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Atoms and Atomic Theory
Chapter 2
2.1 Early Chemical Discoveries and
the Atomic Theory
Law of Conservation of Mass
The total mass of
substances present
after a chemical
reaction is the
same as the total
mass of substances
before the reaction.
In short, Mass of Reactants = Mass of Products.
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Law of Constant Composition
Joseph Proust (1754–1826)
Also known as the law of definite proportions.
• All samples of compounds have the same
composition- the same proportions by mass of
the constituent elements.
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2-6
Dalton’s Atomic Theory
1. Each element is composed of small particles
called atoms. Atoms are neither created nor
destroyed in chemical reactions.
2. All atoms of a given element are identical and
differ from all other elements
3. Compounds are formed when atoms of more
than one element combine in simple numerical
ratios.
When two elements form more than one
compound (if the mass of one element is
kept constant) the ratios of the masses
of the other element are in small whole
numbers
8
Law of Multiple Proportions
John Dalton 1803
If two elements form more than a single compound, the
masses of one element combined with a fixed mass of the
second are in the ratio of small whole numbers.
•
In forming carbon
monoxide, 1.0 g of carbon
combines with 1.33 g of
oxygen.
•
In forming carbon dioxide,
1.0 g of carbon combines
with 2.66 g of oxygen.
Multiple Proportions Example
• Consider water (H2O) and hydrogen
peroxide (H2O2)
• The mass ratio for water is 2:16
or simplified 1:8
• The mass ratio for hydrogen peroxide is
2:32 or 1:16
• The mass ratio of oxygen in both
compounds is 2:1
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2.2 Electrons and Other
Discoveries in Atomic Physics
Review
• +/- Attract
• -/- and +/+ Repel
• All matter is made of charged particles
• + > - Positive
• + < - Negative
• + = - Neutral
Cathode ray tube
The Electron
• Streams of negatively charged particles were
found to come from cathode tubes.
• J. J. Thomson is credited with their discovery
(1897).
• Thomson measured the charge/mass ratio of the
electron to be 1.76  108 coulombs/g.
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The Atom, circa 1900:
• “Plum pudding” model,
put forward by Thompson.
• Positive sphere of matter
with negative electrons
imbedded in it.
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Millikan Oil Drop Experiment
Robert Millikan showed
ionized oil drops can be
balanced against the pull
of gravity by an electric
field.
Once the charge/mass ratio
of the electron was
known, determination of
either the charge or the
mass of an electron would
yield the other.
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Millikan Oil Drop Experiment
Robert Millikan determined
the charge on the
electron in 1909.
He determined the charge
of an electron to be
1.60 X 10-19C
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Millikin’s experiment
• He then used
Thomsons charge-tomass ratio to
calculate the mass of
an electron
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1.60 x 10 C
electron mass 
8
1.76 x 10 C/g
• 9.10 x 10-28 g
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Radioactivity:
• The spontaneous emission of radiation by an
atom.
• First observed in 1896 by Henri Becquerel
while he was studying the properties of a
uranium compound.
• Marie and Pierre Curie further experimented
and discovered radium and poloniium
• All three shared the Nobel prize in 1903.
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Radioactivity
• Two types of radiation were discovered by Ernest
Rutherford:
4
2
–  particles He
0
–  particles
-1 e
–  rays (Paul Villard)
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2.3 The Nuclear Atom
Discovery of the Nucleus
Ernest Rutherford shot
 particles at a thin
sheet of gold foil and
observed the pattern of
scatter of the particles.
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The Nuclear Atom
Since some particles
were deflected at large
angles, Thompson’s
model could not be
correct.
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The Nuclear Atom
• Rutherford postulated a very small,
positive, dense nucleus with the electrons
around the outside of the atom.
• Most of the volume of the atom is empty
space.
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Rutherford
protons 1919
James Chadwick
neutrons 1932
The nuclear atom – illustrated by the helium atom
Subatomic Particles
• Protons and electrons are the only particles that
have a charge.
• Protons and neutrons have essentially the same
mass.
• The mass of an electron is so small we ignore it.
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2.4 Chemical Elements
• Atomic Number: Number of protons (Z)
– Also equal to number of electrons
• Mass Number: Total protons and neutrons (A)
• Number of Neutrons = A-Z
• Atomic Mass Unit (u): 1/12 the mass of a
carbon-12 atom
– Mass of a proton/neutron is ~1u
– Mass of an electron is ~ 1/2000u
2-4 Chemical Elements
To represent a particular atom we use symbolism:
A= mass number
Z = atomic number
E= element
Symbols of Elements
Elements are symbolized by one or two letters.
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Atomic Number
All atoms of the same element have the same
number of protons:
The atomic number (Z)
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Atomic Mass
The mass of an atom in atomic mass units (amu) is
the total number of protons and neutrons in the
atom.
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Isotopic Notation
20
10
Ne
21
10
Ne
22
10
Ne
Isotopes: Same atomic number but different
mass number
Percent Natural Abundance: On Earth,
90.51% Ne-20
0.27% Ne-21
9.22% Ne-22
How many protons, neutrons, and electrons
are in
(a)an atom of 197Au
(b) an atom of strontium-90?
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Ions
• Atoms that have gained or lost electrons and
carry a charge
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2.5 Atomic Mass
Why is carbon’s mass in the PT not 12.000?
– Must take into account all naturally occurring
isotopes.
– Find average atomic mass
(
Atomic Mass =
Fractional
Abundance
of Isotope 1
X
Mass of
Isotope 1
)+ (
Fractional
Abundance
of Isotope 2
X
Mass of
Isotope 2
)+
Naturally occurring chlorine is 75.78% 35Cl, which has an
atomic mass of 34.969 amu, and 24.22% 37Cl, which has an
atomic mass of 36.966 amu. Calculate the average atomic mass
(that is, the atomic weight) of chlorine.
Average atomic mass = (0.7578)(34.969 amu) + (0.2422)(36.966 amu)
= 26.50 amu + 8.953 amu
= 35.45 amu
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2.6 Introduction to the Periodic
Table
Alkali Metals
The Periodic table
Noble Gases
Main Group
Alkaline Earths
Halogens
Transition Metals
Main Group
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Lanthanides and Actinides
General Chemistry: Chapter 2
Slide 41 of 27
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2.7 The Concept of the Mole and
the Avogadro Constant
• Mole: the amount of a substance that
contains the same number of entities (atom or
molecules) as there are in exactly 12g of
carbon-12
• NA = 6.02214179 x 1023 units/mole
Molar Mass
• The molar mass, M, is the mass of one
mole of a substance. (Get from Atomic
Mass on PT)
• Lithium average atomic mass: 6.941u
• Lithium molar mass: 6.941 grams/mole
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General Chemistry: Chapter 2
Slide 44 of 27
2.8 Using the Mole Concept in
Calculations
• Conversion factor
1 mole S = 6.022 x 1023 S atoms
1 mole S = 32.065 grams S
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Canada Inc.
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