it does not crack and
imparts unrivalled visual properties to the paint overlaid on it but it too is banned from
sale in some regions.
Lead batteries.
Most (90%) of the lead mined today is used in the production of the ubiquitous lead
battery that is used in every motor vehicle and as standby power sources in the event of
power supply failure. These batteries are very reliable and simple and the materials
from which they are constructed can be recycled an unlimited number of times. These
include the sulfuric acid electrolyte, the lead plates that are part of the electrodes, the
lead dioxide and lead sulfate from the electrodes as well as the polypropylene battery
casing. Over 90% of all lead acid batteries sold are eventually recycled and plants that
once smelted lead ores are now largely involved in recycling. The main drawbacks of
lead batteries are their weight and limited lifetime.
Lead sheeting.
Lead sheeting is the ideal material for sealing tiled roofs and in similar building
applications because of its flexibility which allows moulding to the required shape and
its low melting point which makes it easy to seal. Lead is extremely resistant to
corrosion and so lead sheeting is used to protect electrical cables, especially from sea
water.
Ballast.
The large number of protons and neutrons in the nucleus of lead atoms endow this
element with exceptionally large density which, combined with its resistance to
corrosion, makes it ideal to use as ballast. It especially is useful in marine applications.
XII - 36
Radiation shielding.
Again because of its nuclear structure, lead can block radiation from sources such as
X-ray machines and nuclear radiation. When undergoing body scans, lead covering is
used to protect organs from dangerous radiation and limit exposure to the region under
examination.
GROUP 15. N, P, (non-metals)
As, Sb, (intermediate) Bi (metal)
Group Overview.
Again, a transition from non-metal to metal is observed down this Group. Arsenic and
antimony are usually regarded as metalloids. Nitrogen is one of the few elements to
occur as a gas at room temperature and pressure. The other elements of Group 15 are
solids. The elements nitrogen and phosphorus exhibit molecular covalent bonding,
arsenic and antimony have network covalent bonding but bismuth has metallic bonding
in the solid state as expected of a metal. Elements of this group commonly have
valencies of 5 or 3 in their compounds. Compounds of nitrogen and phosphorus are of
vital importance in biological systems - nitrogen as a component of all proteins and
both nitrogen and phosphorus as components of DNA. Phosphate groups are an
essential component of the system by which cells store and use energy. Elemental
nitrogen is rather unreactive because it occurs as the highly stable N2 molecule which
constitutes 78 % by volume of the atmosphere.
Simple compounds of nitrogen found in the natural environment as part of the nitrogen
cycle include ammonia (NH3), ammonium salts (containing the NH4+ ion), nitrate salts
(containing the NO3– ion) and nitrite salts (NO2– ion). The process whereby
atmospheric nitrogen is converted to such salts requires considerable energy input and
is called NITROGEN FIXATION. It is accomplished by lightning strikes and by
various microorganisms. Today, about 50 % of all nitrogen fixation is man-made
through synthesis of ammonia from nitrogen and hydrogen gases.
Phosphorus occurs in four different allotropic forms, all of which react vigorously with
oxygen in air, and thus must be stored under water. Phosphorus occurs most
commonly in the natural environment as phosphates which contain the PO43– ion.
Washing powders often contain phosphates.
Arsenic, antimony and bismuth occur less frequently and are of less importance in
biological systems.
Being mostly non-metals, many compounds of Group 15 are covalently bonded,
although compounds containing ions such as As3+, Sb3+, Bi3+ and N3– commonly occur.
Bismuth forms the usual salts containing the Bi3+ ion as expected of a metal.
XII - 37
FOR THOSE WHO WANT TO KNOW MORE - nitrogen, phosphorus.
NITROGEN
The element nitrogen occurs as a diatomic gas, N2, at atmospheric conditions but can
be liquefied by using high pressures and lowering its temperature. Liquid nitrogen
boils at –176 o C at atmospheric pressure. Nitrogen constitutes almost 80 % of the
atmosphere but is quite unreactive due to the very strong triple bond joining the two N
atoms of the N2 molecule. The bond in the nitrogen molecule is the strongest of any
molecule. The nitride ion, N3–, does occur in some ionic compounds and the black
tarnish that appears rapidly on the surface of freshly-cut lithium metal is the compound
lithium nitride. Being easy to produce, liquid nitrogen finds many uses such as
freezing biological specimens for future use and freezing water pipes to allow
disconnection for maintenance. Every doctor’s surgery has a container of liquid
nitrogen used to freeze and remove skin cancers. Many applications where strong
magnetic fields are required make use of the low temperature attainable from liquid
nitrogen. This allows greatly enhanced magnetic fields to be produced because
electrical resistance in metals which carry the electrical current through the
electromagnets decreases at low temperatures.
Gaseous nitrogen is used as an inert atmosphere in food packaging whereby excluding
the reactive element oxygen prolongs the storage life of perishable items.
Breaking the triple bond
Nitrogen atoms are one of the essential components of all the vital molecules involved
in living systems. For example the basic molecules of living cells - the enzymes, DNA
and RNA - all contain N atoms in their structures in combination with atoms of other
elements, predominantly H, C and O.
Both DNA and proteins contain N atoms within their structures where one of their
roles involves both intermolecular and intramolecular hydrogen bonding. The allimportant shapes of proteins are largely maintained by intramolecular hydrogen bonds
between the amino acids which constitute the protein chain. The opening and rejoining
of the links between the double strands of DNA are dependent on hydrogen bonds
breaking and reforming between N atoms and some O atoms on the two strands.
However, extracting the N atoms from the molecular form of nitrogen, N2, requires the
input of a lot of energy, 944 kJ per mole of N2 molecules split. Although nitrogen
molecules are abundant in the atmosphere, this large energy requirement to convert
them to a more accessible form is a limiting factor for all living systems. The triple
bonds in the elemental N2 molecule must be replaced preferably by single bonds in
nitrogen compounds. This feat is achieved in nature partly by lightning strikes which
produce vast amounts of energy and can split N2 and O2 molecules and the atoms
XII - 38
released may combine to form oxides of nitrogen. The main source of converting
atmospheric nitrogen to more accessible compounds is done by some plants called
legumes which host bacteria that can break the triple bond and incorporate the N atoms
into compounds that the plant can use. Subsequently animals eat the plants and gain
the desired nitrogen compounds. The process whereby nitrogen molecules are
converted to compounds in this way is called nitrogen fixation. The food chain in
which animals eat the plants to derive their source of nitrogen and other animals eat
those animals then continues the nitrogen cycle. In due course, the death of the
nitrogen fixing plants and all the other forms of life return the fixed nitrogen to the soil
or the ocean. Until a method of artificially fixing nitrogen was devised in the early 20th
century, this vital element was mainly available by recycling using plant mulch and
manure from animals for example. One problem encountered by the First Fleet when
establishing the settlement in Sydney was lack of manure due to the shortage of
animals available.
Due to the very strong triple bond in N2, nitrogen availability for living creatures
including humans was once very limiting for food production. As world population
increased and farming expanded in the 19th century, the traditional means of recycling
nitrogen in fields as mulch and manure became inadequate. Significant deposits of
nitrates existed in Chile in particular and also use was made of guano (bird faeces)
which have accumulated for centuries in some South American locations. This
material is rich in nitrogen compounds and was ideal to use as a fertilizer. So valuable
was this source that wars were fought over it in the latter part of the 19th century in
South America where large deposits occurred.
The ionic compound potassium nitrate occurs in deposits where it has crystallised from
water as the water evaporated, usually in very dry environments such as the lakes in the
high regions of inland South America. However the natural and recycled sources of
fixed nitrogen were insufficient to feed an increasing world population and it is only as
a result of the production of synthetic ammonia that it is possible to feed 7 billion
people today. The driving force to develop the process used known as the Haber
Bosch synthesis of ammonia was the cornering of the guano supplies from Chile by
Britain. The German chemist Fritz Haber succeeded in combining hydrogen and
nitrogen gases over an iron catalyst at high temperatures and pressures in the lab in
1905 and the process was developed commercially in conjunction with Carl Bosch.
The Haber Bosch synthesis requires vast amounts of energy due to the high
temperatures and pressures needed and about 2 % of the total energy used world wide
is consumed by this process. About half of all the nitrogen now fixed on the planet is
produced by this process and about 40 % of the nitrogen atoms in each of us was fixed
by the Haber synthesis. Without the Haber Bosch synthesis of ammonia, mass
starvation would occur - it is estimated that the world’s population would be about 3.5
billion today instead of 7 billion. Following the commercialisation of the process,
XII - 39
nitrate production in Chile fell from 2.5 million tons annually to 800,000 tons within a
decade. Today annual production of ammonia by the process is more than 100 million
tons. Haber was awarded the Nobel Prize for chemistry in 1918 for his discovery.
This was a controversial decision by the Nobel Prize committee as Haber had also
developed the war gases used in the First World War and others that would be used in
concentration camps during the second world war. Despite his services to Germany,
Haber who was Jewish had to flee and died of a heart attack on his way to Israel in
1934.
Biofixation of nitrogen.
The mass use of ammonia fertilizers has had severe detrimental environmental
consequences. Fertilizers are sprayed on fields in far greater quantities than actually
used by crops and much runs off to contaminate waterways and to change acidity of
soils. It is estimated that only about 20 % of the fertilizer deployed is actually used with
the bulk being lost by various means. To avoid these undesirable consequences, much
research is in progress trying to harness the methods that some plants already use to
derive their nitrogen requirements. These methods utilise an enzyme called
nitrogenase which is able to split the N/N triple bond to make compounds that can be
taken up by the plant which typically hosts the bacteria in root nodules. These nodules
protect the bacteria from oxygen in which nitrogenase is ineffective. Legumes have
this mechanism but not the cereal crops such as wheat and the research aims to equip
cereal plants with the ability to host nitrogen fixing bacteria.
Explosives.
The majority of the ammonia synthesised is not used for food production but as the
basis for making explosives in which nitrogen compounds find considerable
application, replacing earlier products such as dynamite. One such is the compound
potassium nitrate which, mixed with charcoal and sulfur, produces gunpowder which is
believed to have originated in China in the 9th century AD. Potassium nitrate is not an
explosive on its own but another compound of nitrogen, ammonium nitrate, NH4NO3,
has widespread use in mining as a cheap and easily detonated explosive. If heated or
mixed with fuel oil, it explodes violently to form nitrous oxide and oxygen. The
hazzards associated with this compound were highlighted in the great explosion that
occurred in Texas Port in 1947 when a shipload of ammonium nitrate fertilizer caught
fire and detonated, causing massive destruction and the death of at least 581 people,
the injury of 5000 more and the destruction of the sea port. Ammonium nitrate is made
from ammonia and nitric acid which is itself made via the Haber Bosch process and is
an important industrial oxidizing acid.
PHOSPHORUS.
Phosphorus compounds like those of nitrogen play a vital part in all living systems.
