Download periodic classification of elements

Classification
PERIODIC CLASSIFICATION OF ELEMENTS
(i)
Classification is the method of arranging similar atoms
together and separating them from dissimilar atoms.
If Lithium is first element then Sodium is the eighth
element and both have similar chemical properties.
(ii) If Fluorine is first element then Chlorine is the eighth
element and both have similar chemical properties.
Need For Classification Of Elements
To make the study of large number of elements easier it is
reduced to the study of small group of elements by
classification of elements in different groups. Many scientists
like Dobereiner, Newland etc. laid the foundation of
classification of elements on the basis of which the modern
classification is developed.
Dobereiner’s Classification or Dobereiner’s
Triads
Dobereiner observed triads. Triads were group of three
elements having similar properties. He stated that, when
elements are arranged in the increasing order of atomic
masses, the atomic mass of the middle element of the triad is
equal to the arithmetic mean of the atomic masses of the
other two elements and these three elements possess similar
chemical properties.
Example
Element
Atomic Mass
Lithium
7
Sodium
23
Potassium
39
Element
=
35.5
Bromine
79.9
Iodine
127
1.
2.
3.
It was found that the Law of Octaves was only applicable
upto calcium, as after calcium every eighth element did
not possess properties similar to the first element.
It was assumed by Newland that only 56 elements
existed in nature and no more elements would be
discovered in the future. But later on, several new
elements were discovered, whose properties did not fit
into then law of octaves.
In order to fit elements into his periodic table, Newland
adjusted two elements in the same slot, but also put
some unlike elements under the same note.
Mendleev’s Classification
7 + 39
2
Mendleev in 1869 arranged elements in the order of increasing
atomic masses and the properties were repeated after regular
intervals.
Mendleev’s Periodic Law
It states that the properties of the elements are the
periodic function of their atomic masses.
Atomic Mass
Chlorine
Anomalies Of Newland’s Law Of Octaves
=
35.5 + 127
2
He was able to make only three triads out of the then known
elements.
But all the elements could not be arranged in such triads.
Newland’s Law Of Octaves
Newland arranged elements in order of increasing atomic
masses. He stated that, when elements are arranged in such a
way, the properties of eighth element (starting from a given
element) are the same as the properties of first element.
Elements are arranged in horizontal rows with seven elements
in each row.
Example
Newland’s Table Of Elements
Li
Be
B
C
N
O
F
Na
Mg
Al
Si
P
S
Cl
K
Ca
Periodic Table
A periodic table is the arrangement of the known elements in
rows and coloumns according to their properties in a tabular
form.
Mendleev’s Periodic Table
Mendleev’s periodic table consisted of eight groups (vertical
columns) and six periods (horizontal rows). At that time 63
elements were known and when they were arranged in the
order of increasing atomic masses, the properties of elements
were repeated after regular periods. He also left some gaps in
the periodic table for the undiscovered elements and predicted
the properties of those elements by studying the properties of
neighbouring elements.
Principles On Which Mendleev’s Periodic Table
(i) Increasing order of the atomic masses.
(ii) Similarities in the properties of the elements of same
group.
Achievements Of Mendleev’s Periodic Table
1.
In his periodic table the elements with similar chemical
properties were grouped together due to which the order
of atomic masses for some elements was wrong i.e,
decreasing order instead of increasing. Like Cobalt
(atomic mass 58.93) placed before Nickel (atomic mass
2.
3.
58.71) and Tellurium (atomic mass 127.6) placed before
Iodine (atomic mass 126.90).
Mendleev left some gaps in his periodic table as he
predicted the existence of some new elements that had
not been discovered at that time. Later on those elements
were adjusted in the table without disturbing the original
table.
When the noble gases were discovered, they could be
placed in a new group without disturbing the existing
order.
arranged in groups (columns) and periods (rows).
