Chapter 4
Structure of the Atom
Section 4.1

Democritus (460-370 BC)
– Matter is composed of empty space through which
atoms move
– Atoms are solid, homogeneous, indestructible and
indivisible
– Different kinds of atoms have different sizes and
shapes
– The differing properties of matter are due to the
size, shape, and movement of atoms
– Apparent changes in matter result from changes in
the groupings of atoms themselves

Aristotle (384-322 BC)
– One of the most influential philosophers
– Wrote extensively on politics, ethics,
nature, physics, and astronomy
– Most of his writings have been lost through
the ages
– Criticized Democritus, saying that he did
not believe that the nothingness of empty
space could exist

John Dalton (1766-1844)
– All matter is composed of extremely small
particles called atoms
– All atoms of a given element are identical.
Atoms of a specific element are different
from those of any other element
– Atoms cannot be created, divided into
smaller particles, or destroyed
– Different atoms combine in simple whole
number ratios to form compounds
– In a chemical reaction, atoms are
separated, combined, or rearranged
The smallest particle of an element that
retains the properties of the element is
called an atom.
 Atoms are extremely small (a copper
atom has a diameter of
0.000000000128 m)
 Atoms can be seen using a scanning
tunneling microscope.
 See them on page 96.

Section 4.2

J.J. Thomson (1856-1940) discovered
the electron through experiments using
a cathode ray tube.

Robert Millikan (1868-1953) determined
that the charge of an electron was
negative and was able to calculate the
mass of an electron (9.11 x 10-28 g)

Earnest Rutherford (1871-1937) is
famous for discovering the nucleus of
the atom. He used an experiment in
which he passed alpha particles through
a thin film of gold foil. He expected the
alpha particles to pass through, but
found instead that they were deflected
at large angles. He determined that
there was a dense positive core in the
atom, which he called a nucleus.
Subatomic Particles
The nucleus is the tiny positive core of the atom
which contains most of the mass of the atom.
The proton (p+) is the positively (1+) charged
particle found in the nucleus of the atom. It has a
relative mass of one.
The neutron (no) is the particle with no charge (0)
found in the nucleus of the atom. It has a relative
mass of one.
The electron (e-) is the negatively (1-) charged
particle found in the electron cloud outside of the
nucleus. It has very little relative mass by
comparison.
Obj. 5…Subatomic Particles
PROPERTIES OF SUBATOMIC PARTICLES
PARTICLE
SYMBOL
Electron
e-
Proton
p+
Neutron
n
0
ELECTRICAL CHARGE RELATIVE MASS
-1
1/1840
+1
1
0
1
ACTUAL MASS
(g)
9.11 x 10-28
1.67 x 10-24
1.67 x 10-24

Since the number of protons is equal to
the number of electrons, the atom is
electrically neutral
Section 4.3

Henry Moseley (1887-1915) discovered
that the atoms of each element contain
a unique positive charge in their
nucleus. The number of protons in an
atom is referred to as the element’s
atomic number.
• Atomic #:
 # of p+ in an atom
 identifies element
(change atomic # = change of element).
 # p+ = # e- in neutral atom
** (+) charge = less e- than p+
** (-) charge = more e- than p+
• Atomic mass:
(a.k.a. mass #)
 mass of the nucleus
 p+ + n0
 units are a.m.u.
(atomic mass unit)
APEMAN – great shortcut
To make it easier, you might want to use
APEMAN….
 Atomic number = Protons or Electrons
 Mass number – Atomic number =
Neutrons
 (mass number must be rounded)


Since an atom is electrically neutral, the
number of protons equals the number of
electrons.

Atomic # = # of protons = # of electrons
Element
Protons
electrons
Pb
Atomic
Number
82
?
?
?
?
8
?
?
?
?
30
Atoms of the same element with
the same number of protons,
but different numbers of
neutrons are called isotopes.
All isotopes of any element
have the chemical properties of
that element.

Most elements are a mixture a isotopes.
The relative abundance of each isotope
is constant.

For example, in bananas,93.25% of the
K atoms have 20 no, 6.7302% have 22
no, and 0.0117% have 21 no. In any
other source of K, the relative
percentage of the isotopes will be the
same.

In order to identify the various isotopes of an
element, chemists add a number after the
elements name. The number added is called
the mass number and it represents the sum
of the number of protons and neutrons in the
nucleus.
Potassium-39 Potassium-40 Potassium-41

P+
 No
 E-

19
20
19
19
21
19
19
22
19
Number of neutrons = mass number – atomic number

Because the mass of an atom is so
small, chemists have developed a
method of measuring the mass of an
atom relative to the mass of a
specifically chosen atomic standard.
The standard is the carbon-12 atom.
Scientists assigned the C atom a mass
of 12. One atomic mass unit (amu) is
defined as 1/12 the mass of a carbon12 atom. 1 amu is approximately equal
to the mass of a single p+ or no.

Atomic mass of an element is the weighted
average mass of the isotopes of that element.
Example: The atomic mass of Chlorine (Cl) is
35.453 amu. Chlorine exists naturally as
75% chlorine-35 and 25% chlorine–37.
.75770 x 34.969 amu = 26.496 amu
.24230 x 36.966 amu = 8.957 amu
Weighted average atomic mass of Cl =
(26.496 + 8.957) = 35.453 amu
• Isotopes are naturally occurring.
• The mass # of an element (periodic table) is the weighted
avg. of all isotopes that exist in nature.
- abundance of isotope is just as important as mass!
• Ex...
Natural copper (Cu) consists of 2 isotopes ...
Copper - 63 (mass = 62 .930 g/mole)
69%
Copper - 65 (mass = 64 .930 g/mole)
31%
• To calculate avg. mass...
mass x abundance for each isotope
Step 1 :
add the two values from step 1 together
Step 2 :
62 .93 x .69 =
43.42
43.42
+ 20.13
64 .93 x .31 =
20.13
63.55 g/mole
The teacher lied…

There are reactions that involve an
atom of one element changing into an
atom of another element. These
reactions, which involve an atom’s
nucleus, are called nuclear reactions.
Radioactivity is the emission of
radiation.
 Radiation is the radioactive material
that is emitted during radioactivity.
 Radioactive atoms emit radiation
because their nucleus is unstable.
Unstable nuclei lose energy by emitting
radiation in a spontaneous process
called radioactive decay.

Types of Radiation

Alpha radiation – made up of 2 p+ and
2 no . Is equivalent to He-4 nucleus.
During alpha decay, a nucleus loses 4
amu and 2 protons, to become a new
element.
Beta radiation – consists of fast moving
electrons called beta particles. Each beta
particle is an electron with a 1- charge. Beta
decay changes a n0 into a p+ and an e-.
 Gamma radiation – Gamma rays are high
energy radiation that possess no mass.
Because they possess no electrical charge,
they are not deflected by electric or magnetic
fields. Gamma rays usually accompany
alpha and beta radiation and they account for
most of the energy lost during the radioactive
decay process. Because gamma rays are
massless, they cannot result in the formation
of a new atom.

• Practice:
208
82
230
90
226
88
Pb
0
-1
Th
230
90
Ra
4
2
+
208
83
Th +
0
0
e
He
+
222
86
Bi
beta

gamma
Rn
alpha
TYPE
ALPHA
()
BETA
()
GAMMA
()
ATOMIC # NEUTRON
SYMBOL CHANGE
CHANGE
4
2
He +2
by 2
by 2
e
by 1
by 1
0
-1
0
0

MASS
CHANGE
by 4
no change
no change no change no change