Bones and teeth contain phosphate compounds, but far more important is the role of
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phosphate groups in DNA molecules and in the energy supply systems for cells based
on the compound adenosine triphosphate, abbreviated as ATP.
Phosphate groups in DNA.
The structure of the backbone of each strand of DNA consists of a series of sugar
groups each of which is bonded to one of four bases, the so-called “letters” of the
genetic code designated as A, C, G and T. Each of these sugar/base groups is joined to
the next one in the DNA strand by a phosphate group. This sequence extends along the
entire strand. In the cell, two complementary strands of DNA form a double helix in
which the two strands are joined to each other through hydrogen bonding between N
atoms on the bases. This is illustrated by the following block diagram.
The strands open by
breaking the hydrogen
bonds to allow the
genetic code to be
transcribed to a molecule,
RNA, which is similar to
DNA but which uses a
different sugar. RNA is
single stranded and also
uses phosphate groups to
join the component
sugar/base groups.
Phosphate groups in the energy supply system, ATP.
All cellular processes need energy to drive them. A molecule containing a large
amount of accessible energy in its bonds is adenosine triphosphate.
XII - 41
The diagram above shows this molecule consists of a sugar molecule bonded to a base
(adenine) at one end and to a chain of three phosphate groups covalently bonded to
each other at the other end. When energy is required, the ATP loses a phosphate group
to become adenosine diphosphate, ADP and releases 30.5 kJ of energy per mole of
ATP reacting. A further reaction can also take place in which a second phosphate
group cleaves off to form adenosine monophosphate and releases 61 kJ of energy per
mole of ADP reacting.
The full sequence can be shown as
ATP 6 ADP + PO43! + 30.5 kJ
6 AMP + PO43! + 61 kJ
The sequence can be reversed to rebuild the ATP molecules from AMP and phosphate
groups using energy supplied from other processes.
Sources of phosphorus compounds.
Normally phosphorus compounds are recycled when living systems die and break
down but by eating plants and animals which consume them, the phosphorus content of
soils diminishes and severely limits plant growth. The lost phosphorus content can be
replaced as for nitrogen previously discussed, by using mulch or manure but for high
intensity farming which is essential now, additional phosphate must be added to the
soil. Sources of phosphate were again bird droppings and when these were exhausted,
phosphate rich rocks were mined and converted to phosphate fertilizers. Initially
suitable sources were to be found on some Pacific islands such as Nauru but now the
main source of economically viable phosphate is from Morocco which controls 75 %
of the worlds known stock. Unlike nitrogen which can be extracted from air, albeit
with a huge energy requirement, there is no alternative source of phosphorous
compounds. There is concern about how agriculture will deal with the ultimate
depletion of the currently available stocks. One approach now under way in England is
to recycle the phosphates which are excreted into sewage. A side benefit of this is to
reduce phosphates entering waterways where they stimulate algal growth which then
leads to excessive bacterial growth for the amount of oxygen in the water, and killing
other marine life. While recycling cannot totally replace the need for additional
phosphate from mining, it can help to reduce that need.
In summary, Group 15 contains two of the most vital elements to life as we know it.
Both constitute a limitation on how many people can be supported on earth. The
Haber process has overcome the problem of lack of suitable nitrogen compounds but
the only long term solution for the limited phosphorous availability seems to be
different farming practices and recycling.
XII - 42
GROUP 16. O, S, Se (non-metals) Te (intermediate) (Po - metal)
Group Overview.
All except the rare and radioactive element, polonium, are non-metals or metalloids.
They exist as 2– charged anions in compounds with metals and form covalently
bonded compounds with other non-metals. Oxygen is the only member to occur as a
gas, the others are solids at room conditions. Oxygen also occurs in small amounts in
the atmosphere as the allotrope called OZONE. Ozone is dangerous to health when
inhaled but plays a vital role in the upper atmosphere where it absorbs much of the
harmful ultraviolet light that would otherwise impinge on the earth’s surface.
Tellurium has network covalent bonding in the solid state while elemental oxygen,
sulfur and selenium are molecular covalently bonded. The common valencies are 6
and 2, but oxygen has the valence of 2 only.
Oxygen is one of the most reactive non-metals, forming oxides with most elements.
The term OXIDATION originally referred to the reaction of substances with oxygen.
Apart from occurring as the diatomic element O2 (20.9 % by volume in air), oxygen
also occurs extensively as part of numerous compounds in the earth's crust and in
water. All aerobic organisms require elemental oxygen to survive as part of the
process of respiration. Compounds of oxygen may be ionic, containing the oxide ion
(O2–), or covalent (e.g. H2O, CO2). Oxygen also occurs as the peroxide ion, O22–, in
ionic compounds or covalently bonded to non-metals such as in the antiseptic,
hydrogen peroxide, H2O2, which has the structural formula H!O!O!H.
Sulfur occurs free in large deposits as the element and in compounds containing, for
example, sulfide (S2– ) and sulfate (SO42–) ions. Covalently bonded sulfur atoms are an
important component of proteins, helping to maintain the required shape of enzymes.
The compound hydrogen sulfide (rotten egg gas), H2S, is a highly toxic gas which is
generated by anaerobic bacteria. It is produced naturally by such bacteria in marshes
and in sewage holding tanks where oxygen is excluded and can sometimes be detected
in the exhaust fumes of modern motor cars.
Selenium and tellurium are very similar to sulfur in their properties and their hydrides,
H2Se and H2Te, have an even more repulsive odour. Selenium is used commonly as a
rectifier for converting AC to DC. Foods containing selenium compounds are in
vogue at present because of the belief that they remove cancer-forming free radicals
from the body.
All Group 16 elements form covalent compounds with other non-metals (e.g. H!O!H)
and anions in ionic compounds with metals (e.g. Mg2+O2–).
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FOR THOSE WHO WANT TO KNOW MORE - oxygen, sulfur.
OXYGEN
Oxygen is the third most abundant element in the universe and in combination with the
most abundant element, hydrogen, it forms water which is the most abundant
compound. On earth, apart from atmospheric O2 gas, much more oxygen is present in
compounds including water and minerals, especially with silicon in silicon dioxide.
Two thirds of the mass of the human body is made up of oxygen atoms in compounds,
especially water.
When earth was formed about 4.5 billion years ago, its atmosphere contained no
elemental oxygen. Microbial life evolved in the absence of oxygen (anaerobic) but
after about 1 billion years, microbial species which contained organelles called
chloroplasts within their cells developed.
Chloroplasts use light energy
(photosynthesis) to split water molecules and release O2 gas in the process which is
carried out by plants and some types of algae. At that stage in earth’s development
there were huge quantities of iron present as dissolved salts and as the oxygen was
released, iron oxides formed and precipitated to form the vast beds which today are the
ore bodies being exploited in places such as Western Australia. Ultimately the freely
available iron was depleted so instead the oxygen gas was released into the atmosphere
where its present composition is 21 %. This change had a drastic effect on the earth’s
future. Much of the preexisting microbial life was in effect poisoned by the new
pollutant and died out. Anaerobic life continued in places shielded from the
atmosphere such as swamps.
Oxygen is one of the most powerful oxidizing agents after fluorine. It reacts with
almost all metals to form oxides, many of which come away from the surface of the
metal which disintegrates. While oxygen gas is essential for aerobic life including
animals, within the body extremely reactive oxygen atoms rather than O2 molecules
damage cells. The body has defence mechanisms devoted to mopping up such oxygen
free radicals as they are called. Damage to cells by oxygen free radicals may
contribute to aging and be the cause of other deleterious effects. Anaerobic organisms
generally cannot survive in the presence of oxygen gas.
Oxygen in the atmosphere is inherently so reactive that constant replenishment from
plants and algae is needed to maintain its concentration. Due to the burning of fossil
fuels, the percentage of oxygen gas in the atmosphere is reducing.
Water.
In space, water can exist as a gas but mostly as ice in small grains or lumps and on
asteroids. Current theory holds that when the sun ignited about 4.5 billion years ago,
water along with much other old material swirling around the sun was vaporised and
XII - 44
condensed further out, possibly where Jupiter is now. Comets which formed far out
may have been the vehicle by which water came to be on earth is such large amounts.
There must also have been some special circumstances on earth that allowed it to retain
its water whereas other planets such as Mars which 3 billion years ago had oceans but
now has none. Recently a comet lander has analysed comet-borne water and found
that it has a H:D ratio different from water on earth so comet delivery is not confirmed
as a source yet.
SULFUR
Sulfur is found as the free element near volcanoes and hot springs. It also occurs as
strata buried during previous geologic eras. The stable form of sulfur is the familiar
yellow coloured powder which contains 8-membered rings of S atoms covalently
bonded to each other. If heated, these rings of S atoms break apart and form long
chains of a treacle-like appearance. If this form of sulfur is cooled, it solidifies into a
less stable allotrope called plastic sulfur.
Sulfur burns with a blue flame forming an acidic oxide, SO2, sulfur dioxide, which is
soluble in water and forms sulfurous acid, H2SO3. In the mid-20th century, burning of
large amounts of sulfur-containing coal and petroleum lead to sulfur-containing acids
contaminating forests and waterways, falling as acid rain and killing plant and animal
life. In the 1970's the countries of North America instituted the first and very
successful emissions trading scheme in which polluters paid for the amount of SO2
they emitted. Like the schemes currently used in some parts of the world to abate CO2,
any unused permits could be sold to other polluters who could not meet the mandated
targets. Reduced levels of acid rain soon followed as it became uneconomic to burn
“sour” coal and oil so that coal mines producing high sulfur content coal closed and oil
refineries installed equipment to remove sulfur before it was incorporated in the end
products. Now so much elemental sulfur is extracted from oil that there is a world glut
of it unused despite its being the starting material for the manufacture of sulfuric acid,
a vital and widely used industrial material.
Elemental sulfur and slow release compounds of sulfur are now added to fertilizer as
sulfur is an essential nutrient for plant growth. Ironically this addition of sulfur to
fertilizer has become necessary as a result of the reduction of sulfur compounds being
emitted industrially.
Compounds.
Cysteine.
Sulfur containing compounds are essential components of living organisms, mainly as
atoms incorporated into amino acids that are the building blocks of proteins. The
enzymes which catalyse all metabolic reactions are proteins and they function because
XII - 45
of the fixed 3-dimensional shape which the long chain of amino acids maintains (see
Topic 13). To keep their required shapes, amino acids at different locations in the
chain intramolecularly hydrogen bond via N–H bonds but also S–S bonds form
between S the atoms of the amino acid which contains them, cysteine. The
combination of the hydrogen bonding and the S/S linkages provide the rigidity needed
for the enzyme to operate as a catalyst. The human body contains about 0.25 % by
mass sulfur in compounds. The S–S bonds can be broken and reformed relatively
easily. Hair contains strands of protein with S/S linkages between them which
provides hair’s strength. These cross linking bonds are broken and reformed when
hairdressers straighten or permanent wave hair. The unpleasant smell from burning
hair is due to sulfur compounds being formed.
