Chapter20

Contents and Concepts
Radioactivity and Nuclear Bombardment
Reactions
1. Radioactivity
2. Nuclear Bombardment Reactions
3. Radiations and Matter: Detection and
Biological Effects
4. Rate of Radioactive Decay
5. Applications of Radioactive Isotopes

Copyright © Cengage Learning. All rights reserved.

20 | 2

Energy of Nuclear Reactions
6. Mass–Energy Calculations
7. Nuclear Fission and Nuclear Fusion


20 | 3

Learning Objectives
1. Radioactivity
a. Define radioactive decay and nuclear
bombardment reaction.
b. Learn the nuclear symbols for positron,
gamma photon, electron, neutron, and
proton.
c. Write a nuclear equation.
d. Deduce a product or reactant in a nuclear
equation.
e. Describe the shell model of the nucleus.
f. Explain the band of stability.

20 | 4

g.
h.
i.
j.

Predict the relative stabilities of nuclides.
List the six types of radioactive decay.
Predict the type of radioactive decay.
Define radioactive decay series.

2. Nuclear Bombardment Reactions
a. Define transmutation.
b. Use the notation for a bombardment
reaction.
c. Locate the transuranium elements on the
periodic table.
d. Determine the product nucleus in a nuclear
bombardment reaction.

20 | 5

3. Radiation and Matter: Detection and Biological
Effects
a. State the purpose of a Geiger counter and
a scintillation counter.
b. Define activity of a radioactive source and
curie (Ci).
c. State the relationship between a rad and a
rem.


20 | 6

4. Rate of Radioactive Decay
a. Define radioactive decay constant.
b. Calculate the decay constant from activity.
c. Define half−life.
d. Draw a typical half−life decay curve of a
radioactive element.
e. Calculate the half−life from the decay
constant.
f. Calculate the decay constant and activity
from the half−life.
g. Determine the fraction of nuclei remaining
after a specified time.
h. Apply the carbon−14 dating method.

20 | 7

5. Applications of Radioactive Isotopes
a. State the ways in which radioactive
isotopes are used for chemical analysis.
b. Describe how isotopes are used for medical
therapy and diagnosis.
6. Mass–Energy Calculations
a. Calculate the energy changes for a nuclear
reaction.
b. Define nuclear binding energy and mass
defect.
c. Compare and contrast nuclear fission and
nuclear fusion.

20 | 8

7. Nuclear Fission and Nuclear Fusion
a. Explain how a controlled chain reaction is
applied in a nuclear fission reactor using a
critical mass of fissionable material.
b. Write the reaction of the nuclear fusion of
deuterium and tritium.


20 | 9

In chemical reactions, only the outer electrons of the
atoms are disturbed.
In nuclear reactions, the nuclear changes that occur
are independent of the chemical environment of the
atom.


20 | 10

Radioactive decay is the process in which a
nucleus spontaneously disintegrates, giving off
radiation.
A nuclear bombardment reaction is a nuclear
reaction in which a nucleus is bombarded, or
struck, by another nucleus or by a nuclear particle.


20 | 11

The phenomenon of radioactivity was discovered
by Henri Becquerel in 1896. Becquerel noted that
photographic plates had bright spots when they
were exposed to uranium minerals. This radiation
was found to be composed of three types when
exposed to an electric field.


20 | 12

We write nuclear equations using nuclide symbols.
Nuclear equations are balanced when the total
mass number and the atomic number on both
reactant and product sides are equal.
Let’s look at the decay of uranium−238.

238
92

U→


Th + He

234
90

4
2

20 | 13

Symbols for other particles are given below:

Proton

1
1

Neutron

1
0

Electron

0
-1

Positron

0
1

Gamma photon

0
0


H or 1P
1
n
e or -0 β
1

e or 0 β
1
γ

20 | 14

A beta particle is an electron. Beta emission
occurs when a nucleus decays by emitting a beta
particle, an electron.
A positron is similar to an electron, but has a
positive charge. Positron emission occurs when a
nucleus decays by emitting a positron.
A gamma photon is a particle of electromagnetic
radiation that has higher energy and a smaller
wavelength than an x−ray.

