CHEM 100. CHPTER 2. KEY CONCEPTS
Radioactivity
Subatomic Particles
Electrons
Electronic Charge
Nuclear atom
Protons
Neutrons
Atomic number (Z)
Size of Atoms
SI Units
Unit Conversions
Mass Numbers
Isotopes
Isotopic symbols
Atomic Mass Units
Mass Spectrometer isotope masses and % composition?
Average atomic weights
Periodic Table
Abundance of Elements
Earth's Atmosphere
Atomic Structure and Subatomic Particles
1. Radioactivity
Henry Becquerel had already noted that uranium emanations of ionizing radiation
could turn air into a conductor of electricity. Using sensitive instruments, radiation
counters, invented by Pierre Curie and his brother, Pierre and Marie Curie measured
the ability of emanations from various elements to induce conductivity. On February
17, 1898, the Curies tested an ore of uranium, pitchblende, for its ability to turn air
into a conductor of electricity. The Curies found that the pitchblende produced a
current 300 times stronger than that produced by pure uranium. They tested and
recalibrated their instruments, and yet they still found the same puzzling results. The
Curies reasoned that a very active unknown substance in addition to the uranium must
exist within the pitchblende. In the title of a paper describing this hypothesized
element (which they named polonium after Marie's native Poland), they introduced
the new term: "radioactive." Radioactivity is a term that use to explain the instability
of certain nucleus of atoms and their disintegration into stable nucleus while giving of
a-(He nuclei), b-(fast moving electrons), and g-(high energy electro magnetic
radiation) radiation
2. Discovery of Electrons
3. Thomson's cathode ray tube experiments
Electrons: Electrons are sub-atomic particles with a mass of 9.11 x 10-28g ( 1/1833
times mass of a proton) and a negative charge of 1.60 x 10-19 c (c=coulombs) or -1.60
x 10-19 c. Electron was first discovered by J.J. Thompson using cathode-ray tubes or
Crook's tubes. According to modern atomic theory an electron travels around a
nucleus made up of protons and neutrons. An electrical current is a stream of
electrons passing through a metal or a conductor.J.J. Thomson used results from
cathode ray tube (commonly abbreviated CRT) experiments to discover the electron.
Thomson and a cathode ray tube from around 1897, the year he announced the
discovery of the electron. Only the end of the CRT can be seen to the right-hand side
of the picture.
The two plates about midway in the CRT were connected to a powerful electric
battery thereby creating a strong electrical field through which the cathode rays
passed. Thomson also could use magnets, which were placed on either side of the
straight portion of the tube just to the right of the electrical plates. This allowed him
to use either electrical or magnetic or a combination of both to cause the cathode ray
to bend. The amount the cathode ray bent from the straight line using either the
electric field or the magnetic field allowed Thomson to calculate the e/m ratio. e/m
ratio stands for charge-to-mass ratio of the electron.
4. Measuring Electronic Charge: Millikan's oil drop experiments
American physicist Robert Andrews Millikan (1868-1953) designed an experiment to
measure the electronic charge. Drops of oil were carried past a uniform electric field
between charged plates. After charging the drop with x-rays, he adjusted the electric
field between the plates so that the oil drop was exactly balanced against the force
of gravity. Then the charge on the drop would be known. Millikan did this repeatedly
and found that all the charges he measured came in integer multiples only of a
certain smallest value, which is the charge on the electron, a negative charge of 1.60
x 10-19 c (c=coulombs) or -1.60 x 10-19 c.
5. Nuclear atom
6. Rutherford’ s experiment
A New Zealander, Rutherford fired Alpha particles at an extremely thin gold foil. He
expected them to go straight through with perhaps a minor deflection.
Most did go straight through, but to his surprise some particles bounced directly off
the gold sheet! This means there something in the atom that deflected the alpha
particles
Rutherford hypothesized that the positive alpha particles had hit a conccentated mass
of positive particles, which he termed the nucleus.