The formation of S/S linkages is the basis for producing rubber from latex which is a
liquid but which contains polymers with S atoms by mixing elemental sulfur with latex
and heating it to convert it into the stretchable material, a process called vulcanising.
The first vulcanised rubbers were developed by Dunlop in the 19th century enabling the
production pneumatic tyres.
Hydrogen sulfide.
Hydrogen sulfide is a highly toxic gas at room conditions. One might expect it to be a
liquid given that H2O from the period above boils at 100 o C but there is no hydrogen
bonding associated with S atoms due to their larger size compared with O atoms, (See
Supplementary Topic 4). It is well known as rotten egg gas because some is formed
from the sulfur-containing amino acids in eggs. Anaerobic conditions in swamps for
example, make use of the redox reaction in which the S atoms in compounds undergo
the oxidation state change which in aerobic conditions would be fulfilled by O atoms.
Unpleasant odours can be detected when microbial life is using this method of redox.
Deep oceans sometimes contain vents which release mineral-rich steam from beneath
the surface. Surprisingly these vents teem with living creatures despite their being too
deep to receive any sunlight which is normally the driver for life. Instead the heat from
the vent and the oxidation of hydrogen sulfide are the basis for the deep water life
forms. It is thought that the earliest forms of life may have started around such vents.
Sulfuric acid.
Sulfuric acid is the most widely used commercial acid due to its low cost. Large
amounts are used in many industrial processes including the production of detergents.
Detergents contain molecules which consist of a hydrocarbon group at one end which
is attracted to dirt and grease by dispersion forces (see Supplementary Topic 4) and an
ionically charged group at the other end which is attracted to polar solvent such as
water. This structure allows molecules of detergent to surround dirt particles and carry
them into water. The most commonly used ionic group used is derived from sulfuric
acid.
XII - 46
Sulfuric acid is used to extract phosphate from calcium phosphate which is mined and
the other end product is calcium sulfate, also known as gypsum, which is used to make
plasterboard. Sulfuric acid is also used to extract metals from their ores by leaching
and then reducing the leachate to form the free metal - copper and nickel in particular
are extracted this way.
Sulfuric acid is made by converting sulfur dioxide over a catalyst to sulfur trioxide
which is dissolved in water.
Sulfur dioxide.
Sulfur dioxide derived from burning sulfur in air or roasting sulfides of iron is the
starting material for the production of sulfuric acid. Sulfur dioxide was used to
sterilize wooden wine barrels between vintages - sulfur was burnt in the barrel as the
SO2 gas kills bacteria. Today sulfites are still used as preservatives in wine as one can
read on he labels.
Dimethyl sulfide.
Most sulfur compounds have foul smells - one of them, dimethyl sulfide, is added to
natural gas which is odourless so that gas leaks are detected. Dimethyl sulfide is
released from the decomposition of marine organisms including seaweeds. It is then
oxidized in the air to other compounds including sulfuric acid as an aerosol which can
act as nuclei for the formation of drops of water which may become clouds. As vast
amounts of dimethyl sulfide are released over the oceans, it is thought that this process
may be significant in controlling the earth’s climate.
GROUP 17. F, Cl, Br, I, (At) also known as the halogens.
Group Overview.
Fluorine and chlorine are diatomic gases at room conditions, bromine is a liquid and
iodine is a low melting solid. All exhibit molecular covalent bonding. The halogens
are very reactive non-metals which can combine with metals to form ionic compounds
containing the halide ion (F–, Cl–, Br– or I–). Species which can enter into such
reactions are called OXIDIZING AGENTS, and all the halogens are therefore good
OXIDANTS. Because of its oxidizing power, chlorine gas is introduced into the
drinking water supply to oxidize organic contaminants such as bacteria and viruses
which could be harmful, as well as to remove algae. Chlorine solutions can also be
obtained for household use from the compound sodium hypochlorite (NaOCl) which is
sold as bleach and also as an agent for maintaining clean swimming pool water. Iodine
dissolved in alcohol (tincture of iodine) is used as a disinfectant for cuts of the skin,
and is used to sterilise dairy equipment. Lack of iodide in food is one of the most
common deficiency disorders in Australia where once it was mandatory to add iodide
to table salt but this practice has ceased.
XII - 47
Some compounds of Group 17 elements are covalent (e.g. HCl) while others are ionic
(e.g. Na+Cl–). They occur as anions in the latter case. The only ionic valence shown is
1 but, apart from fluorine, they have numerous other valencies when covalently bonded
e.g. as the polyatomic anions in the salts NaClO, NaClO2, NaClO3, NaClO4.
Important compounds of halogens include hydrogen chloride (HCl) which is a gas, but
when dissolved in water it breaks down (ionizes) to form a solution of H+ and Cl–
called hydrochloric acid. The halide ion Cl– is an essential component of the nervous
system and the fluoride ion, F– plays an important role in developing strong enamel on
teeth. As natural levels of F– in drinking water are often too low, the salt sodium
fluoride, NaF, is commonly added to drinking water to prevent tooth decay as a public
health measure.
FOR THOSE WHO WANT TO KNOW MORE - fluorine, chlorine, bromine.
FLUORINE
Fluorine is the most reactive element because of its small size and proximity to the
noble gas neon so consequently it has the largest effective nuclear charge. Conversely,
having reacted with other atoms, the resulting compounds of fluorine are usually
extremely stable given the large bond strength of covalent bonds to F atoms.
Fluorine is so reactive that it is difficult to store and handle and extremely hazardous if
contact is made with living organisms. It is an even stronger oxidant the chlorine and
oxygen and it reacts with all the other elements except a few of the noble gases.
Fluorine is mined as the mineral fluorite which is essentially calcium fluoride. The
mineral can be crushed and used directly as flux in steel making. Fluxes lower the
temperature at which a metal melts. Elemental fluorine is extracted from fluorite by
heating it with sulfuric acid.
Fluorine is unusual in that it only exists in a single isotopic form, a property made use
of in the separation of isotopes of uranium - see uranium hexafluoride below.
Hydrogen fluoride.
Hydrogen fluoride dissolves in water to form hydrofluoric acid which is analogous to
hydrochloric acid but unlike the other hydrogen halides, hydrogen fluoride is a weak
acid. Even so, if contact results in it penetrating beneath the skin, hydrofluoric acid
reacts with any calcium ions which control vital functions of the body and converts
them to solid calcium fluoride thereby disrupting those functions.
Hydrogen fluoride is produced from fluorite by mixing it with sulfuric acid.
Hydrofluoric acid must be stored in plastic containers and cannot be stored in glass
which it dissolves. Use is made of its ability to attack glass when glass etching is
XII - 48
required for example, the security coding on car windows. Hydrofluoric acid is
essential in many industrial processes, for example in the smelting of ores of
aluminium by electrolysis and production of refrigerants.
Sulfur hexafluoride.
While fluorine is extremely reactive, it follows that its compounds are extremely
stable. One of many extremely stable covalent fluorine compounds is sulfur
hexafluoride, SF6, which has an application as a spark retardant in high voltage
electricity transformer stations. The extreme stability of SF6 which is a gas prevents
any spark from ionizing the air and thus being propagated and damaging the electrical
equipment. However sulfur hexafluoride is an extremely potent greenhouse gas with a
lifetime of 3000 years so its use must be carefully monitored.
Fluorocarbon refrigerants.
Compounds of carbon with fluorine have been used as refrigerants for decades but it
has been discovered that when released they destroy ozone molecules in the upper
atmosphere, opening the way for UV light to penetrate to lower levels with serious
health and environmental implications. Refrigerants are compounds that are gases at
room conditions and which can be liquefied by compressing and cooling externally.
The liquid refrigerant is then passed through a small hole in a sealed unit and it
converts back to the gaseous state with the absorption of heat. This is the basis for the
operation of all refrigerators. A desirable refrigerant needs to undergo these phase
conversions readily and to be non-toxic in case of leakage. One of the early
refrigerants was ammonia but it fails the toxicity test. Hydrocarbons in which the H
atoms have been replaced by F and Cl atoms are ideal refrigerants but since the
discovery of the damage they were causing to the ozone layer, the original versions
have been banned worldwide. The Montreal protocol devised in 1987 brought an
international agreement to phase out their production. The replacements are
chlorofluorohydrocarbon compounds which are similar but retain one or two H atoms
on each of the carbon atoms. These compounds usually break down too rapidly to
reach the upper atmosphere but if they do, they are found to be extremely potent
greenhouse gases with very long lifetimes. It is estimated that 15 % of the total
greenhouse gases released by humans is made up of such compounds.
Polymers of fluorocarbons.
One of the most versatile polymers is teflon, a polymer of tetrafluoroethylene which is
an ethylene molecule in which all four H atoms have been replaced by F atoms. In the
polymer both C atoms of each ethylene molecule join to other tetrafluoroethylene
molecules. The double bond is broken in this process and the electrons used to form
the two new bonds. Thus the structure of the polymer is an unbroken chain of C
atoms, each of which is bonded to two F atoms and its formula could be represented as
!(CF2)n! where n is a very large number. The chemical name for the polymer is
XII - 49
polytetrafluoroethylene or PTFE. Teflon is a very versatile polymer. It is extremely
resistant to chemical attack and can be machined to any shape. Thin teflon tape is used
to seal joints in plumbing. It has self-lubricating properties which find application as a
material for bearings and in a powdered form, as an enhancement with other lubricants.
One of its best known applications is the surface of non-stick saucepans.
Fluoride in teeth.
Unlike the element, having achieved the noble gas configuration of neon, fluoride ions
are very stable. The compound calcium fluoride, CaF2, is insoluble. The enamel on
the surface of teeth can be given a protective coating of glass-like calcium fluoride by
bringing F!ions into contact with teeth which consist of calcium compounds. The
fluoride content of most natural water supplies is insufficient to achieve this so most
public water utilities add additional fluoride ion as sodium fluoride. Dentists also
typically finish an examination by coating the teeth with a fluoride-rich gel.
Uranium hexafluoride.
Uranium hexafluoride, (UF6), is a stable gas above 57o C. It is vital in the separation
processes used to separate the uranium isotope U235 which undergoes fission, from the
non-fissile U238 which occur as a mixture in uranium ores. By converting all the
uranium to UF6 and either using gas diffusion or gas centrifugation , the slightly lighter
isotope can be separated based on the mass difference. In gas diffusion, the U235F6 will
move faster through a membrane than U238F6 and by setting up many gas diffusion
chambers in series, gradually the diffused gas becomes richer in the desired compound.
Gas centrifugation uses centrifuges to spin the gas samples at high speed and the
heavier 238UF6 is concentrated lower in the container and mixture richer in the lighter
compound can be drawn off from the top. Again, a large number of the centrifuges
must be used in sequence to achieve the desired level of enrichment. Both processes
can only work because the F atom exists as just one isotope so the mass difference
between the 235UF6 and 238UF6 is solely attributable to the difference in the masses of
the two U isotopes.