20 | 15

?

Radon−222 is a radioactive noble gas
that is sometimes present as an air
pollutant in homes built over soil with
high uranium content (uranium−238
decays to radium−226, which in turn
decays to radon−222). A radon−222
nucleus decays to polonium−218 by
emitting an alpha particle. Write the
nuclear equation for this decay
process.


20 | 16

From the periodic table, we can see that the atomic
number of radon is 86 and the atomic number of
polonium is 84. For the alpha particle symbol, both
He and α are correct.
222
86
222
86

Rn → 4 α +
2

Rn → 4 He +
2

218
84

Po

218
84

Po

To check, total the mass numbers and atomic
numbers on each side of the reaction.
Mass numbers:
222 = 4 + 218
Atomic numbers:
86 = 2 + 84

20 | 17

?

Iodine−131 is used in the diagnosis
and treatment of thyroid cancer. This
isotope decays by beta emission. What
is the product nucleus?


20 | 18

From the periodic table, we find that the atomic
number of iodine is 53. The beta particle symbol is
correct as either e or β.

I → -0 e + A X
1
Z

131
53

I → -0 β + A X
1
Z

131
53

Now find the atomic and mass number of the
product:
131 = 0 + A
53 = –1 + Z
A = 131
Z = 54
Next, use the atomic number to find the symbol: Xe.

I → -0 e +
1

131
53

131
54


Xe

I → -0 β +
1

131
53

131
54

Xe
20 | 19

Nuclear Stability
It is reasonable to wonder how a nucleus with
positively charged protons is held together, given
that positively charged particles repel each other.
The stability of the nucleus is due to the strong
nuclear force. The nuclear force acts only at very
short distances, about 10−13 m. At this distance it is
stronger than the electric repulsion.


20 | 20

The shell model of the nucleus is a nuclear
model in which protons and neutrons exist in
levels, or shells, analogous to the shell structure
that exists for electrons.


20 | 21

Just as certain very stable numbers of electrons
(2, 10, 18, and so on) occur when a shell is filled,
so there are magic numbers for nucleons.
A magic number is the number of nuclear
particles in a completed shell of protons and
neutrons.
For protons, the magic numbers are 2, 8, 20, 28,
50, and 82. For neutrons, the magic numbers also
include 122.

20 | 22


20 | 23

A plot of number of
protons versus
number of neutrons
for each stable
nuclide yields a band
of stability, the
region in which stable
nuclides lie.


20 | 24

For stable nuclides with Z ≤ 20, the ratio of
neutrons to protons is between 1 and 1.1.
For stable nuclides with Z > 20, the ratio of
neutrons to protons increases to about 1.5. This is
believed to be due to the increasing repulsion
between protons, which requires more neutrons to
increase the strong nuclear force.
No stable nuclide exists for Z > 83, perhaps
because the proton repulsion becomes too great.

20 | 25

?

Predict which nucleus in each pair
should be more stable and explain why.
a. astatine−210 and lead−207
b. molybdenum−91 and
molybdenum−92
c. calcium−37 and calcium−42


20 | 26

a. Astatine−210 has 85 protons and 125 neutrons.
Lead−207 has 82 protons and 125 neutrons.
Lead−207 is more stable because it has a
magic number of protons. Also, At has > 83
protons.
b. Molybdenum−91 has 42 protons and 49
neutrons.
Molybdenum−92 has 42 protons and 50
neutrons.
Molybdenum−92 is more stable because it has
a magic number of neutrons.

20 | 27

c. Calcium−37 has 20 protons and 17 neutrons.
Calcium−42 has 20 protons and 22 neutrons.
Calcium−42 is more stable because it has an
even number of neutrons. (Both have a magic
number of protons.)


