The Structure of the Atom
Chapter 4
The Roots of Atomic Theory
Democritus
 Aristotle
 John Dalton

Ideas versus Science
 Why was it hard for Democritus to defend
his ideas?

Dalton’s Atomic Theory
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All elements are composed of tiny indivisible
particles called atoms.
Atoms of the same element are identical. The
atoms of any one element are different from
those of any other element.
Atoms of different elements can chemically
combine with one another in simple wholenumber ratios to form compounds.
Chemical reactions occur when atoms are
separated, joined, or rearranged. Atoms of one
element are never changed into another element
as a result of a chemical reaction.
Defining the Atom
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An atom is the smallest particle of an element
that retains the properties of that element.
100,000,000 atoms = 1 centimeter
The Electron
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Using a vacuum pump, scientists passed
electricity through glass tubes with an
anode at one end and a cathode at the
other. Cathode rays led to the discovery
of negatively charged particles called
electrons.
Electrons
JJ Thomson did experiments using
cathode rays, discovered that opposite
charges attract and like charges repel
 Robert Millikan determined electrons
negative charge and very small mass
 found in cloud (sublevels) outside the
nucleus; valence electrons are the
outermost electrons
 Plum Pudding Model
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Atomic Nucleus
Ernest Rutherford proposed that the atom
is mostly empty space (gold foil
experiment)
 all the positive charge and almost all the
mass is concentrated in a small region
 Rutherford called this region the nucleus
 nucleus is the center core of an atom
composed of protons and neutrons

Rutherford’s Nuclear Model
The atom is composed of a dense,
positively charged nucleus surrounded by
negative electrons.
 What force causes the deflection of alpha
particles?
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Protons and Neutrons
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atoms are electrically neutral
protons are positively charged
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almost 2000 times the mass of an electron
neutrons have no charge, but equal mass to a
proton
protons and neutrons are collectively called
nucleons
Summary of Atom Anatomy
The nucleus of the atom
is made of protons
and neutrons.
Electrons orbit the
nucleus in a cloud.
Protons
+ charge
Neutrons
no charge
Electrons
- charge
Atomic Number
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elements are different due to different numbers
of protons
number of protons is the atomic number
number of protons equals number of electrons in
a neutral atom
Mass Number
total number of protons and neutrons in an
atom is the mass number
 number of neutrons = mass # - atomic #
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oxygen has an atomic # 8, mass # 16
atomic number can be written as the
subscript and mass number as the
superscript
 can also designate element by mass
number
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gold-197
Isotopes
atoms with same number of protons but
different number of neutrons are called
isotopes
 different mass numbers; chemically alike
due to same # of protons and electrons
 hydrogen-1 (mass number 1, no neutrons)
 hydrogen-2 (mass number 2, 1 neutron)
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deuterium
Atomic Mass Unit
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defined as one twelfth the
mass of a carbon-12
atom (amu)
mass of a single proton
(or neutron) is about 1
amu
Why isn’t atomic mass
always a whole number?
Atomic mass is the
weighted average mass
of the atoms in a naturally
occurring sample of the
element.
Isotope Abundance
Analyzing an element’s mass can indicate
the most abundant isotope for that
element.
 Using the atomic mass, you can usually
assume that the most common isotope is
the atomic mass rounded to the nearest
whole number.
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Example Problems

Find the weighted average of 3
assignments…
Term paper
 Midterm
 Final
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20%
30%
50%
A = 4.0
B = 3.0
C = 2.0
Student receives a “B” on term paper, “C” on
midterm, and “B” on final. What is the overall
grade for this student?
Example Problems

Calculate the average atomic mass of
nitrogen…
Nitrogen-14
 Nitrogen-15
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99.63%
0.37%
14.003
15.000
What is the average atomic mass?
Example Problems

Three isotopes of Element X have atomic
masses and relative abundances of
23.985 amu (78.99%), 24.986 amu
(10.00%), and 25.982 amu (11.01%).
Calculate the atomic mass.
Radioactivity
Nuclear reactions involve an atom of one
element changing into an atom of another
element.
 radioactive atoms undergo changes that
alter their identities
 unstable nuclei lose energy by emitting
radiation through radioactive decay
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Radioactivity
Alpha (helium-4 nucleus)
 Beta (fast moving electron)
 Gamma (high energy, no mass)
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