Chemistry: Atoms First
Second Edition
Julia Burdge & Jason Overby
Chapter 20
Nuclear Chemistry
M. Stacey Thomson
Pasco-Hernando State College
Copyright (c) The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
1
What Is Radioactivity?
• Radioactivity is the release of tiny, highenergy particles or gamma rays from an
atom
• Particles are ejected from the nucleus
Tro: Chemistry: A
Molecular Approach
2
Nuclear Decay
Some nuclei are unstable, and will,
over time, emit particles and/or
electromagnetic radiation until they
become stable.
The spontaneous emission of
particles or electromagnetic radiation is
known as radioactivity.
All elements with Z > 83 are radioactive.
Tro: Chemistry: A
Molecular Approach
3
20.1
Nuclei and Nuclear Reactions
During a nuclear reaction, the products and
reactants will contain different elements as the
nuclei change.
There are several types of particles or forms of
electromagnetic radiation that may be emitted
during a nuclear reaction.
You should learn the names, symbols and the
mass number and charge of each of the particles.
4
Nuclei and Nuclear Reactions
The symbols for subatomic particles include:
proton
1
1
1
1
neutron
1
0
electron
0
1
e
0
1

positron
0
1
e
0
1

α particle
4
2
H
p
n

4
2
He
5
Nuclei and Nuclear Reactions
In balancing a nuclear reaction, simply balance the total of all
atomic numbers and total of all mass numbers for the products and
reactants.
212
84
Po
208
82
Pb
+
4
2
He
Mass number:
212
208 + 4 = 212
Atomic number:
84
82 + 2 = 84
6
Example 20.1
Identify the missing species X in each of the following nuclear equations:
208
(a) 212
Po

84
82 Pb  X
0
(b) 90
38 Sr  X  -1 
(c) X  188 O  -10
7
20.2
Nuclear Stability
Review the information from Chapter 2 on Nuclear stability.
Principle factor for nuclear stability is neutron-to-proton
ratio (n/p)




There are more stabile nuclei with 2, 8, 20, 50, 82, or
126 protons or neutrons
More with even #’s
All with atomic number > 83 are radioactive
All isotopes of Tc and Pm are radioactive
8
Nuclear Stability
The figure shows the number
of neutrons vs. the number
of protons in various
isotopes.
 Stable nuclei are located in
an area of the graph known
as the belt of stability.
 Most radioactive nuclei lie
outside the belt.
 Above the belt of stability,
the nuclei have higher
neutron-to-proton ratio.
9
Types of Nuclear Decay
Above the belt, isotopes
decay by:
beta emission
10
Tro: Chemistry: A
Molecular Approach
11
Types of Nuclear Decay
Below the belt, isotopes
decay by:
positron emission
electron capture
12
A Low Neutron to Proton Ratio
If the N/Z ratio is too low, and the
nuclide has too many protons. These
nuclides tend to undergo either positron
emission or electron capture.
Tro: Chemistry: A
Molecular Approach
13
Positron Emission
Positrons result from a proton changing into
a neutron.
A positron has a charge of +1 and negligible
mass.
anti-electron
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Molecular Approach
14
Positron Emission
• When an atom loses a positron from the
nucleus, its
mass number remains the same
atomic number decreases by 1
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Molecular Approach
15
Tro: Chemistry: A
Molecular Approach
16
Electron Capture
Electron capture occurs when an inner orbital
electron is pulled into the nucleus. A
proton combines with the electron to make a
neutron. This decreases the atomic number,
and increases the N/Z ratio.
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Molecular Approach
17
Electron Capture
As a result of electron capture:
 mass number stays the same
 atomic number decreases by one
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Molecular Approach
18
Particle Changes
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Molecular Approach
19
Alpha (α) Emission
Many nuclides that are too heavy to be
stable (Z>83) undergo alpha emission.
An  particle contains 2 protons and 2
neutrons, and is the same as a helium
nucleus.
Tro: Chemistry: A
Molecular Approach
20
Tro: Chemistry: A
Molecular Approach
21
Alpha Emission
Loss of an alpha particle means:
atomic number decreases by 2
mass number decreases by 4
As a result, the N/Z ratio increases.
222
88
Ra ® He +
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Molecular Approach
4
2
22
218
86
Rn
Gamma (γ) Emission
During a nuclear reaction, high energy
electromagnetic radiation, called
gamma rays, is often emitted.
Generally occurs after the nucleus
undergoes some other type of decay
and the remaining particles rearrange.
Tro: Chemistry: A
Molecular Approach
23
Gamma Emission
• Gamma (g) rays are high energy photons of
•
•
light
No loss of particles from the nucleus
No change in the composition of the nucleus
 same atomic number and mass number
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Molecular Approach
24
Other Properties of
Radioactivity
• Radioactive rays can ionize matter
cause uncharged matter to become charged
basis of Geiger Counter and electroscope
• Radioactive rays have high energy
• Radioactive rays can penetrate matter
• Radioactive rays cause phosphorescent
chemicals to glow
basis of scintillation counter
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Molecular Approach
25
Penetrating Ability of Radioactive Rays