20 | 28

There are six common types of radioactive decay.
1.

Alpha emission
Emission of an alpha particle from an unstable
nucleus.

226
88

Ra →


222
86

Rn + He
4
2

20 | 29

2.

Beta emission
Emission of a beta particle from an unstable
nucleus. Beta emission is equivalent to a
neutron converting to a proton.

14
6

C → N+ e


14
7

0
−1

20 | 30

3.

Positron emission
Emission of a positron particle from an
unstable nucleus. Positron emission is
equivalent to a proton converting to a neutron.

Tc →

95
43


95
42

Mo + e
0
1

20 | 31

4.

Electron capture
The decay of an unstable nucleus by capture
of an electron from an inner orbital of the
atom. Electron capture is equivalent to a
proton converting to a neutron.

40
19

K+ e→


0
−1

40
18

Ar
20 | 32

5.

Gamma emission
Emission from an excited nucleus of a gamma
photon, corresponding to radiation with a
wavelength of approximately 10−12 m.
Technetium−99m is an example of a
metastable nucleus; it is in an excited state
and has a lifetime of ≥ 10−9 s.


20 | 33

6.

Spontaneous fission
The spontaneous decay of an unstable
nucleus in which a heavy nucleus of mass
number greater than 89 splits into lighter
nuclei and energy is released.

236
92

U→


Y+

96
39

136
53

I+ 4 n
1
0

20 | 34

Nuclides to the left of the band of stability have a
neutron−to−proton ratio, N/Z, that is too large.
They decay by beta emission, which reduces the
N/Z ratio by converting a neutron to a proton.
Nuclides to the right of the band of stability have
an N/Z ratio that is too small. These nuclides
decay by either positron emission or electron
capture. Either process increases the N/Z ratio by
converting a proton to a neutron.


20 | 35


20 | 36

?

Thallium−201 is a radioactive isotope
used in the diagnosis of circulatory
impairment and heart disease. How do
you expect it to decay?

Thallium−201 has 81 protons and 120 neutrons.
N/Z < 1.5 (too small).
Thallium−201 will decay by either electron capture or
positron emission—probably electron capture, given
that it is a heavy element.

20 | 37

Radioactive Decay Series
A sequence in which one radioactive nucleus
decays to a second, which then decays to a third,
and so forth, until a stable nucleus of lead is
formed.
Three radioactive decay series are found naturally:
uranium−238, uranium−235, and thorium−232.


20 | 38

The radioactive decay series
for uranium−238 ends with
lead−206


20 | 39

You have two samples of water, each
made up of different isotopes of
hydrogen: one contains hydrogen−1
and the other contains hydrogen−3.
a. Would you expect these two water
samples to be chemically similar?

?

b. Would you expect these two water
samples to be physically the same?
c. Which one of these water samples would
you expect to be radioactive?

20 | 40

a. Yes, isotopes have similar chemical properties.
b. No, the hydrogen−3 water has more mass than
the hydrogen−1 water.
c. The hydrogen−3 (tritium) water should be
radioactive.


20 | 41

Nuclear Bombardment Reactions
Nuclear bombardment reactions are not
spontaneous. They involve the collision of a
nucleus with another particle.
Transmutation is the change of one element into
another by bombarding the nucleus of the element
with nuclear particles or nuclei.


20 | 42

When Rutherford allowed alpha particles to collide
with nitrogen nuclei, he found that a proton was
ejected and oxygen was formed.

14
7

N + He → O + H


4
2

17
8

1
1

20 | 43

James Chadwick proposed the existence of the
neutron based on the result of bombarding
beryllium−9 with alpha particles. The product
included neutral radiation we now know as
neutrons.