CHLORINE
Chlorine was known but not recognised as an element prior to it being named by
Humphrey Davy in 1810 - the name is derived from the Greek meaning “green” on
account of the pale yellow green colour of gaseous chlorine. It is produced by
electrolysis of concentrated sodium chloride solution in a cell in which the anode and
cathode are separated by a membrane which allows electrical neutrality of the
electrolyte to be maintained but prevents the gaseous products formed at the electrodes
from mixing. At the positive electrode, chlorine gas forms and at the negative
electrode hydrogen gas and hydroxide ions form, both from the water. Metallic
sodium cannot form by electrolysis of an aqueous solution, only by electrolysis of
molten sodium chloride.
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Chlorine like all the halogens is a diatomic element and it is a powerful oxidising
agent. Because the element is so reactive, the compounds it forms are consequently
generally stable. This is especially useful with regard to organic chloro compounds in
which Cl atoms replace H atoms. Chlorine atoms are incorporated into hundreds of
compounds used for example in the building industry and in the manufacture of
pharmaceuticals. Apart from such compounds where Cl atoms are part of the final
product, chlorine is used in many processes to form intermediate compounds without
being part of the final product. Two common examples are the polymers nylon and
polyurethane, neither of which contain Cl atoms but synthesis of both requires
chlorine. The following discusses a few of the many compounds which incorporate
chlorine atoms.
Sodium chloride.
Sodium chloride is easily obtained from underground deposits or by evaporation of salt
water. It is the basic feedstock for both elemental sodium and chlorine and the
manufacture of their compounds. Because chlorine is very reactive, the Cl! ion formed
when the Cl atom has gained an electron is very stable and along with sodium ions is
an essential component for maintaining the correct water balance in the blood stream.
[See sodium chloride under Group 1 elements.]
Sodium hypochlorite.
This compound, NaClO, is sold as common household bleach and is also used in water
treatment, especially swimming pools. It is made by reacting chlorine gas with sodium
hydroxide solution, the chlorine being released in solution as it is used. The Cl2
molecules kill microbial life because of their strong oxidizing properties. Chlorine gas
is a very toxic substance if inhaled and it has been used as a war gas - it damages the
linings of the lungs and causes a fatal build up of fluid. Consequently for most
disinfection purposes it is much safer to use a solution of sodium hypochlorite rather
than elemental chlorine gas.
Polyvinylchloride.
An organic compound of chlorine, vinyl chloride is very stable and forms the polymer
polyvinyl chloride, commonly known at PVC.
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Because of its extreme stability, PVC is used in many building products such as
drainage pipes, guttering and window frames. Production of PVC is the largest single
use for chlorine.
Hydrogen chloride.
The hydride of chlorine, hydrogen chloride, HCl, reacts vigorously with water to form
the strong acid hydrochloric acid in solution. Hydrochloric acid is one of the most
widely used in industrial applications.
Stomach acid.
Hydrochloric acid is secreted in the stomach as part of the digestion of food. It plays a
role in enabling the enzymes to break down amino acids and when food is being
digested, the pH of the stomach juices falls to the range 1 - 2. After a meal has been
digested, the pH increases to its normal value of 4 - 5.
DDT (dichlorodiphenyltrichloroethane).
DDT is a potent insecticide widely used in agriculture until the 1970's by which time
its propensity to accumulate in fat cells in living creatures lead to its disuse in most
situations. However in some locations, DDT is still used as a means of controlling
mosquitoes that carry malaria parasites.Apart from DDT, a number of other
organochloro compounds have been used as very effective insecticides but again, due
to their long lifetimes and unwanted indiscriminate effects, most have been banned
from sale.
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BROMINE.
Bromine, symbol Br, is a red-brown liquid at room conditions, boiling at 59o C. Like
the other halogens, it is a corrosive oxidizing agent and its reactivity lies between that
of chlorine and iodine. The halogen Group illustrates how the strength of
intermolecular attractions through dispersion forces increases with the number of
electrons (see Supplementary Topic 4) as reflected in the boiling points. The main
forces acting between the molecules in these non-polar diatomic elements are
dispersion forces. Thus at room conditions fluorine and chlorine are both gases,
bromine is a liquid and iodine is a solid.
Like chlorine and iodine, bromine is used as a disinfecting agent. Many of the uses
made of bromine in the past have been discontinued recently on health and
environmental grounds - for example some fire extinguishers once contained organic
bromo compounds but their ozone depleting properties have caused this use to cease.
Organic bromo compounds.
The main application for bromine is as a fire retardant. Organic molecules containing
Br atoms replacing some H atoms, when heated, release Br atoms which have the
property of extinguishing a fire. Organic bromo compounds are incorporated into
plastics which normally will burn readily if heated, for example in TV sets and laptop
computers if a fault develops and overheating occurs. Also bromo organic compounds
can be soaked into materials to reduce flammability. In these roles, bromine
compounds have undoubtedly saved many lives and much damage.
Drilling fluids.
When drilling oil wells and similar, the pressure outside the well must be maintained to
match that of fluids or gas inside the well. This is achieved using concentrated
solutions of bromide salts which are very dense because of the large size and mass of
Br! ions.
Mercury scavenger.
Bromide salts such as calcium bromide are sprayed onto coal just prior to its being
burnt in coal fired power stations. These salts remove the mercury which is always
present in coal and normally is released through the flu gases unless captured.
Photography.
Classical black and white photography using film is based on the ability of light to
reduce the silver ions in silver salts to silver metal. This is why silver salts and their
solutions are kept stored in dark containers to reduce light exposure. The most
favoured silver compound for photography is silver bromide, a cream, insoluble salt.
When light is focussed by the camera lens onto film which contains silver bromide in a
solid emulsion attached to a transparent base, the Br! ions are converted to Br atoms
XII - 53
and release an electron which the Ag+ ion accepts and converts to an Ag atom. The
more light striking a given spot on the film, the more Ag atoms are produced.
Developing processes amplify the effect and fixing removes any unreacted silver
bromide from the film. The image is reversed so the areas that would be white appear
as black on the negative produced while areas that received no light from the lens
appear transparent. To obtain a positive print, the process is used again, shining light
through the negative to expose silver bromide layered onto paper and developing the
print in the same way as was done for the negative. The Br atoms released in these
processes are removed in a layer containing a bromine acceptor on the top of the
emulsion . The exposed silver appears as black rather than shiny because the particle
size of the individual grains of silver is very small.
GROUP 18. He, Ne, Ar, Kr, Xe, Rn - the noble gases or inert gases.
Group Overview.
All occur as monatomic gases at room conditions - the only monatomic elements. They
have virtually no reactions due to the very stable atomic structure which has the
maximum effective nuclear charge for that period and which is therefore associated
with very large energy requirements for the gain or loss of electrons required for
reactions to occur.
Helium is found trapped in oil wells along with gas or crude oil. It occurs there
because helium is a product of the radioactive decay of large, unstable nuclei such as
uranium. Helium is an extremely useful substance because it has the lowest boiling
point of any element. It constitutes a significant part of the mass of the sun where the
nuclear fusion reaction of hydrogen to form helium provides most of the energy
released.
The gases neon, argon and krypton are all present in the atmosphere in small amounts argon to the extent of 0.9 %. Neon and argon are used in situations where an inert
atmosphere is required, such as in neon lighting tubes and in argon arc welding. The
element radon is a powerful carcinogen which is ubiquitous in all minerals where it too
arises from radioactive decay processes. Radon presents a particular hazard for miners
involved in uranium mining, but is also released from the burning of coal and even
from the clay of house bricks.
FOR THOSE WHO WANT TO KNOW MORE - helium.
HELIUM.
Helium is named after Helios, the sun god because it was first discovered by
spectroscopic analysis of sunlight in which previously unknown spectral lines were
observed. Later it was recognised as a gas released from some radioactive minerals
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where it originates from alfa particles expelled when unstable nuclei decompose. Alfa
particles contain two protons and two neutrons but no electrons so is in effect a helium
nucleus. The required electrons are picked up rapidly in air to form He atoms, atomic
number = 2 and mass number = 4. This nuclear arrangement of 2 p + 2 n has a
particular stability which accounts for why so much helium was formed in the short
interval following the hypothesised big bang. Helium is rare on earth because it is so
light that it escapes gravity and is only found trapped in the impermeable formations
along with natural gas and oil from which it is commercially separated. Helium is used
to dilute pure oxygen breathed by deep sea divers as it is non-toxic. When inhaled, it
causes one’s voice to take on a squeaky Donald Duck sound due to the much greater
speed of sound in the lighter atmosphere as the timbre of the voice is very dependent
on the speed of the sound waves. Its most obvious although trivial presence is as the
gas used to fill party balloons and blimps but it has a common industrial use as a gas to
protect metals from reacting with oxygen or nitrogen while being welded. Because of
its inertness, helium is used to provide an inert atmosphere when growing and handling
crystals of semiconductors such as silicon.
Cryogenic applications.
Helium has the lowest boiling point of any substance because of its small atomic size
and extremely weak intermolecular interactions. It liquefies at !269o C, a property
essential in superconducting magnets such as those used for magnetic resonance
imaging (MRI). At such low temperatures, electricity travels through copper wires
with no resistance allowing the generation of powerful magnetic fields that then can
interact with hydrogen atoms from which images of bodily structures and organs can
be obtained unobtrusively and without tissue damage. Likewise, many instruments
used for scientific research rely on the low temperatures achievable with liquid helium.
Although helium is one of the two most common elements in the universe, it is rare on
earth and there is concern that supplies of helium are not being husbanded carefully
enough.
The "d-block" elements.
Block Overview.
This is a block of elements located between Groups 2 and 13 following Ca, Sr and Ba.
In the current IUPAC system of Group numbering, they are numbered as Groups 3 to
12. They first appear after Ca in the fourth period of the Table, and there are ten dblock elements in each period where they occur. Note that there are no d-block
elements in Periods 1, 2 or 3. They are all metals, most being hard with high melting
points, although mercury is a liquid at room temperature. Well known elements from
this block include chromium, iron, manganese, nickel, cobalt, copper, zinc, cadmium,
tungsten, platinum, silver, gold and mercury. Some of the special characteristics of
these elements include the following:
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(i) They all form cations in salts (e.g. MnCl2) but also can form covalent bonds to nonmetals such as oxygen (e.g. Mn in the polyatomic ion MnO4–).
(ii) They often have more than one ionic valence state (eg Fe occurs as the ions Fe2+
and Fe3+ in its compounds) as well as a number of valence states in covalent
compounds.
(iii) Their compounds are frequently coloured (eg copper(II) sulfate crystals - blue)
whereas compounds of other metals are usually white.
FOR THOSE WHO WANT TO KNOW MORE - vanadium, chromium,
manganese, nickel, technetium, tungsten, gold, mercury.
