Download Chapter 4: Elements and the Periodic Table Development of atomic

Chapter 4: Elements and the Periodic Table
Development of atomic theory
Name
Date
Theory
Experiment
Democritus
Aristotle
John Dalton
400 BC
350 BC
1803 AD
atomos – smallest particle
hyle – continuous matter
billiard ball – indivisible,
smallest particle
JJ Thomson
1897
Ernest Rutherford
1911
Niels Bohr
1913
Erwin Schrödinger
1926
James Chadwick
1932
plum pudding – electrons
in positive pudding
mass
/ charge) ratio e–
(
nuclear model – small,
dense nucleus with esurrounding empty
space
energy levels (planetary
model of the atom)
wrote an equation to find
electron probability
clouds
discovered the neutron
none, philosophical argument
none, philosophical argument
laws of: conservation of matter
definite proportion
multiple proportion
Cathode ray tube / e- beam
(a Crookes tube)
α – particle / gold foil
bright line spectra of the elements
used a mathematical equation
bombarded Be foil with
α – particles
Dalton’s Atomic Theory
1. All elements are composed of extremely small particles called atoms that cannot be divided
2. Atoms of the same element are exactly alike and have the same mass while atoms of different
elements are different and have different masses
3. An atom of one element cannot be changed into an atom of a different element nor can atoms be
created or destroyed in any chemical change, only rearranged
4. Every compound is composed of atoms of different elements combined in a specific ratio
Modern atomic model
Atoms are made of three particles
Particle
protons
neutrons
electrons
Symbol
(p+)
(n0)
(e–)
}
nucleus
Charge
1+
0
1–
Relative Mass (amu)
1
1
1
1 836
Particle charges
Note in the chart above that the charge of protons and electrons is equal size but opposite in sign
Neutrons have no charge
Since atoms have a neutral charge they must contain the same number of p+ and n0
Particle mass comparison
Note in the chart above that the mass of protons and neutrons is about equal (about 1 amu)
Electrons have very little mass (p+ and n0 have about 1836 times more mass than an e–)
Size and scale of atoms
Atoms range in size from about 75 pm (75 x 10–12 m) for hydrogen to about 265 pm for cesium
Nuclei range from about 1.75 fm (1.75 x 10–15 m) for hydrogen to about 15 fm for uranium
This makes a hydrogen atom about 43 000 times larger than its nucleus and a cesium atom is about
35 000 times larger than its nucleus and uranium atoms about 18 500 times their nuclei
As a scale model
If the nucleus of a hydrogen atom was the size of a baseball, the electrons would be one mile away
Isotopes: atoms of the same element (the same atomic number or number of protons in their nucleus)
that have differing numbers of neutrons (or different mass numbers)
Atomic number: the number of protons in the nucleus of an atom
Atomic number defines what element an atom is
Mass number: the number of protons and neutrons in the nucleus of an atom
Mass number defines what isotope an atom is
Hyphen notation: a means of writing a single nuclide in an isotope
Example: carbon–13 has 6 p+ (it is carbon) and 7 n0 (it is one of the nuclides of carbon)
Calculating p+, n0, and e– in a nuclide
Because atoms are neutral, they have the same number of p+ and e–
Subtracting the atomic number (p+) from the mass number (p+ + n0) gives the number of n0
Nuclear symbol: the hyphen notation carbon–13 can be written as a nuclear symbol, C
Example: Find the number of p+, n0, and e– in a nuclide of boron–11
B, therefore 5 p+, 5 e–, and 6 n0 (because 11 – 5 = 6)
Counting atoms in chemical formulas
Chemical compounds have definite composition and so their formulas have small, whole
number ratios of atoms
Water = H2O
Sodium tetraborate = Na2B4O7
Magnesium chlorate = Mg(ClO3)2
Example: count the atoms of each element in magnesium
chlorate, Mg(ClO3)2
Mg – magnesium: 1
Cl – chlorine: 2
Mg(ClO3)2 is shown here →
O – oxygen: 6
so, there are two Cl and six O
The Periodic Table – organizing the elements
1869 – scientists looked for ways to organize the 63 known elements in order to find patterns
Dmitri Mendeleev
Made cards showing known elements and listed chemical and physical properties
Chemical properties – showed the oxygen atom ratio in compounds
Physical properties – listed melting point, density, color, etc.