Be + 4 He → 12C + 1 n
4
2
6
0

9

The first radioactive nucleus produced in the
laboratory was phosphorus−30.
27
13

Al + 2 He → 15P + 0n
4

30

1

Phosphorus−30 decays by positron emission.
30
15

30
0
P → 14 Si + 1 e
20 | 44

In the abbreviated notation for nuclear
bombardment reactions, the starting nucleus is
written first. It is followed by, within parentheses,
the bombarding particle, a comma, and then the
ejected particle. Finally, the product nucleus is
written.


20 | 45

For example, for the bombardment of nitrogen−14
with an alpha particle, which leads to the ejection
of a proton, the reaction is written as follows:
14
7

N + 4 He → 17O + 1H
2
8
1

The abbreviated notation is
14
7

(

4
2

1
1

)

17

N He, p 8 O


20 | 46

The following symbols are used for nuclide
particles when writing them using the abbreviated
notation for a nuclear bombardment reaction.
Neutron, n
Proton, p
Deuteron (hydrogen−2), d
Alpha (helium−4), α


20 | 47

?

Sodium−22 is made by the
bombardment of magnesium−24 (the
most abundant isotope of magnesium)
with deuterons. An alpha particle is the
other product. Write the abbreviated
notation for the nuclear reaction.
Reaction :

24
12

Mg + H →
2
1

Abbreviate d notation :

24
12

Na + He

22
11

4
2

Mg( d, α ) Na
22
11

20 | 48

?

A neutron is produced when lithium−7
is bombarded with a proton. What
product nucleus is obtained in this
reaction?

Reaction : Li + H → Be + n
7
3

1
1

7
4

1
0

The product is 7 Be.
4


20 | 49

When heavy nuclei are bombarded, the
bombarding particles are scattered or deflected. To
produce transmutation, the bombarding particles
must be accelerated.
A particle accelerator is a device used to
accelerate electrons, protons, alpha particles, and
other ions to very high speeds.


20 | 50

A cyclotron is a type of particle accelerator
consisting of two hollow, semicircular metal
electrodes called dees (because the shape
resembles the letter D), in which charged particles
are accelerated by stages to higher and higher
kinetic energies.


20 | 51


20 | 52

Transuranium elements are elements with atomic
numbers greater than that of uranium (Z = 92), the
naturally occurring element of greatest atomic
number.
In 1940, the first transuranium element was
produced at the University of California, Berkeley,
when element 93 (later named neptunium) was
documented. It was created by bombarding
uranium−238 with neutrons, producing
uranium−239, which then decayed by beta
emission to give neptunium−239.

20 | 53

Radiations and Matter
Radiation from nuclear processes affects matter in
part by dissipating energy in it.
The dissipation can ionize atoms and molecules
and, in some cases, excite electrons in matter.
When these electrons undergo transitions to their
ground states, light is emitted.
Because nuclear radiations can form ions and
break chemical bonds, they adversely affect
biological organisms.

20 | 54

Radiation Counters
There are two types of devices:
ionization counters and scintillation counters.


20 | 55

The Geiger counter is an ionization counter used
to count particles emitted by radioactive nuclei. It
consists of a metal tube filled with gas, such as
argon.


20 | 56

A scintillation counter detects nuclear radiation
based on flashes of light generated in a material
by the radiation. A phosphor is a substance that
emits flashes of light when struck by radiation. In
the scintillation counter, the flashes of light are
detected by a photomultiplier tube.


20 | 57

The activity of a radioactive source is the
number of nuclear disintegrations per unit time
occurring in a radioactive material.
The curie (Ci) is a unit of activity equal to 3.700 ×
1010 disintegrations per second.


20 | 58

Biological Effects and Radiation Dosage
The rad (from radiation affected dose) is the
dosage of radiation that deposits 1 × 10−2 J of
energy per kilogram of tissue.


20 | 59

The rem is a unit of radiation dosage that is used
to relate various kinds of radiation in terms of
biological destruction. It equals the rad times a
factor for the type of radiation, called the relative
biological effectiveness (RBE).
rem = rad × RBE
Beta and gamma radiation have an RBE of about
1, neutron radiation has an RBE of about 5, and
alpha radiation has an RBE of about 10.