Nucleus: The mass and the positive (+) charge of an atom are concentrated in the
center. This center is called nucleus, and it has radius of about 10-13 cm. Nucleus
contains protons and neutrons which are equal in mass. Number of protons in a
nucleus is called the atomic number (Z)
Proton
Proton is a sub-atomic and sub-nuclear particle. A proton has a mass of 1.67 x 10-24 g
(which is 1833 times heavier than an electrons) and carries a positive charge of 1.60 x
10-19 c which is equal to the negative charge found on an electron . Protons gives a
positive charge to the nucleus of an atom. In a neutral atom, number of electrons and
protons are equal . Number of protons in a nucleus is called atomic number (Z)
Henry Moseley's X-ray experiment: The metal in the anode of the cathode-ray tube
gives off x-ray when the when the cathode rays hit the anode. Moseley measured the
frequency of X-rays given off and found that about half-the-mass of the atoms making
the anode is directly proportional to the square-root of the frequency. Moseley
showed that the energies were given in good approximation by:
EK = 3/4 (Z - b)2 EI,
in which Z is the atomic number of the element, b is an empirical screening constant
roughly equal to , and EI is the ionization energy of the hydrogen atom, 13.6 eV. This
number (Z) he called atomic number later found to be the number of protons in the
nucleus.
Neutrons
Chadwick's bombardment of 9Be with -particles: There must be uncharged particles
in the nucleus to account for the missing mass of atoms after considering number of
electrons and protons. Chadwick observed a particle in the nucleus of the same mass
as a proton( 9Be + 4He --> 12C + 1n), but without a charge in a nuclear reaction of 9Be
with "-particles. His experiment led to the discovery of neutron. Neutrons are subatomic and sub-nuclear particles. A neutron has a mass of 1.67 x 10-24 g which is equal
to that of a proton, but it is neutral. Neutrons along with protons contribute to the
mass or bulk of the matter or atoms.
General structure of the atom
Atom is the fundamental unit of matter. An atom has a nucleus and electrons. The
electrons are presumed to be rotating around the nucleus in orbits similar to planets
in the solar system. Nucleus contains protons and neutrons, which have equal mass.
Volume or the radius of an atom is, determined by the outermost orbit of electrons,
in the range of 10-8 cm. Number of protons in the nucleus is called atomic number (Z),
which characterizes the type of element. It 's possible for atoms of an element to
have different masses (i.e. different # of neutrons). They are called isotopes.
The Size of Atoms and Units Used to represent them
Mass and charges of electron, proton and neutron
Atoms are made of small particles called protons, neutrons, and electrons. Each of
these particles is described in terms of measurable properties, including mass and
charge. Mass is the amount of matter that an object contains. The proton and neutron
have roughly the same mass and have approximately one thousand times the mass of
the electron. The proton and electron have equal, but opposite, electrical charges. A
neutron does not have an electrical charge.
Rest mass of Proton is 1.672 x 10-27 kg
Rest mass of neutron is 1.675 x 10-27 kg
Rest mass of electron is 9.109 x 10-31 kg
Charge on an electron is - 1.60 x 10-19 C
It is very hard for anyone to really understand how small something like the atom or
its nucleus really is. The only good way to visualize it is to make a comparison to the
relative sizes of things we see in our daily life.
If an atom were the size of the period at the end of a sentence or a pixel on your
screen, a person would be 1000 miles tall! That is how small the atom is. Nucleus has
a radius of about 10-13 cm. An tom has a radius of about 10-8 cm or angstroms.
If an atom were the size of a football stadium, with the electrons out around the
upper deck, the nucleus down at midfield would be smaller than the coin flipped at
the start of the game. An atom is roughly about 0.2 nm across (nm means nano meter,
where nano is the metric prefix meaning 10-9) although the electrons spend most of
their time in a region about 0.1 nm = 0.0000000001 meters in diameter.
Units and Conversion Factors
SI Units
SI - System International
– Systematic subset of the metric system. Only uses certain metric
•
units.
Mass
kilograms
Length
meters
Time
seconds
Temperature
kelvin
Amount
mole
Other SI units are derived from SI base units.