When the cards were placed in order by atomic mass, Mendeleev noticed that certain
patterns repeated (Li, Na, and K for example) so he started a new column to match
properties
He noticed two important things:
A few element properties did not match exactly in order of atomic mass – he moved these
He noticed three missing elements and predicted their properties
All three of these elements were easily found by other scientists (Sc, Ga, and Ge)
Organization of the modern periodic table
Periods – horizontal rows on the periodic table (sometimes called series)
There are 7 periods on the modern periodic table
Each period starts at the left with a very active metal, then come less active metals, then the
metalloids, then the nonmetals with the noble gases finishing each period
Groups – vertical columns on the periodic table (sometimes called families)
There are 18 groups on the modern
periodic table
Sometimes, the lanthanides and actinides
are placed differently
Short form: the lanthanides and
actinides are placed below the usual
18 groups
Long form: lanthanides are between Ba
and Lu and actinides are between Ra
and Lr
Blocks – remember, blocks help you remember electron configurations
from http://en.wikipedia.org/wiki/Block_%28periodic_table%29
Groups (or families) -- vertical columns on the table are numbered in different ways
Modern group labels (as seen above) are the integers 1 – 18 but many chemists still use the
older American scheme (refer to http://en.wikipedia.org/wiki/Periodic_table_%28large_version%29)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
IA
IIA
IIIB
IVB
VB
VIB VIIB
←
VIIIB
→
IB
IIB
IIIA
IVA
VA
VIA VIIA VIIIA
There are also trivial names for main group elements (s- and p-blocks)
IA
Alkali metals
IIA
Alkaline earth metals
IIIA
Icosagens
IVA
Crystallogens
VA
Pnictogens
VIA
Chalcogens
VIIA
Halogens
VIIIA
Noble gases
Transition metals (d-block elements)
Inner transition metals or rare earth metals (f-block elements)
Metals, metalloids, and nonmetals (see http://en.wikipedia.org/wiki/Periodic_table_%28metals_and_nonmetals%29)
Periods (or series) – horizontal rows are numbered to show the highest energy level
Chemical symbols – one or two letter symbols for each element
The first letter is always capitalized
If there is a second letter in the symbol, it is always small case
Two capital letters in a row denote two separate elements in a compound
Example: Co = cobalt while CO is carbon monoxide
Average atomic mass – most elements have an isotopic mixture
B is about 80% 11B and 20% 10B so that the average is 10.8 amu
Cl is about 75% 35Cl and 25% 37Cl for an average of about 35.5 amu
Typical information on modern Periodic Tables
Atomic number
Chemical symbol
Element name
Atomic mass
Formation of elements in stars
In the extreme temperatures found in stars, matter exists in an ionized state called a plasma
There are actually four states of matter: solid, liquid, gas, and plasma
Big bang: the only elements that formed during the big bang were H, He, and a very small
trace of Li
How a sun the size of ours generates He by nuclear fusion
from: MSFC Solar Physics (http://solarscience.msfc.nasa.gov/interior.shtml)
Nuclear fusion in stars the size of our sun can create He, C, N, and O
More massive stars can create elements as heavy as Mg and Si
The most massive stars have cores with elements all the way up to Fe
To produce elements more massive than Fe requires a supernova with enough energy to
reach billions of degrees K – enough energy to make all the elements heavier than Fe in the
very massive star’s final hours of life
The elements: metals, metalloids, and nonmetals
from: Wikipedia (https://en.wikipedia.org/wiki/Properties_of_metals,_metalloids_and_nonmetals)
The properties of metals
Physical properties of metals
Malleable – can be hammered or rolled into thin sheets or other shapes
Ductile – can be pulled or drawn into long wires
Conductivity – most metals are good conductors of heat and electricity
Luster – most metals are very shiny or have high metallic luster
Magnetic – many metals (but not all) are attracted to magnets
Chemical properties of metals
Reactivity – metals react by losing electrons to form positive ions