20 | 60

The effect of radiation on a person depends on the
dosage and the length of time of the exposure. A
series of small doses have less overall effect than
a large dose given all at once.
A single dose of 500 rems is fatal to most people.
Detectable effects are seen at dosages as low as
30 rems. Background radiation averages about 0.1
rem per year but varies dramatically by location.


20 | 61


20 | 62

?

If you are internally exposed to 10 rads
of α, β, and γ radiation, which form of
radiation will cause the greatest
damage?

The α radiation has the highest RBE, so it will
cause the greatest damage.


20 | 63

Rate of Radioactive Decay
The rate of radioactive decay is the number of
nuclei disintegrating per unit time. It is proportional
to the number of nuclei in the sample.
Rate = kNt
Nt = the number of radioactive nuclei at time, t.
k = the radioactive decay constant or rate
constant for radioactive decay; it is
characteristic of the nuclide.


20 | 64

?

The thorium−234 isotope decays by
emitting a beta particle. A 50.0−μg
sample of thorium−234 has an activity
of 1.16 Ci. What is the decay constant
for thorium−234?


20 | 65

First, we find the number of nuclei of thorium−234.

1 mol
6.022 × 10 23 nuclei
-6
Nt = 50.0 × 10 g ×
×
232.04 g
1 mol
Nt = 1.298 × 10 nuclei
17

Next, we convert the activity from curies to
disintegrations per second.
10 disintegrations
3.700 × 10
s
Rate = 1.16 Ci ×
1 Ci
10 disintegra tions
Rate = 4.292 × 10
s

20 | 66

Finally, we use the rate equation, understanding
that 1 disintegration = 1 nuclei.

rate
k=
Nt
disintegrations
4.292 × 10
s
k=
1.298 × 1017 nuclei
10

k = 3.31 × 10 -7 /s


20 | 67

Half−life is the time it takes for one−half of the
nuclei in a sample to decay.
Half−life is related to the decay constant by the
following equation:

t1

2


0.693
=
k

20 | 68

After one half−life, half of the sample (0.5)
remains.
After two half−lives, one−fourth of the sample
(0.25) remains.
After three half−lives, one−eighth of the sample
remains.
This relationship is summarized in the following
equation and in the graph on the nextnslide.

 1
Fraction remaining =   ,
2
where n = number of half - lives


20 | 69


20 | 70

?

Thallium−201 is used in the diagnosis
of heart disease. This isotope decays
by electron capture; the decay constant
is 2.63 × 10−6/s. What is the half−life of
thallium−201 in days?


20 | 71

t1

2

t1

2

t1

0.693
=
k

0.693
=
-6
2.63 × 10
s

1 min
1h
1 day
= 2.63 × 10 s ×
×
×
60 s 60 min 24 h
5

2

t 1 = 3.05 days
2


20 | 72

?

Iodine−131 is used in the diagnosis
and treatment of thyroid disorders. The
half−life for the beta decay of
iodine−131 is 8.07 days.
a. What is the decay constant (in units
per second)?
b. What is the activity (in curies) of a
1.0−μg sample of iodine?


20 | 73

0.693
a. k =
t1
2

0.693
k=
24 h 60 min 60 s
8.07 days ×
×
×
1 day
1h
1 min
-7
k = 9.94 × 10 /s


20 | 74

b.

1 mol
6.022 × 10 23 nuclei
Nt = 1.0 × 10 -6 g ×
×
126.90 g
1 mol

Nt = 4.745 × 1015 nuclei

Rate = kNt
9.94 × 10 -7
15
Rate =
× 4.745 × 10 nuclei
s
9 nuclei
Rate = 4.72 × 10
s


20 | 75

The rate constant is related to the fraction of nuclei
remaining by the following equation:

 Nt 
ln
 N  = - kt

 0
N0 is the original number of nuclei.
Nt is the number of nuclei at time t .
Nt
is the fraction of nuclei remaining at time t .
N0

Nt = 4.745 × 1015 nuclei

20 | 76

?