SCANDIUM (Z = 21) and YTTRIUM (Z = 39)
See “rare earths” in f-block
VANADIUM (Z = 23).
Vanadium is a silver-grey metal located in the first row of the d-block, between
titanium and chromium. Consequently it has many similarities to these two elements.
Vanadium added in very small amounts to steel forms alloys with vastly improved
strength and heat resistance, the latter being an essential for high speed drills.
Oxidation states and colours.
Vanadium has four common oxidation states, +II, +III, +IV and +V. Vanadium ions in
solution in each of these oxidation states have their own characteristic colours as
below:
+II
lilac
+III green
+IV blue
+V yellow
The range of colours of vanadium compounds lead to its being named after the
goddess of beauty, Vanadis. Coloured compounds are characteristic of d-block
elements and result from the large number of electrons and transitions between orbitals
that are available in their outer levels. This is in contrast to s and p-block elements
which rarely form coloured compounds.
Vanadium flow battery.
Transitions between the various oxidation states of the V atom is the basis for the
vanadium flow battery. In the usual rechargeable or non-rechargeable batteries one or
both of the electrode materials undergoes a redox reaction. These redox reactions
involving the electrodes proceed in one direction when discharging and in the reverse
direction when charging. In the vanadium flow battery, the electrodes are inert and
merely act as part of the external electron transport. The redox reactions take place
entirely through vanadium ions in the electrolyte in each half cell changing their
XII - 56
oxidation states. Thus it is oxidation and reduction of the electrolyte rather than the
electrodes that provides the electron flow.
The following diagram illustrates a vanadium battery.
When discharging, in the negative half cell vanadium ions undergo the change from
the +IV (VO2+) to the +V (VO2+) states with the release of electrons through the
external circuit where they move to the positive electrode. There another vanadium
solution undergoes the change from the +III state to the +II state. The electrodes made
of an inert material play no part in the operation of the cell which is maintained by
pumping electrolytes of each type through the relevant half cell. For this reason, it is
known as a flow battery. To reverse the reaction and recharge the cell, the spent
electrolyte solutions are pumped back through their half cells while the voltage needed
to reverse the reaction is applied to the electrodes. The charge balance within the cell
is achieved by H+ ions migrating through a suitable membrane that keeps the vanadium
ions from intermixing. This type of cell has many advantages - it can be reused an
unlimited amount of times and can hold/supply large amounts of electricity, limited
only by the size of the electrolyte tanks. The main drawbacks are related to the bulk
which limits their use to fixed applications such as back up for the grid rather than for
transport.
Sulfuric acid manufacture.
One of the main uses for vanadium is for the oxide, vanadium pentoxide, which is used
as a catalyst for the conversion of SO2 to SO3 in the contact process by which sulfuric
acid is produced. In this process, sulfur is burnt or sulfide compounds are heated to
release sulfur dioxide gas which needs to be further oxidized to sulfur trioxide before
reacting with water to form sulfuric acid. Without the catalyst the process is too slow
to be commercially viable.
XII - 57
CHROMIUM (Z = 24).
Chromium is a shiny metallic element found in ores often in company with iron. The
name comes from the Greek, “chroma”, meaning “colour”. This relates to the many
chromium compounds which are brightly coloured and are used as pigments in paints
and elsewhere. More recently these uses have diminished due to health concerns about
ingested chromium compounds.
When chromium is mixed with iron, stainless steel results. First developed in 1913,
stainless steel largely resists rusting unlike normal steel because the chromium atoms
form an extremely hard oxide on the surface and this prevents the attack on the Fe
atoms by water and oxygen. Up to 30 % of chromium is mixed with iron to make
stainless steel and the higher the chromium content, the more resistant it is to
corrosion. Unlike iron, chromium is not magnetic and high quality stainless steel can
be detected by its inability to be attracted to a magnet. Alloys of chromium and iron,
sometimes with other metals, are used for high temperature environments such as the
valves in car engines. The property of extreme hardness coupled with the ability to
resist corrosion is the reason that chromium plating is much used for both protection
and appearance. When a thin layer of chromium atoms is plated onto normal steel and
polished, a bright shiny surface results giving the familiar chrome plating used to
decorate and protect many types of metal objects and notably, on motor cars from
previous eras.
Chromium also is used in the tanning of leather. Chromium ions seep into the hide and
cross link the collagen polymers of the skin, making it more pliable and water resistant
and able to absorb coloured dyes.
Rubies and emeralds are crystals of aluminium oxide and they owe their colours to Cr
atoms occupying places within the crystals. This is possible because Al and Cr atoms
are of similar size so a chromium ion can easily replace an aluminium ion in a crystal
lattice. Ruby crystals were used in the first lasers where the properties of the imbedded
Cr ions allow the production of the coherent light which lasers produce.
However chromium in one of its many oxidation states is very toxic and can cause a
range of medical problems including potentially cancer. Chromium in oxidation state
VI also known as hexavalent chromium as for example in dichromate ions, Cr2O72–,
was shown to be associated with widespread illnesses among the population of the
town of Hinkley in California by Erin Brockovitch. In a landmark class action in 1996,
a fine of $333M was awarded against a gas company which had been dumping
polluted water into the groundwater which was later drunk by local well users. Her
work which was also featured in the film bearing her name as the title, has lead to
stringent limits on the amount of hexavalent chromium allowed to be released into the
XII - 58
environment worldwide. One consequence has been a much reduced use of chrome
plating which, along with tanning, used to be responsible for some of that release.
MANGANESE (Z = 25).
Manganese, symbol Mn, shows all the characteristic properties of d-block elements
especially the wide range of colours of its compounds and the range of valence states
(oxidation numbers). The element has a silvery grey appearance and occurs as
compounds in minerals, the dioxide MnO2 being the most common. Manganese is a
vital atom in the photosynthetic processes used by all plants to generate oxygen. Small
amounts are also essential for human health, playing a vital part in development,
metabolism and antioxidant processes.
Applications.
Most of the manganese mined is used in the production of steel and as a component of
alloys with iron and aluminium. Manganese dioxide is used as one component in the
standard zinc dry cell. The brown staining liquid from leaking dry cells is this oxide
material.
Oxidation states.
Manganese atoms can exhibit oxidation states from !III through to +VII. The most
common and examples of their occurrence are listed below.
+II
+IV
+VI
+VII
MnCl2
MnO2
K2MnO4
KMnO4
manganese(II) chloride
manganese(IV) oxide or manganese dioxide
potassium manganate
potassium permanganate
pink
brown
green
purple
The range of oxidation states available to Mn atoms in compounds is the basis for its
use as an oxidant, especially as the permanganate ion and as manganese dioxide. The
oxidizing power of these compounds increases as the number of O atoms bonded to the
Mn atom increases.
Coloured compounds.
The d-block elements are remarkable for having many coloured compounds as distinct
from the compounds of the s and p blocks. This property is a consequence of the large
number of outer electrons and available orbitals. The colours of solutions of the above
compounds is listed on their right.
NICKEL (Z = 28).
Nickel is another metal that resists corrosion because a protective oxide layer forms on
its surface. Nickel plating is similar to chromium plating in that it affords a corrosive
metal such a iron a shiny and protective surface. Nickel is used as one component of
XII - 59
the common rechargeable batteries known as NICAD BATTERIES (for
nickel/cadmium) batteries as well as other types. While on its own, nickel has many
applications, its most widespread use is as a component of many alloys (mixtures of
metals), especially stainless steel which is an alloy of nickel, iron and chromium. There
are over 3000 different alloys of nickel mixed with other metals, each tailored for
specific purposes. A particularly valuable alloy of mostly nickel with copper is monel
which is very strong and highly resistant to corrosion. It has particular uses in marine
environments where it is very resistant to corrosion in salt water. An alloy of nickel
with iron, aluminium and cobalt known as ALNICO can be magnetised to form
stronger and more lasting permanent magnets which have only recently been
superceded by rare earth magnets. Some alloys of nickel are known as super alloys
because of their strength and extreme tolerance of heat. The turbine blades of jet
engines make use of these alloys which contain as many as ten other elements apart
from nickel and are able to resist the extreme temperature and high speeds of rotation
experienced. Modern jet engines would not be possible without various nickel alloys
used in their construction. The compositions of each alloy is designed for a specific
purpose in the engine.
Catalysts.
Apart from its role in alloys, nickel in a finely powdered form is used as a catalyst for
many chemical reactions which would be too slow without catalytic participation. One
common application as a catalyst is in the hydrogenation of vegetable fats (which are
usually liquids at room temperature) to form solid margerine as a substitute for butter.
Shape memory alloys.
Some alloys of various metals have a shape memory whereby if the metal is distorted
from its initial shape such as a straight wire to a twisted or bent shape and then warmed
to a specific temperature, the wire returns to its previous exact state. One such alloy
contains a mixture of about 55 % nickel and the remainder is titanium, so it is known as
nitinol. This self-expanding property of nitinol lends it to a particularly useful
application as stents to expand blocked arteries in the heart. The stent can be made to
the desired state at body temperature and then rolled into a suitable volume in order to
be inserted through an artery in the groin and into the desired location in the heart.
Once warmed to the original body temperature at which the stent was made, it opens up
to the required shape in situ. This type of stent is replacing the older balloon
expandable type previously used.
TECHNETIUM (Z= 43).
The existence of element 43 was predicted by Mendeleev in his Periodic Table (1871).
He named it as ekamanganese using his system for undiscovered elements, in this case
because its position is one place below manganese in the Table. However technetium is
not to be found in the normal manner of the elements because it has no stable isotopes,
XII - 60
the longest lived being 98Tc of half life 4.2 × 106 years. Instead, it can only be obtained
from decay products of fuel rods in nuclear reactors which gives it the name technetium
and symbol Tc. There are numerous other isotopes of Tc each with its own
characteristic half life. Due to its relatively short half life compared to the age of the
earth, no significant amounts of technetium can be found naturally occurring but bulk
quantities are extracted from spent fuel rods of nuclear reactors.
Nuclear medicine applications.
One isotope of technetium, 99Tc, can exist is what is called a meta stable state where the
nucleus has a higher energy and it is designated as 99mTc. This isotope has a very short
half life of 6 hours and when it decays to the more stable form, 99Tc, it emits a gamma
ray. Gamma rays (ã rays) are high energy electromagnetic radiation like visible light
but of much higher energy. Gamma rays can pass through significant thicknesses of
materials without being absorbed and in the case of tissues, without doing significant
damage to cells. Use is made of this property to scan organs such as the heart by
injecting a solution of a salt of 99mTc into appropriate blood vessels and detecting the ãrays as they emerge with a ã ray camera. The resulting pictures show areas where for
example, plaque has built up in arteries or damage has occurred to the heart muscle.
Having such a short half life means the radioactive isotope is quickly cleared from the
body so it does little ongoing damage. This procedure is so widely used that millions of
scans of this type are conducted world wide ever year. The problem with using 99mTc
for diagnostic radiation testing is its half life is so short that it must be used quickly
after preparation. To overcome this to a certain extent, it is made in situ from a
radioactive isotope of the element molybdenum, 99Mo, which decays to form 99mTc.