Periods
In periods, elements are arranged in order of increasing
atomic number. The number of elements in a period is fixed
by the maximum number of electrons. A new outer shell is
there in each period. Sixth period is the longest & complete
period as it contains Lanthanide Series.
Period
No. Of Elements
1st
2(Shortest Period)
8
2nd
3rd
8
18
4th
18
5th
32(Longest Period)
6th
23(Incomplete)
7th
There are two more rows at the bottom of the periodic table.
These rows consist of fourteen elements after lanthanum
(called lanthanides) and fourteen elements after actinium
(called actinides). These are placed separately in the periodic
table to save space and to avoid unwanted sidewise expansion
of the periodic table.
Groups
Anomalies In Mendleev’s Periodic Table
1.
Not Good For The Prediction Of New Elements
The atomic masses do not increase in a regular manner in
going from one element to the next. So it was not
possible to predict how many elements could be
discovered between two elements especially when we
consider the heavier elements.
2.
The Position Of Isotopes Could Not Be Explained
Mendleev was not able to place the elements in the
periodic table which are having isotopes. (Isotopes are
the atoms having same atomic number but different
atomic masses.
3. Position Of Hydrogen
Hydrogen is placed in group 1. However it resembles the
elements of group 1 as well as the elements of group 17.
Therefore the position of hydrogen was not correctly
defined.
Modern Periodic Law
The modern periodic law was proposed by Moseley. It states
that the properties of the elements are the periodic
function of their atomic numbers. All the anomalies of
Mendleev’s periodic table were removed in modern periodic
table. When elements are arranged according to increasing
atomic number, there is a sequence or periodicity in the
electronic configuration of the elements. The cause of
periodicity is the recurrence of similar electronic
configurations. The properties of the elements get repeated
after intervals of 2, 8, 8, 18, 18, 32 because similar electronic
configurations recur only after these intervals. The elements
having similar electronic configuration show similar properties.
Modern Periodic Table
The modern form of periodic table is based on atomic number
of elements. It was prepared by Bohr. The arrangement of
electrons in modern periodic table is based on their atomic
number and electronic configuration. The elements are
In groups, elements which are having same number of valance
electrons are placed in one group. Outer shell electrons are
identical in each group. The elements within a group show: (i) Similar properties due to similar electronic configuration.
(ii) A gradation of properties due to slowly varying attraction
between nucleus and the valence electrons.
(iii) According to the new recommendations of International
Union of Pure and Applied Chemistry (IUPAC) there are 18
groups numbered 1 to 18. The group 1, 2 and group 13 to
17 contain Normal elements. Group 18 elements are
known as Noble gases or Inert gases & elements in
group 3 to 12 are Transition element. Elements from
93 onwards are purely synthetic and are called Transuranium elements. These are all radioactive in nature.
Trends In Groups And Periods
1.
Valance Electrons
Valence electrons are the electrons present in outermost
shell of an atom.
Trend Of Valence Electrons In Groups
All elements of a group of the periodic table have the
same number of valance electrons.
Group
Group
Group
Group
Group
Group
Group
1
2
13
14
15
16
17
1
2
3
4
5
6
7
valance
valance
valance
valance
valance
valance
valance
electron
electron
electron
electron
electron
electron
electron
To Find Out That A Particular Element Belongs To
Which Group
(i)
The group number of elements having up to two valence
electrons is equal to the number of valence electrons.
(ii) The group number having more than two valence
electrons is equal to the number of valence electrons +10.
Example
Element
Number
Hydrogen
Number Of Valence
Electrons
1
Group
1
Helium
Aluminum
Silicon
Phosphorous
Sulphur
Chlorine
Argon
2
3
4
5
6
7
8
+
+
+
+
+
+
10
10
10
10
10
10
=
=
=
=
=
=
Trend Of The Size Of Atom In Groups
The size of the atoms increases on going down in a
group, because in moving top to bottom in a group, a new
shell of electrons is added in each element. Due to the
increase in number of shells, the size of the atom increases.