A 0.500−g sample of iodine−131 is
obtained by a hospital. How much will
remain after a period of one week? The
half−life of this isotope is 8.07 days.


20 | 77

First, we find the value of k.

0.693
k=
t1
2

0.693
k=
1 week
8.07 days ×
7 day
0.601
k=
week


20 | 78

Next, we find the fraction of nuclei remaining.

 Nt 
ln  ÷ = − kt
 N0 
0.601
 Nt 
ln  ÷ = −
× 1 week
week
 N0 

 Nt 
ln  ÷ = − 0.601
 N0 
 Nt 

 = 0.548
N 
 0
54.8% of nuclei remain.

20 | 79

Radioactive Dating
Because the rate of radioactive decay is constant,
this rate can serve as a sort of clock for dating
objects.
Carbon−14 is part of all living material. While a
plant or animal is living, the fraction of carbon−14
in it remains constant due to exchange with the
atmosphere. Once dead, the fraction of carbon−14
and, therefore, the rate of decay decrease. In this
way, the fraction of carbon−14 present in the
remains becomes a clock measuring the time
since the plant’s or animal’s death.

20 | 80

The half−life of carbon−14 is 5730 years. Living
organisms have a carbon−14 decay rate of 15.3
disintegrations per minute per gram of total
carbon.
The ratio of disintegrations at time t to time 0 is
equal to the ratio of nuclei at time t to time 0.


20 | 81

?

A sample of wheat recovered from a
cave was analyzed and gave 12.8
disintegrations of carbon−14 per
minute per gram of carbon. What is the
age of the grain?
Carbon from living material decays at a
rate of 15.3 disintegrations per minute
per gram of carbon. The half−life of
carbon−14 is 5730 years.


20 | 82

Ratet = 12.8 disintegrations/min/g
Rate0 = 15.3 disintegrations/min/g
t1/2 = 5730 y

 Nt  rate t 12.8 disintegrations/min/ g

 N  = rate = 15.3 disintegrations/min/ g = 0.8366

0
 0
 Nt 
ln  ÷
 Nt 
ln  ÷
 N0 
ln ( 0.8366 )
 N0  = 
=
t=
0.693
k
÷ −  0.693 
−

 5730 y 

 −t 1 ÷



2 
3
t = 1.48 × 10 y

20 | 83

?

Why do you think that carbon−14
dating is limited to materials that are
less than 50,000 years old?

After 50,000 years, about ten half−lives would have
passed, meaning there would be almost no
carbon−14 present to detect and measure. (Only
about 0.1% carbon−14 would remain.)


20 | 84

Applications of Radioisotopes: Chemical
Analysis
A radioactive tracer is a very small amount of
radioactive isotope that is added to a chemical,
biological, or physical system so as to study the
system.
Another example of the use of radioactive tracers
is in isotope dilution, a technique to determine
the quantity of a substance in a mixture or in the
total volume of solution by adding a known amount
of isotope to it.

20 | 85

Neutron activation analysis is an analysis of
elements in a sample based on the conversion of
stable isotopes to radioactive isotopes by
bombarding a sample with neutrons.


20 | 86

Applications of Radioisotopes: Medical
Therapy and Diagnosis
Radioisotopes are used for diagnosis of many
medical conditions. For example, they are used to
develop images of internal body organs so those
organs’ functioning can be examined. More than
100 different radioactive isotopes have been used
in medicine.
Radioimmunoassay is a technique for analyzing
blood and other body fluids for the presence of
very small quantities of biologically active
substances.