The molybdenum isotope is mostly made in older style nuclear reactors and it has a half
life of 2.75 days so it can supply the required 99mTc for about a week before a new
supply is required. There is concern that there may be world wide shortages of supply
of 99mTc because most of the reactors that are used to produce the molybdenum isotope
are at the end of their operating lives. In Australia, the Opal reactor at ANSTO in
Sydney produces the molybdenum isotope for distribution throughout the country and
to nearby Pacific nations.
TUNGSTEN (Z = 74).
The symbol for tungsten, W, comes from the name wolfram by which it was known in
some European countries. Tungsten is one of the hardest elements and it has the
highest melting point of any metal, 3422o C. Its density is almost twice that of lead and
nearly twenty times greater than water as a result of the large number of protons and
neutrons in its atoms. Once a method of drawing tungsten into wires was developed, it
found a widespread use as the filament in incandescent electric light globes, replacing
tantalum which had been used earlier. The globe contains argon to prevent any
tungsten oxide forming which darkens the glass. Because of the very high melting
point of tungsten, an electric current passed through it causes the filament to glow
XII - 61
without melting. However, in the process, 95 % of the electrical energy is converted
to heat rather than light so this type of globe is now rapidly being displaced by more
efficient sources. It is still used in X-ray tubes in which electrons from a heated
tungsten filament hit a target, itself often made from tungsten, and X-radiation is
released. Tungsten is mainly used in various alloys which benefit from the increased
hardness and heat resistance, for example in high speed steel which contains as much as
20 % of tungsten.
Tungsten carbide.
This compound of formula WC is made by heating a mixture of tungsten and carbon to
temperatures up to 2000o C. It too has a very high melting point, 2870o C, and is
extremely hard. These properties make tungsten carbide an ideal material for industrial
grinding and cutting. Drill bits and saw blades are often tipped with tungsten carbide.
It is also used in armoury such as bullets and protective sheeting for military vehicles
such as tanks. The ball tip in the end of ball point pens is often made from tungsten
carbide.
GOLD (Z = 79).
Gold is one of the few elements that occur as the free element in nature and its symbol,
Au, is from the Latin, aurum. It is a soft, dense metal and along with copper, one of the
two coloured metals. Gold occurs as nuggets in surface layers of the ground and also
as inclusions in rocks such as quartze. It also occurs free in large veins underground the largest piece found was about 1.5 m long discovered at Hill End. Its limited supply
and demand for jewelry and as a currency hedge and industrial usage have all
contributed to the high price gold commands. Gold is extremely unreactive and does
not corrode in air or water. It is an excellent conductor of electricity and is used in
electronics where reliable contacts are essential. All mobile phones contain tiny
amounts of gold. Because of this and other valuable metals present, mobile phones are
crushed and several elements reclaimed. Despite the small amount of gold present,
crushed phones contain a much greater concentration than in the original ore from
which they were extracted.
Medical applications.
Because gold is so inert, it has uses in medicine. Gold nanoparticles have the ability to
bind to antibodies and drugs and therapeutic agents and deliver them to the desired part
of the body, evading possible defence mechanisms. In cancer therapy, gold
nanoparticles have been found to be taken up more by tumours than healthy tissue so
anti-cancer agents can be concentrated near to the site where they are needed. In some
applications, adsorbed gold nanoparticles at the tumour sites can be irradiated with near
infra red light which then is converted to heat of sufficient intensity to destroy
malignant tissues.
Gold nanoparticles are also finding applications in the new area of gene therapy.
XII - 62
MERCURY (Z = 80).
Mercury, named after the god of speed, has the symbol which is derived from the Greek
name, hydragyrum. It has been known for thousands of years as it is easily obtained by
heating the ore cinnabar which is mercury(II) sulfide and which occurs in highly
concentrated deposits. Mercury is the only metal that exists as a liquid at room
conditions. Its shiny appearance is well known as are some of its common uses such as
in thermometers and blood pressure monitors. Less well known is that the largest use
of mercury today is for the extraction of gold by small scale miners who mix mercury
with low concentrations of gold such as from panning. Mercury can dissolve a number
of metals including gold to form an amalgam and then be removed by heating to leave
the concentrated gold, a process aided by mercury’s relatively low boiling point of 630
o
C. This process was also used by large scale miners in the past but now combination
of gold with cyanide is used commercially. In past eras, mercury and its compounds
were used as medicines and as cosmetics. Mercury is used in tiny amounts in all
fluorescent light tubes and the compact fluorescent light bulbs which have replaced
tungsten filament globes. The topical antiseptic, mercurochrome, is found in most
home medicine kits. Until a few decades ago, the most common material used for
dental fillings was an amalgam of mercury with silver, tin and copper. About 50 % of
the filling is mercury. Small amounts of mercury metal do leach out of these fillings so
people sometimes opt to have them replaced using modern materials which adhere to
the tooth rather than being wedged in as was the case with amalgams.
Health concerns.
Mercury ingested or inhaled is a deadly nerve toxin and in recent years, most of its
former uses have been supplanted by safer alternatives. Currently an effort is being
made to ban many of its traditional applications and to ultimately discontinue mining it.
To reduce pollution, fluorescent light tubes are recycled to remove the mercury content.
Coal fired power stations release mercury into the atmosphere and efforts are now made
to remove this source of pollution. Even thermometers containing mercury are being
replaced by alternatives.
Organo compounds of mercury such as methyl mercury are the most dangerous. This
danger was made very evident in Japan in 1956 when residents of a town called
Minamata fell ill in large numbers. The cause was a mystery but food poisoning of
some type was suspected as even local cats were dying. Ultimately it was traced to the
release of methyl mercury from an industrial plant into the nearby bay. The mercury
compound was concentrated by small aquatic organisms which in turn were eaten by
larger creatures up the food chain to fish and shellfish which contained dangerous
levels. Many thousands of people died or were severely affected by eating the
contaminated seafood. Despite the cause being known, the dumping of methyl mercury
into the bay there continued until 1968. Mercury poisoning by organomercury
compounds has subsequently been named as Minamata disease.
XII - 63
The f-block elements.
Block Overview.
The f-block is located near the bottom of the Table and contains 14 elements in each of
Periods 6 and 7 using the 4f and 5f orbitals respectively. The 14 elements using the 4f
orbitals following lanthanum are known as the lanthanides and those using the 5f
orbitals following actinium are known as the actinides although neither lanthanum nor
actinium are f-block elements. They are all metals and many of the actinides have
unstable nuclei. Particularly important actinides are the elements uranium (U) and
plutonium (Pu) because of their relevance to the nuclear fuel cycle. The many outer
level electrons in the lanthanides impart unique magnetic properties to these elements
and colours to their compounds . The lanthanides are known as the RARE EARTHS,
a grouping which is frequently expanded to include the elements scandium and yttrium
from Group 3. The rare earths mostly are not rare but they generally do not occur in
sufficiently large concentrations to be mined profitably. Mining and purifying these
elements is a very polluting operation which is now almost exclusively done in China
where environmental controls are weak. However, some of these rare earth elements
have taken on great importance in modern technology and have become indispensable.
For example, rare earth elements are vital in the production of lasers and tiny magnets
used in computer drives as well as the larger powerful magnets used in electric motors
and generators.
(a) The lanthanides.
Lanthanum (La) - not strictly an f-block element as it is in the d-block in Group 3;
cerium (Ce); promethium (Pm) - not mined but synthesised in nuclear reactors;
praseodymium (Pr); neodymium (Nd); samarium (Sm); europium (Eu); gadolinium
(Gd); terbium (Tb); dysprosium (Dy); holmium (Ho); erbium (Er); thulium (Tm);
ytterbium (Yt); lutetium (Lu). Among the many useful properties of these elements and
their compounds are the following:
Coloured compounds.
All of the elements in this row of the f-block have coloured compounds and when their
ions are excited they emit light of very specific wavelengths in their atomic spectra.
Erbium is particularly useful as an amplifier in the fibre optic cables used for internet
transmission. Transmission of light along fibre optic cables requires the signal to be
amplified at regular intervals and erbium compounds are the ideal materials to do this.
Erbium ions are incorporated into the walls of the cable at intervals and are energised to
an excited state with laser light. Light travelling along the fibre stimulates the ions to
release the stored energy which is of exactly the required wavelength to boost the
signal.
XII - 64
Small crystals of neodymium compounds are used to produce the green light from
lasers which for example are used in the familiar green pointers. Various mixtures of
rare earth elements and their compounds are the essential ingredients of phosphors used
in screens - for example phosphors based on terbium provide the yellow-green colours
while europium in conjunction with the rare earth from Group 3, yttrium, provides the
red phosphor. The amounts of these elements and their compounds used in the screens
are tiny but the quality of colour TV prior to their inclusion was very inferior. Terbium
compounds mixed with the element europium are also used in the long life fluorescent
light globes to provide the more favoured warm coloured light.
Magnetism.
A mixture of the elements neodymium, boron and iron allows the manufacture of
magnets which are 12 times stronger than those made from of the same weight of iron.
Consequently magnets made from neodymium can be much smaller than comparable
iron magnets and are essential for making the tiny motors used in the hard drives of
laptops. More significantly, the power of such magnets allows the efficient production
of electricity by wind turbines and has made electric vehicles practical. The new
generation of wind turbines is able to dispense with the gear boxes which used to be
needed to synchronise the blade revolutions with the requirements of the electrical
generators they drive by using 648 neodymium and dysprosium magnets set in a
doughnut shape directly on the rotating axle of the blade. Dysprosium in small amounts
when added to neodymium alloy magnets prevents the loss of magnetism which occurs
when the temperature exceeds 300 o C. Magnets made from samarium and cobalt resist
demagnetising when exposed to nuclear radiation which is particularly important in
nuclear power stations. Holmium is another 4f element which is used in high strength
magnets.
Other applications.
Cerium is used for fine polishing of liquid crystal displays and as a catalyst in the
catalytic converters fitted to the exhaust systems of motor cars.
Gadolinium compounds are used in magnetic resonance imaging as a contrast dye.
Lanthanum and cerium mixed are used as one of the electrodes in the nickel metal
hydride rechargeable battery commonly used to power electric cars.
Yttrium is incorporated into high temperature resistant ceramics that are essential in jet
engine turbine blades and rocket exhaust systems and similar applications.
Promethium is used in atomic batteries for specialised purposes.
XII - 65
(b) The actinides.
Actinium (Ac); (like lanthanum, this element is not strictly a member of the f-block and
belongs to Group 3 in the d-block but is usually grouped with the adjacent f-block
elements); thorium (Th); protactinium (Pa); uranium (U); neptunium (Np); plutonium
(Pu); americium (Am); curium (Cu); berkelium (Bk); californium (Cf); einsteinium
(Es); fermium (Fm); mendelevium (Md); nobelium(No); lawrencium (Lr).