Some metals are useful because they are extremely reactive (Li in batteries)
Some metals are useful because they exhibit very low reactivity (Au and Pt)
Corrosion – destruction of a metal because of its reactivity with oxygen from the air
Metals in the Periodic Table
Alkali metals – metals found in group 1 (or IA, s block)
Highly reactive losing one electron
Have low density and react violently with water
Sodium and potassium ions are needed by the body and are a requirement for life
Alkaline earth metals – metals found in group 2 (or IIA, s block)
Also very reactive losing two electrons
Like group 1 metals, group 2 metals are never found as uncombined elements in nature
Magnesium (when mixed with a small amount of aluminum) makes lightweight wheels
and ladders
Calcium ions are needed by the body for bone and muscle growth
Ca and Mg are found in dairy products and in leafy green vegetables
Transisition metals – metals found in groups 3 through 12 (or IIIB through IIB, d block)
These metals are usually much less reactive and often useful as conductors or for strong
building materials
Iron is needed to make hemoglobin in the blood, a requirement for carrying oxygen
Inner Transisition metals – metals found in the two rows at the bottom of the short form of
the Periodic Table and do not have group numbers
Lanthanides – the top row, these metals tend to be soft, malleable, shiny, and have high
conductivity
Lanthanides are often found mixed in nature and are hard to separate because they have
very similar properties
They are often used to make alloys
Nd and Sm are used to make very powerful magnets used for modern speakers
Actinides – the bottom row of which only Ac, Th, Pa, and U occur naturally on earth
Most of the actinides are synthetic elements formed in particle accelerators
Mixed group metals – metals found in the bottom left corner of the p block
The most familiar of these metals are Al, Sn, and Pb
Pb was used in the past to make lead pipes for water, but since it was discovered that lead
was very toxic it is no longer used for this purpose
The symbol for lead comes from its old name, plumbum, which also explains why
people who used to work with lead pipes for water lines are called plumbers
Sn is used to line iron cans to prevent the iron from rusting
Al is used for many purposes because it is very lightweight and very common in the
Earth’s crust
Nonmetals and metalloids
Physical properties of nonmetals
Brittle – if hammered, most nonmetals will shatter and they do not bend well
Cleave – if the right pressure is applied, many nonmetals will break along a flat plane
Conductivity – most nonmetals are poor conductors of heat and electricity
Luster – most nonmetals are dull but may have a sheen or luster
Of the 16 nonmetals 10 are gases, 5 are solids (I, Se, S, P, and C), and 1 is a liquid (Br)
Chemical properties of metals
Reactivity – nonmetals react by gaining electrons to form positive ions or sharing electrons
to form molecules
Nonmetals in the halogen family react with metals to form salts (NaCl)
Oxygen reacts with metals to form rust or calx (Fe rusts and Mg forms a white calx)
Families of nonmetals in the Periodic Table, the p block
Carbon family – the crystallogens found in group 14 (or IVA)
Have four electrons to gain, lose or share
Carbon is the only nonmetal in this family
Especially important because it can make chains and so it plays a big role in the
chemistry of living things
Nitrogen family – the pnictogens found in group 15 (or VA)
Nitrogen forms a diatomic molecule (N2)
Most living things need nitrogen but are unable to get it from the air
Some plants form a symbiotic relationship with bacteria that can fix N2
Nitrogen is a major component of most fertilizers
Phosphorus is the only other nonmetal in group 15
Phosphorus compounds are used to make matches
Phosphorus is needed for the backbone of DNA but this is the element that is the
limiting reactant for life
Oxygen family – the chalcogens found in group 16 (or VIA)
Oxygen also forms a diatomic molecule (O2)
Most living things need oxygen
We breathe oxygen from the air
Ozone (O3) in the upper atmosphere protects us from UV radiation
Sulfur is a common element
Often forms very smelly compounds like H2S, the rotten egg smell