Conversion factors for common mass, area, volume and length units
Length
12 inches = 1 foot
3 feet
= 1 yard
220 yards = 1 furlong
8 furlongs = 1 mile
1760 yards = 1 mile
220 yards = 1 furlong
8 furlongs = 1 mile
36 sections = 1 township
Area
144 sq. inches = 1 square foot
9 sq. feet = 1 square yard
4840 sq. yards = 1 acre
640 acres
= 1 square mile
5280 feet
= 1 mile
1 sq.mile = 1 section
1760 yards = 1 mile
1 sq.mile = 1 section
36 sections = 1 township
Volume
1728 cu. inches = 1 cubic foot
27 cu. feet = 1 cubic yard
Mass
Troy Weights
437.5 grains = 1ounce
16 ounces = 1 pound (7000 grains)
24 grains
= 1 pennyweight
14 pounds = 1 stone
20 pennyweights = 1 ounce (480 grains)
100 pounds = 1 hundredweight [cwt]
12 ounces
= 1 pound (5760 grains)
20 cwt
= 1 ton (2000 pounds)
Capacity (Dry)
Capacity (Liquid)
16 fluid ounces = 1 pint
2 pints = 1 quart
4 gills
= 1 pint
8 quarts = 1 peck
2 pints
= 1 quart
4 pecks = 1 bushel
4 quarts
= 1 gallon (8 pints)
Apothecaries' Measures
Apothecaries' Weights
60 minims = 1 fl.dram
20 grains = 1 scruple
8 fl.drams = 1 fl.ounce
3 scruples = 1 dram
16 fl.ounces = 1 pint
8 drams = 1 ounce (480 grains)
12 ounces = 1 pound (5760 grains)
Atomic Numbers and Mass Numbers
Atomic number (Z): Number of protons in a nucleus of an atom is called atomic
number (Z), Z is characteristic to an element. An atom of oxygen always have eight
protons and atomic number equal to eight. However, It is possible for an element to
have atoms with different masses by having different number of neutrons in the
nucleus. Atoms of an element having different number of neutrons in the nucleus are
called isotopes.
Mass Numbers (A): Number of neutrons and protons together in a nucleus of an atom
is called mass number (A), An element could have different atoms with different
A values. An Therefore It is possible for an element to have atoms with different
masses by having different number of neutrons in the nucleus. Atoms of an element
having different number of neutrons in the nucleus are called isotopes. Number of
neutrons and protons together in a nucleus of an atom is called mass number (A), An
element could have different atoms with different A values. An Therefore It is
possible for an element to have atoms with different masses by having different
number of neutrons in the nucleus. Atoms of an element having different number of
neutrons in the nucleus are called isotopes.
7. Isotopes and Isotopic symbols & Mass
Isotope: Atoms of the same element with different masses or different number of
neutrons in the nucleus. E.g. 1H (hydrogen with one proton and one electron)and 2H
(deuterium with one proton, one neutron and one electron).
Isotopic symbol: Element symbol (X) indicating number of protons or atomic number
(Z) written as left subscript, and mass number (A), total of number of protons and
neutrons written as left superscript.
A
12
X
E.g carbon-12:
C
Z
6
or simply written as 12C because once atomic symbol is known atomic number is
known from the periodic table.
The scale on the detector plate of mass spectrometer is calibrated using 12C isotope
and gives mass of each isotope.
Atomic mass unit (amu) or amu scale is defined as the one-twelfth (1/12) mass
of 12C isotope.
1 amu = (1/12) mass of 12C isotope
The position of the peaks in the mss spectrum thus directly gives its mass in amu, and
their relative compositions (abundance) can be directly obtained from the intensity of
each peak.
Mass spectrometer provides mass of isotopes of elements and their natural abundance
or relative composition of an element.
For example, hydrogen is a mixture of 1H (hydrogen)and 2H (deuterium) in the ratio
99.985% and 0.015%, respectively. Hydrogen also has a third artificial isotope or
radioactive isotope, Tritium-3H, which does not normally appear in the mass spectrum
since it has already decayed or converted to other two isotpoes in naturally occurring
hydrogen.
Explain how the isotope masses (in amu) and % compositions are measured
using Mass spectrometer.
Mass spectrometer is the instrument used to measure the masses of different isotopes
of an elements. It is essentially an atomic balance. Atoms of an element are made up
of different isotopes ( i.e. atoms having different masses because of different
numbers of neutrons in their nuclei When a sample of an element is injected into a
mass spectrometer, it is vaporized into atoms and then to positive ions by knocking
off electrons. These ions are made to accelerate through negatively charged plates
(Mass spectrometer, page 54). Thus a fast moving ion beam is obtained by having
these ions passes through small slits in these plates. This beam is then made to travel
through a strong magnetic field which deflects them onto a detector plate. Each line
on the detector plate correspond to an isotope and the intensity of the line is
proportional to relative composition or the natural abundance of the particular
isotope.