All the actinides are radioactive. Only uranium and thorium occur naturally in large
quantities so the others require nuclear reactors or particle accelerators to produce them
from smaller atoms. Plutonium is one such element produced in nuclear reactors and it
like uranium is used in both power stations and nuclear weapons. Thorium is currently
being used in a new prototype nuclear power plant where it would have many
advantages over the conventional fuels, especially from safety aspects. Thorium
reactors do not produce plutonium which was required for nuclear bombs and it seems
that this is the main reason thorium reactors were not the first choice for power
generation. Americium is used as the ionizing source in most household smoke
detectors.
FOR THOSE WHO WANT TO KNOW MORE - uranium
URANIUM
Uranium occurs as two common isotopes, 235U and 238U. Both are slightly radioactive
but only 235U is fissile - its nucleus when subjected to impacting neutrons breaks into
two smaller nuclei such as 92Ba and 141Kr and releases 3 neutrons per decaying nucleus.
Accompanying this fission process is the release of much energy as heat. The fission
process occurs naturally in the ore but normally only at a very slow rate. This property
is harnessed in nuclear reactors and also atomic bombs. In order to produce significant
fission, it is necessary for neutrons to be available to travel through the 235U nuclei in
sufficient quantities for the reaction to be self sustaining. Remember that neutrons
carry no electrical charge so can penetrate the positively charged nucleus without
experiencing electrostatic repulsions. When a 235U nucleus undergoes fission the three
neutrons released can, if the uranium is packed suitably, cause more nuclei to break
apart and start a self-sustaining chain reaction. In a nuclear reactor, the process is
controlled by surrounding the fuel with a moderator such as water to slow the neutrons
to a suitable speed lest they travel through nuclei without setting off fission. To stop
the reaction, control rods of material such as boron or rare earths which absorbs
neutrons readily, are suspended above the fuel rods and they can be lowered as needed.
In an atomic bomb, two sub-critical pieces of 235U are held well apart. To set off the
reaction, they are rammed together to form a critical mass in which the extra neutrons
are enough to provide an uncontrolled fission reaction. In a nuclear power station, the
heat from the fission reaction is captured and used to convert water to steam at high
XII - 66
pressure and this turns a generator to produce electricity. One kilogram of uranium fuel
can produce the same amount of electricity as 1500 tonnes of coal.
The other common isotope, 238U, sometimes called depleted uranium, is not fissile but
has some applications resulting from its extremely high density which is 70% greater
than that of lead. Military uses include providing armour for tanks and casing
explosive shells.
Separating uranium isotopes.
Uranium ore contains 99.3 % 238U and just 0.7 % 235U which must be enriched in order
to have suitably concentrated fuel. Being isotopes, they are chemically identical so
physical methods are the only way to do this. The physical difference between the
isotopes which is exploited is their differing masses. Although only small, by repeating
mass separating processes many times, a suitable degree of enrichment is obtained.
The first method used was gas diffusion in which the uranium is converted to gaseous
UF6 and the slightly more rapid diffusion of 235UF6 over many repeated diffusion steps
is able to be enriched to 3 %. For weapons grade, a higher concentration is needed.
More recently, repeated sequential centrifuging of uranium compounds has been
developed as a more efficient method.
Dating.
The very long half life of 238U, 4.5 × 109 years, makes it ideal for dating old rocks as
uranium is reasonably prevalent in the earth’s crust. The isotope 235U has a half life of
704 million years likewise is suitable for dating rocks of lesser age.
Hydrogen
Hydrogen does not fit into any Group of the Table due to its unique property of having
just one electron. Like Group 1 elements, it forms the +1 ion, but only in association
with a molecule such as water because H+ would be a naked proton, incapable of free
existence. Hydrogen is also similar to Group 17 elements (halogens), existing as an H–
ion in ionic compounds, e.g. sodium hydride (NaH). However, ionic hydrides are
unstable, reacting with water to form hydrogen gas and OH– ions.
Metals vs non-metals - distribution throughout the table.
The metallic elements are located on the left hand section of the Table and the nonmetals are on the right. Although the Periodic Table was originally devised purely on
the basis of macroscopic properties, the true underlying basis for it is now understood
in terms of the structure of the atoms of the elements. Metallic properties increase
down each Group, even amongst those on the extreme left, because the atomic radius
increases down each Group and outer electrons are more easily removed the further
they are from the nucleus of the atom. The non-metals are on the right because they
XII - 67
have more electrons in the same outer orbit which in turn are held more tightly by the
increasing nuclear attraction from the greater number of protons, causing the atomic
radius to decrease from left to right contrary to what one might have expected. Thus
atoms of non-metals are more able to gain electrons to form anions rather than lose
them to form cations. At the conclusion of each Period of the Table, the noble gases
represent the most stable outer electron arrangement as the next element has its last
electron located in a new energy level further from the nucleus and held less strongly,
allowing that element to take on the properties of a metal again. This sequence of metal
v non-metal is repeated for each Period of the Table. The following table gives a
guide to the general distribution of the metals compared with the non-metals in the
Periodic Table. Between the metals and non-metals, towards the middle of the Table,
some of the elements show properties of both and are called metalloids or semi-metals.
The predictive power of the Periodic Table.
When Mendeleyev devised his version of the Periodic Table, he was able to use it to
predict that a number of then unknown elements existed and further, was able to
make accurate predictions about the properties of those elements and some of their
compounds. However, the predictive power of the Periodic Table is by no means
limited to those initial triumphs. It has guided much successful chemical research
and still does today. The following is an example of where researchers have
employed a knowledge of the Periodic Table to guide their work.
The Nobel prize winner of 2010 in chemistry, Ei-ichi Negishi, won his prize for
work on palladium catalysis in such applications as the anti-cancer drug Taxol. In an
interview with New Scientist (16 October 2010) he said: “Years ago when I started
in chemistry, I was awed by the row of transition metals [containing palladium].
They are the most gifted bunch in the periodic table. I work with the Periodic Table
in front of me at all times and approach all challenges in terms of three particles:
positively charged protons, negatively charged electrons and neutrally charged
neutrons. That’s science.”
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DISTRIBUTION OF METALS AND NON-METALS WITHIN THE PERIODIC TABLE
H
FF
Group
18
non-metal
Group 1
Group 2
Group
13
Group
14
Group
15
Group
16
Group
17
He
F
non-metal
Li
G
Be
G
B
:::
C
:::
N
FF
O
FF
F
FF
Ne
F
metal
metal
non-metal
non-metal
non-metal
non-metal
non-metal
non-metal
Na
G
Mg
G
Al
G
Si
:::
P
FF
S
FF
Cl
FF
Ar
F
metal
metal
metal
metalloid
non-metal
non-metal
non-metal
non-metal
K
G
Ca
G
Ga
G
Ge
:::
As
:::
Se
FF
Br
FF
Kr
F
metal
metal
metal
metalloid
metalloid
non-metal
non-metal
non-metal
Rb
G
Sr
G
In
G
Sn
G
Sb
:::
Te
:::
I
FF
Xe
F
metal
metal
metal
metal
metalloid
metalloid
non-metal
non-metal
Cs
G
Ba
G
Tl
G
Pb
G
Bi
G
Po
G
Rn
F
metal
metal
metal
metal
metal
metal
non-metal
Fr
G
Ra
G
metal
metal
Bond type in the element
G
metallic
FF molecular covalent F
::: network covalent
monatomic (noble gases only)
XII - 69
Objectives of this Topic.
When you have completed this Topic, including the tutorial questions, you should have
achieved the following goals:
1.
Understand that the Periodic Table was originally devised on the basis of
families of elements with similar properties, arranged in order of increasing
atomic weight (but subsequently found to be increasing atomic number).
2.
Know the electronic basis for the similarities and trends within Groups of the
Periodic Table.
3.
Commit to memory the elements of Groups 1, 2, 13-18.
4.
Know the general distribution of metals and non-metals within the Table.
5.
Have some familiarity with the main chemical properties of each Group.
Further resources.
Video
The video shown in the lecture can be viewed at
1. http://www.youtube.com/watch?v=M-lnauoORdA
On YouTube where there are many other videos relating to the Periodic Table also
available for viewing.
Audio
The BBC series titled “Elements” is available from their website as Elements Podcasts.
Recommended follow up chemcal modules:
Section: Properties of Atoms
Module: Atomic Properties
Topics covered: The concept of core charge and its relationship to fundamental atomic
properties.
Module: Electronic Structure of Atoms and Ions
Topics covered: Trends in atomic properties in relation to the Periodic Table
Section: Properties of Molecules
Module: Electronegativity and Polar Molecules
Topics covered: Electronegativity in relation to periodic table; polar and non-polar
molecules.
XII - 70
SUMMARY
The classification of elements into groups with similar chemical and physical properties
was begun prior to any knowledge of the structure of atoms and when the existence of
undiscovered elements was overlooked. Purely on these bases, it was apparent that
some elements having similar properties could be grouped together as families. With
the measurement of atomic weights of elements came attempts to find correlations of
properties with atomic weight order, but these were hampered by the many elements
that had not been isolated at that time and also by inaccurate atomic weight
determinations. Mendeleev recognised these deficiencies and turned them to
advantage, still using atomic weight order but giving primacy to assigning elements
with similar properties to the same Group. He left gaps in the arrangement of the
elements where needed and predicted which elements were still to be discovered and
their likely properties. This arrangement of the elements, called the Periodic Table, was
further refined when the structure of the atom was elucidated and it was then realised
that the order of the elements in the Periodic Table should be atomic number order
rather than atomic weight order. From today’s knowledge of atomic structure, the
reason for similarities of chemical properties within any Group of elements has been
clearly established as a consequence of each element in any Group having the same
outer-shell electron arrangement. However, gradations in properties within a Group are
normally observed, with the most metallic elements being at the bottom. Within any
row (Period) of the Table, the elements on the left are metals and on the right are nonmetals, a gradual increase in non-metallic properties from left to right being exhibited.
Some elements towards the middle of the Table show properties of both metals and
non-metals. These trends can also be related to aspects of atomic structure, in
particular the decreasing effective nuclear charge of atoms down each Group which
leads to less energy being required to remove electrons and thus form cations - a
property of metals. Elements to the right of the Table are non-metals because they have
more electrons contained within the same outer level of the atom and are accompanied
by a corresponding increase in the number of protons in the nucleus and thus an
increasing effective nuclear charge, resulting in all the electrons being held more
tightly. Thus more energy is required to remove electrons from non-metals - instead,
formation of anions or covalent bonding are their energetically preferred options. The
least reactive of all elements - the noble gases - have the largest effective nuclear
charge which explains their near complete lack of reactivity.
The Periodic Table is an elegant example of how observations and the collection of
data, development of hypotheses, predictions made from them and testing of each
hypothesis followed by discarding or refining it leads to increased understanding in
science.