Is used to make rubber for tires (vulcanization process)
Sulfur is used to make sulfuric acid (H2SO4), an important chemical in industry
Halogen family – group 17 (or VIIA), F2, Cl2, Br2, I2, and At2
These nonmetals form salts (halogen means ‘salt forming’)
All the halogens are very reactive
Fluorine is the most reactive element and will burn in water, ground glass, and metals
Fluorides are added to water and toothpaste to strengthen teeth
Chlorine is poisonous but is used to kill bacteria (pools and water) and to bleach clothes
Chlorides (like calcium chloride) are used to melt ice on walkways and roads
Silver bromide is used in photography
Noble gases – group 18 (or VIIIA)
Noble gases are found in the air but are very unreactive
He was first discovered in the sun (by spectroscopy)
Ne is used (as well as other noble gases) for neon signs
Ar is used in light bulbs
Hydrogen – usually listed above group 1 (or IA) which are metals, but H is a nonmetalγ
Hydrogen is unique in that it is the only reactive element with only one electron shell
The first energy level can only hold two electrons, so H can either gain or lose electrons
Hydrogen atoms make up about 90% of the atoms in the universe
Only 1% of the mass of Earth’s crust, the oceans, and atmosphere are H
Hydrogen is rarely found in the elemental state on Earth, most if found in water (H2O)
Metalloids
Metalloids form a stair-step border between metals and nonmetals on the Periodic Table
The properties of metalloids are between those of metals and notmetals
They are brittle, hard, and somewhat reactive
All the metalloids form solids at room temperature
The most common metalloid is silicon (Si), most commonly found in SiO2 in sand
Semiconductors – materials that conduct electricity under some conditions but not under
other conditions
The most useful property of the metalloids is their varying ability to conduct electricity
The ability of many metalloids to conduct electricity can depend on temperature,
exposure to light, or the presence of impurities
Computer chips, lasers, transistors, and solid state light emitting diodes (LED lights)
Radioactive elements
Henri Becquerel discovered radioactivity in 1896 while investigating a mineral that
contained uranium (92U)
Becquerel assumed that sunlight was the source of energy that could expose photographic
film or plates
One cloudy day, he placed the mineral next to a plate wrapped in protective paper but
later discovered that even in the dark drawer the mineral gave off energy that exposed
the plates
This led to the question, what was the energy source that produced the penetrating
radiation?
Becquerel presented his findings to the Curies (Marie and Pierre)
After study, Marie and Pierre concluded that the energy source was a reaction taking
place in the nuclei of the uranium atoms
Radioactivity is the name Marie gave to this spontaneous emission of energy by
unstable atomic nuclei
Marie found that some minerals containing uranium were more radioactive than pure
uranium causing her to conclude that these minerals contained small amounts of other
radioactive elements
The Curies eventually isolated two new elements, polonium and radium
Types of natural radioactive decay
Nuclear
Type of Radiation
Symbol
Symbol
Penetrating Power
Alpha decay
α particles
He Very low – blocked by a sheet of paper
Beta decay
β particles
e
Medium – blocked by a thin Al sheet
Gamma radiation
γ radiation
γ
High – thick Pb or concrete required
A nuclear equation can represent nuclear reactions
Ra
He
Th →
Using radioactive isotopes
Tracers – radioisotopes used to track chemical reactions or industrial processes
Phosphorus–32 is used to trace where and how plants use phosphorus for growth
Radioactive gases can help detect leaks in pipes
Gamma radiation can be used to take photographs of metals to detect weak spots
Diagnosis – radioisotopes used to track chemical reactions or industrial processes
Technetium–99 is used to diagnose problems in bones, liver, kidneys, and the digestive
system
Iodine–131 is used to detect problems with thyroid function
Treatment – radioisotopes used to track chemical reactions or industrial processes
High-energy gamma rays can be used like a surgical knife to kill cancer cells