Why atomic masses are important to chemical calculations or stoichiometry?
According to Law of Conservation of Mass atomic masses are conserved in chemical
reactions. Chemical reactions are simply reorganizations of atom to form new
compounds. Atomic mass is the ideal parameter to monitor the balance between
materials consumed and produced in chemical reactions.
Stoichiometry is the word used to describe chemical calculations that are carried out
to obtain the weights of reactants and products of a reaction. It is important since it
enables us to calculate exact amounts to be used to avoid pollution and to conserve
our resources.
A special instrument called mass spectrometer is used to get accurate atomic masses (
ie. mass of each isotope and their abundance in an naturally occurring element).
Initially the development of the periodic table by Mendeleev was based on atomic
weights. However, later atomic weights were replaced by the atomic number since
the atomic number seems to correlate more closely with chemical properties
elements than atomic weights.
Why the atomic weights reported in the periodic table of elements are not whole
numbers? How these weights are calculated from isotope masses and %
composition?
Average atomic weights: Average atomic weight is calculated based on the masses
of isotopes and their relative composition.
Most of the elements have two or more isotopes. The atomic weight reported on
the periodic table are average weights based on the masses of isotopes and their
relative composition. The equation for calculating average atomic mass (AAM) for an
element with two isotopes is:
Ma x a + M b x b
----------------= AAM
100
Ma
= mass of isotope a
Mb
= mass of isotope b
a
= relative percent composition of a
b
= relative percent composition of b
AAM = Average atomic mass (Reported on the Periodic Table)
This equation applies to an element with two isotopes. Extra isotopes, Mc and c1 , Md
and d , etc, could be added to the numerator of equation for elements having more
than 2 isotopes. Isotopic masses and relative composition are obtained from a mass
spectrometer.
Click here to do tutorials on atomic weights and isotopic abundance. Use back button
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Gallium in nature consists of two isotopes, gallium-69, with a mass of 69.926 u and
a fractional abundance of 0.601; and gallium-71, with a mass of 70.925 u and a
fractional abundance of 0.399. Calculate the weighted average atomic mass of
gallium.
In this problem isotopic masses of 69Ga and 71Ga isotopes and their fractional
abundance are given. The average atomic mass or weighted average atomic mass of
Ga can be calculated using the equation after converting fractional abundance to
percent abundance or composition .
A.A.M =
Ma x a + Mb x b
----------100
Ma (69Ga ) =68.926 u,
a = percent abundance of 69Ga = 0.601 x 100
Mb (71Ga ) = 70.925 u,
b = percent abundance of 71Ga = 0.339 x 100
We can obtain an equation with one unknown, AAM.
AAM) = 68.926x(0.601 x 100)+70.925 x(0.339x100)
100
AAM (Ga) = 4142.5 + 2829.9
100
AAM (Ga) = 6972.3 = 69.723
100
AAM (Ga) = 69.723 u (amu)
Weighted average atomic mass of gallium = 69.723
Amount of Substance-The mole
Why we need to use the concept of "mole" of atoms ( 6.022 x 1023 particles) in
chemical stoichiometry?
Chemical equations are written in terms of atoms and molecules. However, we
cannot pick atoms individually and do reactions. Chemists always use mass in grams as
the amount in the reaction. Therefore, we need a conversion factor to convert atoms
and molecules to grams. Mole is the connection or the conversion factor between
atoms and grams.
Avogadro's Number
The name "Avogadro's Number" is just an honorary name attached to the
calculated value of the number of atoms, molecules, etc. in a gram mole of any
chemical substance. Of course if we used some other mass unit for the mole such as
"pound mole", the "number" would be different than 6.022 x 1023.
Atoms and molecules are weighed in the mass spectrometer in amu (atomic
mass units) not in grams. However, if you take atomic mass in grams the number of
atoms is simply 6.022 x 1023 atoms or particles. In other words gram atomic weight or
gram molecular weight contains 6.022 x 1023 atoms, molecules or particles. This
number is called Avogadro's number or mole of particles. (Simply the mole). Mole is a
convenient number to convert grams into number of atoms or vice versa. Mole is also
called the Chemist's dozen since it bring atomic particle to a size that could handle
easily similar to dozen of eggs. 1 mol = M.W. (molecular weight) taken in grams 1 mol
= 6.022 x 1023 particles 1 mol = 6.022 x 1023 atoms 1 mol = 6.022 x 1023 molecules 1
mol = 6.022 x 1023 ions
Define the most important conversion factors used in chemical stiochiometry.