Before commencing the questions associated with Topic 12, complete any
remaining questions from Topics 11, 7, 8, 9 and 10 in that order.
XII - 71
TUTORIAL QUESTIONS - TOPIC 12
1. List the physical properties of metals compared with non-metals.
2. How do the chemical properties differ for metals compared with non-metals with
regard to the following:
(a) Type of ion formed in salts
(b) Reaction with acids
3. Disregarding hydrogen and helium, for the Periodic Table Groups 1, 2, 13 - 18, what
electronic structural feature do all the atoms in any given Group have in common?
4.
(a) Write the names and symbols for the elements of Group 1
(b) List some properties of Group 1 elements which indicate they are all metals.
(c) What valence do all Group 1 elements exhibit in their compounds?
5.
(a) Write the names and symbols of the elements of Group 2.
(b) What properties would indicate that beryllium could in some respects be
more appropriately regarded as a non-metal?
(c) Aside from beryllium, list properties of the other members of Group 2
which indicate they are metals.
(d) What valence do all Group 2 elements show in their compounds?
6.
(a) Write the names and symbols of the elements of Group 13.
(b) Is boron a metal or a non-metal? Give reasons for your answer.
(c) Why is aluminium a useful structural material even though it reacts readily
with water and oxygen?
(d) What is the usual valence shown by elements of Group 13 in compounds?
7.
(a) Write the names and symbols of the elements of Group 14.
(b) Which elements of Group 14 could be best regarded as metals rather than
non-metals? Give evidence to support your answer.
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(c) Diamond and graphite are both forms of pure carbon. Compare their
physical properties and explain the differences in terms of arrangement of their
atoms.
8.
(a) Write the names and symbols of the elements of Group 15.
(b) What type of bonding is present in elemental nitrogen?
(c) Give the names and formulas for some species to which nitrogen is converted
in the process of nitrogen fixation.
(d) Summarise the metallic/non-metallic properties of the Group 15 elements.
(e) In what way does bismuth differ from the other members of Group 15?
9.
(a) Write the names and symbols for the elements of Group 16.
(b) Summarise the metallic/non-metallic properties of Group 16. List properties
which support your answer.
10.
(a) Write the names and symbols for Group 17 elements (halogens).
(b) Explain why the halogens all have low melting and boiling points.
(c) Why are halogens strong oxidising agents?
(d) List all the elements that occur naturally as diatomic molecules.
(e) What properties confirm that Group 17 elements are all non-metals?
11.
(a) Write the names and symbols for Group18 elements (noble gases).
(b) Why do all the elements of Group 18, the noble gases, occur as monatomic
gases in nature?
12.
(a) Where in the Table do the elements of the d-block appear?
(b) While the d-block elements are all metals, these elements show some
properties in general that differ from the metals of Groups 1 and 2. What are
these different properties?
13.
Summarise the occurrence of metals and non-metals in the Periodic Table.
XII - 73
14.
Where in the Periodic Table are those elements which are (a) most easily
oxidised and (b) those most easily reduced located? Explain why this is so.
15.
Define the terms “effective nuclear charge” and “screening” in the context of
atomic structure. How does each relate to the properties of the elements as
arranged in the Periodic Table?
ANSWERS TO TUTORIAL TOPIC 12
1.
Metals: shiny when freshly cut, malleable, ductile, good conductors of heat and
electricity
Non-metals: dull solids, powders or gases, brittle, poor conductors of heat and
electricity
2.
(a) Metals form cations in reactions that produce salts while non-metals form
anions in those reactions.
(b) Many metals react with acids forming cations as part of a salt while nonmetals do not react with acids.
3.
All the elements in any Periodic Table Group have the same arrangement of
electrons in their outer level.
4.
(a) lithium (Li), sodium (Na), potassium (K), rubidium (Rb), caesium (Cs)
(b) They all have the physical properties listed for metals above; they all form
cations in reactions with acids to produce salts.
(c) Group 1 elements always have a valency of 1 in their compounds.
5.
(a) beryllium (Be) magnesium (Mg) calcium (Ca) strontium (Sr) barium (Ba)
(b) Beryllium does not form cations in its compounds, instead it is usually
covalently bonded.
XII - 74
(c) They all display the physical properties of metals given above; they all form
cations in reactions with acids to produce salts.
(d) Group2 elements always have a valency of 2 in their compounds.
6.
(a) boron (B), aluminium (Al), gallium (Ga), indium (In), thallium (Tl)
(b) Boron is a non-metal as it is a black powder, does not form salts with acids
and is only covalently bonded in its compounds.
(c) Aluminium reacts with oxygen in the air to produce an oxide which adheres
strongly to the surface of the metal and protects it from further corrosion in the
atmosphere.
(d) Group 13 elements mostly have a valency of 3 in their compounds.
7.
(a) carbon (C), silicon (Si), germanium (Ge), tin (Sn), lead (Pb)
(b) tin and lead behave more like metals than the rest of Group 14 because they
have the physical properties of metals and form cations in some of their
compounds with non-metals.
(c) Carbon atoms in diamond are arranged in a very stable tetrahedral structure
with each C atom bonded to four other C atoms. The very stable structure of
diamond imparts the properties of considerable hardness and non-conduction of
electricity. In graphite, each C atom is bonded to just three other C atoms in a
planar arrangement, leaving one unused valence electron on each atom. These
unused electrons form weak partial bonds to C atoms in the planes above and
below. Consequently graphite is soft and the planes of C atoms are easily peeled
apart (as in lead pencils). Also graphite can conduct electricity because the
electrons between the planes are so weakly held that they are mobile under the
influence of an electrical voltage.
8.
(a) nitrogen (N), phosphorus (P), arsenic (As), antimony (Sb), bismuth (Bi)
(b) Elemental nitrogen consists of molecules consisting of two N atoms
covalently bonded by a triple bond. This is termed molecular covalent bonding.
XII - 75
(c) ammonia (NH3), ammonium ion (NH4+), nitrogen dioxide (NO2), nitrate ion
(NO3–), nitrite ion (NO2–).
(d) Nitrogen and phosphorus are clearly non-metals. Arsenic and antimony
have properties of both metals and non-metals. Bismuth is more metallic than
non-metallic.
(e) Bismuth forms many ionic compounds containing the Bi3+ cation and it has
metallic bonding in the elemental state.
9.
(a) oxygen (O), sulfur (S), selenium (Se), tellurium (Te).
(b) All the elements of Group 16 are non-metals. They have the usual physical
properties of non-metals, do not dissolve in acids and they form anions when in
compounds with metals. Their compounds with other non-metals are covalently
bonded.
10.
(a) fluorine (F), chlorine (Cl), bromine (Br), iodine (I).
(b) The halogens occur as diatomic molecules which only have weak forces of
attraction between their molecules. Consequently it requires relatively little
energy (thus lower temperature) for the molecules to separate into the liquid
phase from the solid (melt) or from the liquid phase into the gas (boil).
(c) The atoms of halogens only need to gain one extra electron to become
isoelectronic with a noble gas. Species which readily gain electrons are good
oxidants.
(d) H2, N2, O2, F2, Cl2, Br2, I2.
(e) Apart from their physical properties, the halogens all form anions in salts
and covalent compounds with other non-metals.
11.
(a) helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), radon (Rn).
(b) The outer electron level of the atoms of all the noble gases is filled with 8
electrons (or 2 electrons for helium), the arrangement that gives the noble gases
their stability. To form diatomic molecules for example, more electrons would
XII - 76
have to enter this level in forming covalent bonds between the atoms and this is
not possible as the level is already filled.
12.
(a) The d-block elements are located in the Periodic Table between Group 2 and
Group 13. Where they occur, there are 10 d-block elements in each Period.
(b) Among the properties that distinguish d-block elements from the metals of
Groups 1 and 2 are that they frequently show a range of valencies and often have
coloured compounds.
13.
Metals are located from the left hand side starting with Group 1 and extending to
include the d-block. The non-metals are located on the right hand side of the
table extending to boron in Group 13 and carbon in Group 14. Metallic
properties increase down each Group of the Table.
14.
(a) The most easily oxidised (i.e. the strongest reductants) are located in the
bottom, left hand part of the Table where the outer electrons of elements are
further out from the nucleus and not held so tightly by attraction to it.
Consequently relatively small amounts of energy are required for an oxidant to
remove an electron. Group 1 elements are the most easily oxidised as they only
need to lose one electron to become isoelectronic with the nearest noble gas.
(b) The most easily reduced (i.e. the strongest oxidants) are located in the top,
right hand corner of the Periodic Table, excluding the noble gases. Fluorine is
the strongest oxidant followed by oxygen and chlorine. Elements in this region
of the table are only one or two electrons short of having the stable structure of
the nearest noble gas, so energy is released when the atom captures an electron
in the process of oxidizing another atom and itself being reduced in the process.
15.
Effective nuclear charge is the actual force experienced by electrons in the outer
energy level of an atom. Outer electrons are said to be screened from the full
attraction they would otherwise feel from the positively charged nucleus because
of those electrons which occupy orbits closer to the nucleus.
Effective nuclear charge increases from left to right across each period of the
Table because, as electrons are added progressively to the outer orbit, all the
outer electrons experience increased attraction to the larger number of protons in
XII - 77
the nuclei. This results in a steadily reducing atomic radius and increasing energy
requirements to remove electrons from atoms to form cations. Thus cations with greater
that +3 charge are rarely formed. In each Period, the last element (a noble gas) has the
maximum effective nuclear charge and so reactions in which electrons are removed
from noble gas atoms require a prohibitively large energy input.
The outer electron from elements containing one more electron than a noble gas
(always Group 1) must have that electron in the next highest energy orbit which
is now screened from the nucleus by the inner electrons. The consequence of this
is not only a larger atomic radius but also much less energy being required to
remove that electron to form a +1 cation.
Effective nuclear charge decreases down each Group due to increased screening
by the ever larger number of inner electrons and so electrons can be removed
from atoms more readily, i.e. elements become more metallic the lower they are
in a Group.
The large effective nuclear charge on atoms to the right of each Period also
explains why elements such as the halogens form anions so easily, actually
releasing energy in the process. Once the noble gas structure is attained, any
additional electrons would have to occupy the next outer orbit and be screened
from the nucleus to such an extent that they would be unstable. This is also the
reason that noble gases do not form anions.
Nuclear
reactors
Apparatus to
m e a s u r e
radioactivity
9
8
High energy
particle
accelerators
Bunsen burner
and
spectroscope
9
9
8
Liquid air
Electrolysis
9
Apparatus to
handle gases
9
8....... Improvements in analytical ....... 8
methods
Prehistoric
Use of furnaces
to smelt ores
9
8..... Methods for analysing minerals - ... 8
especially use of the blowpipe
DISCOVERY OF THE ELEMENTS - NUMBER OF KNOWN ELEMENTS vs
YEAR
this file won’t print the labels - use the file called 04bridgingcourse append1.