2
13
14
15
16
17
18
Trend Of Valence Electrons In Periods
On moving from left to right in a period the valence
electrons increase in a linear order. In every period, the
first element has one valence electron and the last element
has eight valence electrons.
Example
In the second period Lithium has one valence electron and
Neon, the last element of the period has eight valence
electrons.
2.
Valency
Number of electrons lost, gained or shared by an atom of
an element to become stable is called valency.
Trend Of Valency In Groups
All the elements in a group have same valency. It is due
to the presence of same number of valence electrons in the
atoms of elements in a group.
Example
Group
Valency
1
1
2
2
13
3
14
4
15
3
16
2
17
1
18
0
Trend Of Valency In Periods
On moving left to right in a period, the valency first
increases then decreases. For example, in the second
period, the valency first increases from 1 to 4 then decrease to
0. It is because the number of electrons increases in a linear
order.
Example
Li
Be
B
C
N
O
1
2
3
4
3
2
Element in same period have different valences.
3.
F
1
Ne
0
Size Of Atom
It is the distance from the centre of the atom to its
outermost shell. So it is also called the radius of atom.
Example
In group 1, size of the last element Francium is largest.
Trend Of The Size Of Atom In Periods
On going left to right in a period, the size of atoms
decreases. It is because on going left to right, the atomic
number of elements increases, i.e. number of protons and
electrons increases. Increased numbers of electrons are added
to the same shell in a particular period. Due to the increase in
the positive charge on the nucleus, (effective nuclear charge)
the increased number of electrons in the valence shell, are
attracted more towards nucleus and therefore the size
decreases.
Example
In the second period Lithium is biggest in size but Fluorine is
smallest in size.
Note
Neon even being the last element of the period is not having
the smallest size. Rather its size is larger due to the presence
of Wander Val forces of noble elements. So the elements
having the smallest sizes will be from the halogen family i.e.,
the 17th group elements.
4.
Electropositive Nature
It is the ability to form positive ions or the ability to loose
electrons easily.
Trend Of The Electropositive Nature Of Atom In Groups
The electropositive nature increases on going down in
a group. This is due to the fact that when an electron shell is
added at each step while moving down in a group the size of
the atom increases. The valence electrons become more and
more away from the nucleus and the hold of the nucleus on
the outermost electrons become less, so, the atom can easily
loose these electrons to form positive ions and become
electropositive.
Example
In group 14, Carbon, the first member is electropositive as
well as electronegative due to valency as 4. The electropositive
character increases on going from top to bottom, shown by
the last member Lead of this group, which is most
electropositive.
Trend Of The Electropositive Nature Of Atom In Periods
Electropositive nature of an element decreases on going from
left to right in a period. This is due to the fact that the size of
the atoms decreases on moving from left to right due to the
increase in the effective nuclear charge. So, the valence
electrons become more and more close to the nucleus and the
hold of the nucleus on the outermost electrons increases. In
this situation it becomes difficult for an atom to loose electrons
to form positive ions and become electropositive.
Example
In second period Lithium is the most electropositive whereas
the electropositive nature decreases on moving towards right
showing that Fluorine is not at all electropositive.
5.
Metallic Nature
The electropositive nature of an element shows that it is a
metal.
Trend Of Metallic Nature In Groups
The metallic character increases on moving down in a
group due to increase in the electropositive nature of
elements.
Example
In group 14 Carbon the first member is a non metal. The
metallic character increases on going from top to bottom,
shown by the last member Lead of this group, which is a
typical metal.
Trend Of Metallic Nature In Periods
The metallic nature decreases on moving left to right in
a period. It is due to the decrease in the electropositive
nature of the elements.
Example
In second period Lithium is the most electropositive hence it is
a metal whereas the electropositive nature decrease on
moving towards right showing that Fluorine is a non metal.
6.
Electronegative Nature
It is the ability to form negative ions. Or it is the ability to
gain electrons easily.