6.022 x 1023 atoms = gram atomic weight
6.022 x 1023 molecules = gram molecular weight
6.022 x 1023 atoms C = 12.01 grams of carbon (C)
6.022 x 1023 molecules H2O = 18.02 g of H2O (water)
6.022 x 1023 = 1 mol 1 g = 6.022 x 1023 amu
1 amu = 1 g/mol
An atom weighs 7.47 x 10-23 g. What is the name of the element this atom belongs
to?
First convert g to amu and look up in the periodic table and find out the element.
Conversion factor: 1g = 6.022 x 1023 amu
7.47 x 10-23 g x
6.022 x 1023 amu
=
44.98
1g
amu
In the periodic table atomic masses increase generally with atomic number. Element
with an atomic mass closer to the value calculated is Sc (Scandium).
The element is Sc.
How many moles of iron (Fe) are present in 180.1g of elemental iron?
a.w. Fe = 55.85 g/mol This is a problem to convert grams to moles. Conversion factor
is 55.85 g Fe = 1 mol
180.1 g Fe
x
1 mol
=
3.225
mol
Fe
55.85 g Fe
What is the mass of 7.5 x 105 atoms of Cu in grams?
This problem requires conversion of atoms to grams. If you remember correctly this is
the definition of the mole. 1 mol = 63.55g Cu = gram atomic weight 1 mol = 6.022 x
1023 atoms 6.022 x 1023 Cu atoms = 63.55 grams Cu Therefore,
7.5 x 105 atom Cu
atoms63.55g Cu
6.022 x 1023 Cu
=7.9 x 10-17g Cu
Describe the families of elements found in the periodic table.
Periodic table is an arrangement of all known element according to their aomic
number and chemical properties. This table contains vertical columns called groups
and horizontal columns called periods. Elements in a group have similar chemical
properties. These groups are number from 1 - 8, left to right and some of groups have
thier own names.
group 1
group 2
group 7
group 8
-
alkali metal: Li, Na, K Rb, Cs, Fr
alkaline earth metals: Be, Mg, Ca, Sr, Ba, Ra
Halogens: Cl, Br, I, At
Noble gases: He, Ne, Ar, Kr, Xe, Rn
In
addition to groups in the periodic table there are three blocks of elements called
transition elements, Lanthanides and Actinides
Most of the elements in the periodic table are metals. They are found to the left of
the table. The non - metals are found to the top right side of the periodic table.
Metals loose electrons to form cations, while non - metals gain electrons to form
anions. The bonding between metals and non-metals are usually ionic. Covalent bonds
are found in compounds formed by non - metal reacting with non - metals.
The 10 Most Abundant Elements in the Earth's Crust
Source: CRC Handbook of Chemistry and Physics, 77th Edition
Element
Oxygen
Abundance
percent by weight
46.1%
Abundance
parts per million by weight
461,000
Silicon
28.2%
282,000
Aluminum
8.23%
82,300
Iron
5.63%
56,300
Calcium
4.15%
41,500
Sodium
2.36%
23,600
Magnesium
2.33%
23,300
Potassium
2.09%
20,900
Titanium
0.565%
5,650
Hydrogen
0.14%
Composition of the Earth's Atmosphere
1,400
Source: Definition of the U.S. Standard Atmosphere (1976)
CRC Handbook of Chemistry and Physics, 77th Edition
Gas
Formula
N2
Abundance
percent by volume
78.084%
Abundance
parts per million by volume
780,840
Nitrogen
Oxygen
O2
20.9476%
209,476
Argon
Ar
0.934%
9,340
Carbon Dioxide
CO2
0.0314%
314
Neon
Ne
0.001818%
18.18
Helium
He
0.000524%
5.24
Methane
CH4
0.0002%
2
Krypton
Kr
0.000114%
1.14
Hydrogen
H2
0.00005%
0.5
Xenon
Xe
0.0000087%
0.087
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