g

0.01 mm
1 mm
Pieces of Lead
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Molecular Approach
26
100 mm
Ionizing Ability of Radiation
Highly energetic radiation interacts with molecules and
atoms by ionizing them. This can have serious biological
effects on cells in living systems. Cell damage, or abnormal
cell replication can occur.
α particles are highly ionizing, but not very penetrating.
They can be stopped by a sheet of paper, clothing, or air. As a
result, they are not very damaging unless ingested or breathed
into the lungs.
β particles have lower ionizing power, but are more
penetrating. A sheet of metal or a thick piece of wood will
stop them.
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Molecular Approach
27
Ionizing Ability of Radiation
γ rays have the lowest ionizing power, but are the most
penetrating. Several inches of lead or slabs of concrete are
needed to stop gamma rays.
Tro: Chemistry: A
Molecular Approach
28
Nuclear Binding Energy
A quantitative measure of nuclear stability is the nuclear binding
energy.The nuclear binding energy is the energy required to break up a
nucleus into its component protons and neutrons.
The measured mass of 19F = 18.99840 amu
mass of 9 protons = 9 x 1.007825 amu = 9.070425 amu
mass of 9 electrons = 9 x 5.4858x10-4 amu = 0.0049372 amu
mass of 10 neutrons = 10 x 1.008665 amu = 10.08665 amu
The calculated mass of 19F = 19.16201 amu
The difference between the mass of an atom and the sum of the masses
of its protons, neutrons, and electrons is called the mass defect.
Mass defect of 19F = 19.16201 amu – 18.99840 amu = 0.16361 amu
29
Nuclear Binding Energy
The loss in mass is converted to energy and can be quantified with
Einstein’s mass-energy equivalence relationship.
ΔE = (Δm)c2
 ΔE = energy of product – energy of reactant
 Δm = mass of product – mass of reactant
For 19F, Δm = 18.99840 – 19.16201 amu = –0.16361 amu
1kg


 28
m   0.16361amu 


2
.
7168
x
10
kg

26
 6.0221418 x10 amu 
30
Nuclear Binding Energy
The loss in mass is converted to energy and can be quantified with
Einstein’s mass-energy equivalence relationship.
ΔE = (Δm)c2
ΔE = (–2.7168 x 10–28 kg)(2.99792458 x 108 m/s)2
ΔE = –2.4417 x 10–11 kg (m/s)2
ΔE = –2.4417 x 10–11 J
31
Nuclear Binding Energy
Plot of nuclear binding energy per nucleon versus mass number.
32
20.3
Nuclear Radioactivity
The disintegration of a radioactive nucleus
often is the beginning of a radioactive decay
series, which is a sequence of nuclear
reactions that ultimately result in the
formation of a stable isotope.
The beginning radioactive isotope is called
the parent and the product isotope is called
the daughter.
33
Kinetics of Radioactive Decay
All radioactive decays obey first-order kinetics.
Nt
ln
 kt
N0
The corresponding half-life of the reaction is given by:
t1/2 
0.693
k
34
Nuclear Radioactivity
A piece of linen cloth found at an ancient burial site is found to
have a 14C activity of 4.8 disintegrations per minute. Determine the
age of the cloth.
Assume that the carbon-14 activity of an equal mass of living flax
(the plant from which linen is made) is 14.8 disintegrations per
minute. The half-life of carbon-14 is 5715 years.
Solution
Step 1:Determine the rate constant from the equation below:
t1/2 
k
0.693
k
0.693
0.693

 1.21 10 4 yr 1
t1/2
5715 yr
35
Dating Based on Radioactive Decay
A piece of linen cloth found at an ancient burial site is found to
have a 14C activity of 4.8 disintegrations per minute. Determine the
age of the cloth.
Assume that the carbon-14 activity of an equal mass of living flax
(the plant from which linen is made) is 14.8 disintegrations per
minute. The half-life of carbon-14 is 5715 years.
Solution
Step 2:Use the equation below to calculate time:
ln
Nt
 kt
N0