Trend Of Electronegative Nature Of Atom In Groups
The electronegative nature decreases on going down in
a group. This is due to the fact that when an electron shell is
added at each step while moving down in a group the size of
the atom increases. The valence electrons become more and
more away from the nucleus and the hold of the nucleus on
the incoming electrons in the outermost shell decreases.
Therefore it becomes difficult for the atom to gain electrons to
form negative ions.
Example
In group 17, Fluorine, the first element is most electronegative
but Iodine the last element is least electronegative.
Trend Of Electronegative Nature Of Atom In Periods
The electronegative nature increases on moving left to
right in a period. This is due to the fact that the size of the
atoms decreases on moving from left to right in a period due
to the increase in the effective nuclear charge. So, the valence
electrons become more and more close to the nucleus and the
hold of the nucleus on the outermost electrons increases. In
this situation it becomes easier for the atom to gain the
incoming electrons and form negative ions.
Example
In second period Lithium is not electronegative whereas the
electronegative nature increases on moving towards right
showing that Fluorine is the most electronegative.
7.
Non Metallic Nature
The electronegative nature of an element shows that it is
a non metal.
Trend Of Non Metallic Nature In Groups
The non-metallic character decreases on moving down
in a group due to the decrease in the electronegative nature
of elements. This is because due to the increase in the size of
the atoms the hold of the nucleus on the incoming electrons
becomes weaker and it becomes difficult for the atom to gain
electrons.
Example
In group 17, Fluorine, the first member is most non-metallic
but Iodine the last element is least non-metallic.
Trend Of Non Metallic Nature In Periods
Non-Metallic character of elements increases on
moving left to right in a period, i.e. electropositive character
decreases and electronegative character increases. This is
because, due to decrease in the size of the atom its ability to
gain electrons increases therefore the non-metallic character
continuously increases.
Example
In second period lithium is metallic whereas the
electronegative nature increases on moving towards right
showing that fluorine is non metallic.
8.
Acidic And Basic Nature Of Oxides
The acidic nature of an oxide is related to its non metallic
nature whereas as the basic nature of an oxide is related
to its metallic nature. It means that metals generally form
basic oxides and non metals generally form acidic oxides.
Trend Of Acidic/Basic Nature Of Oxides In Groups
On moving top to bottom in a group the acidic nature
of the oxides decreases whereas the basic nature of
the oxides increases. It is due to the fact that on moving
down in a group the metallic character increases hence basic
nature of oxides increases whereas the non metallic character
decreases hence the oxides become less acidic.
Example
In 1st group Francium the last element makes the most basic
oxide whereas the in 17th group Fluorine the first element
forms the most acidic oxide whereas the last element Astatine
forms the least acidic oxide.
Trend Of Acidic/Basic Nature Of Oxides In Periods
On moving left to right in a period the acidic nature of
the oxides increases whereas the basic nature of the
oxides decreases. It is due to the fact that on moving left to
right in a period the metallic nature of the elements decreases
hence the basic nature of oxides decreases whereas non
metallic nature increases hence the acidic nature of the oxides
increases.
Example
For example on moving from left to right in 2nd period, Lithium
forms basic oxide whereas Fluorine forms acidic oxide.
Advantages Of Long Form Of Periodic Table
(i)
The periodic table has made the study of chemistry
systematic and easy.
(ii) It is easier to remember the properties of an element if
it’s position in the periodic table is known.
(iii) The type of compounds formed by an element can be
easily predicted by knowing its position in the periodic
table.
(iv) Many elements have been discovered with the help of
periodic table.
(v) The atomic masses of many elements have been
corrected on the basis of their position in the periodic
table.
(vi) This classification is based on the atomic numbers which
is the more fundamental property of elements.
(vii) The Lanthanides and actinides which have properties
different from other groups are placed separately at the
bottom of the periodic table.
(viii) The position of placing isotopes is fully justified.
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