4.8
ln
 1.21 10 4 yr 1 t
14.8
36
t = 1.0 x
104
yr
Worked Example 20.3
A wooden artifact is found to have a 14C activity of 9.1 disintegrations per second.
Given that the 14C activity of an equal mass of fresh-cut wood has a constant
value of 15.2 disintegrations per second, determine the age of the artifact. The
half-life of carbon-14 is 5715 years.
Strategy The activity of a radioactive sample is proportional to the number of
radioactive nuclei. Thus, we can use ln([A]t/[A]0) = –kt with activity in place of
concentration:
14C activity in artifact
ln 14
= –kt
C activity in fresh-cut wood
To determine k, though, we must solve t½ = 0.693/k, using the value of t½ for
carbon-14 (5715 years) given in the problem statement.
37
20.4
Nuclear Transmutation
Nuclear transmutation differs from radioactive decay in that
transmutation is brought about by the collision of two particles.
14
7
N +

4
2
17
8
O +
1
1
p
Particle accelerators made it possible to synthesize the so-called
transuranium elements, elements with atomic numbers greater than
92.
38
Nuclear Transmutation
Write an equation for the process represented by:
106
46
Pd( , p)
109
47
Ag
Solution
Step 1:Determine the bombarding particle and the emitted particle:
106
46
Pd( , p)109
47 Ag
bombarding particle
emitted particle
Step 2:Write the equation:
106
46
Pd  He 
4
2
109
47
Ag  p
1
1
39
Worked Example 20.5
Write the balanced nuclear equation for the reaction represented by 26 Fe(d,  ) 25 Mn
where d represents a deuterium nucleus.
56
54
Strategy The species written first is a reactant. The species written last is a
product. Within the parentheses, the bombarding particle (a reactant) is written
first, followed by the emitted particle (a product).
Solution The bombarding and emitted particles are represented by 21 H and 24 ,
respectively.
56
25
Fe  21H 
54
25
Mn  24
Think About It Check your work by summing the mass numbers and atomic
numbers on both sides of the equation.
40
Nuclear Transmutation
Schematic of a cyclotron particle accelerator.
41
Nuclear Transmutation
Section of a particle accelerator.
42
20.5
Nuclear Fission
Nuclear fission is the process in which a heavy nucleus (mass
number > 200) divides to form smaller nuclei and one or more
neutrons.
235
92
U + 01n
90
38
Sr +
143
54
Xe + 3 01n
43
Nuclear Fission
Relative yields of the products resulting
from the fission of 235U as a function of
mass number.
44
Nuclear Fission
235U
is capable of a self-sustaining sequence of nuclear fission
known as a nuclear chain reaction.
45
Nuclear Fission
The minimum mass of fissionable
material required to generate a selfsustaining nuclear chain reaction is
the critical mass.
46
Nuclear Fission
Schematic of a nuclear fission reactor.
47
20.6
Nuclear Fusion
Nuclear fusion is the process of combining small nuclei into larger
ones.
The following reactions are believed to take place in the sun:
Because fusion reactions take place at very high temperatures, they
are often called thermonuclear reactions.
48
Nuclear Fusion
Promising fusion reactions include:
Due to the high temperature
requirements, containment
is an issue.
49
Nuclear Fusion
A promising
design employs
high power lasers
A small scale fusion
reaction was carried
out at the Lawrence
Livermore National
Laboratory (right)
Technical difficulties
still need to be
overcome before it can
be put to practical use
50
20.7
Use of Isotopes: Chemical Analysis
Radioactive and stable isotopes have applications in science for
molecular structure determination.
Two proposed structures for
thiosulfate ion:
1)
By using radioactive sulfer35 isotope, the isotope acts
as a “label” for the S atoms.
2)
Based on studies, structure
2 has been confirmed.
51
Isotopes in Medicine
Radioactive and stable isotopes have
applications in science and medicine.
Radioactive isotopes are used as tracers.
Use of tracers for diagnosis include:
 Sodium-24 – blood flow
 Iodine-131 –thyroid conditions
 Iodine-123 – brain imaging
52
20.8
Biological Effects of Radiation
The fundamental unit of radioactivity is the curie (Ci)
1 Cu = 3.70 x 1010 disintegrations per second.
53
Biological Effects of Radiation
A common unit for the absorbed dose of radiation is the rad
(radiation absorbed dose).
1 rad = 1 x 105 J/g of tissue irradiated
The rem (roentgen equivalent for man) is determined from the
number of rads:
Number of rems = number of rads x 1 RBE
RBE = the relative biological effectivness.
54
Biological Effects of Radiation
55