20 | 87

Energy of Nuclear Reactions
Nuclear reactions involve changes of energy on a
much larger scale than occur in chemical
reactions. This energy is used in nuclear power
reactors and to provide the energy for nuclear
weapons.


20 | 88

Mass–Energy Calculations
When nuclei decay, they form products of lower
energy. The change of energy is related to
changes of mass, according to the equation
derived by Einstein, E = mc2.


20 | 89

∆E = (∆m)c2
We can compute the change in energy for a
nuclear reaction by calculating the change in
mass. The change in mass must be given in
kilograms to satisfy Einstein’s equation.
The masses of some elements and other particles
are given in Table 20.3.


20 | 90


20 | 91

?

Consider the following nuclear
reaction, in which a lithium−7 nucleus
is bombarded with a hydrogen nucleus
to produce two alpha particles:
7
3

Li + 1H → 24 He
1
2

What is the energy change of this
reaction per gram of lithium?
Nuclear masses:
7
3

Li, 7.01436 amu

1
1

H, 1.00728 amu

4
2

He, 4.00150 amu


20 | 92

First we find the change in mass for one mole of
lithium−7.
Mass of products:
2(4.00150 × 10−3 kg) = 8.00300 × 10−3 kg
Mass of reactants:
7.01436 × 10−3 kg + 1.00728 × 10−3 kg
= 8.02164 × 10−3 kg
∆m = –1.864 × 10−5 kg

20 | 93

∆E = (–1.864 × 10−5 kg)(2.998 × 108 m/s)2
∆E = –1.675 × 1012 J

ΔE
– 1.675 × 1012 J
=
7
g 3 Li 7.01436 g 7 Li
3
ΔE
= – 2.388 × 1011 J/g 7 Li
3
7
g 3 Li


20 | 94

Nuclear Binding Energy
The equivalence of mass and energy explains the
mass defect—that is, the difference between the
total mass of the nucleons that make up an atom
and the mass of the atom. The difference in mass
is the energy holding the nucleus together.
The binding energy of a nucleus is the energy
needed to break a nucleus into its individual
protons and neutrons.


20 | 95

Both the binding energy and the mass defect are
indications of the stability of the nucleus.


20 | 96

The maximum binding
energy per nucleon
occurs for nuclides
with mass numbers
near 50


20 | 97

Nuclear fission is a nuclear reaction in which a
heavy nucleus splits into lighter nuclei and
releases energy.
This process sometimes occurs spontaneously, as
with californium−252.
252
98

Cf →


142
56

Ba +

106
42

1
Mo + 40 n

20 | 98

In other cases, a nucleus undergoes fission after
being bombarded by neutrons.
When bombarded by a neutron, uranium−238
gives three possible sets of products.
142
54

n+

235
92

U


139
56

Xe +

94
36

144
55

1
0

Xe +

Xe +

90
37

90
38

Sr + 4 n
1
0

1
Kr + 30 n

1
Rb + 20 n

20 | 99


20 | 100

Nuclear fusion is a nuclear reaction in which light
nuclei combine to give a more stable, heavier
nucleus plus possibly several neutrons. This
process releases energy.

H + H → He + n

2
1


3
1

4
2

1
0

20 | 101


20 | 102

Nuclear Fission; Nuclear Reactors
When the uranium−235 nucleus splits, it releases
two or three neutrons. These neutrons are
absorbed by other uranium−235 nuclei, which then
release even more neutrons.
A nuclear chain reaction is a self−sustaining
series of nuclear fissions caused by the absorption
of neutrons released from previous nuclear
fissions.


20 | 103

Representation of
a chain reaction
of nuclear
fissions.


20 | 104

To sustain a chain reaction in a sample of
fissionable material, a minimum amount of the
particular fissionable material is needed—the
critical mass.
If the mass is much larger (a supercritical mass),
the number of nuclei that split will multiply rapidly.
An atomic bomb is detonated by creating a
supercritical mass of fissionable material.


20 | 105

A nuclear fission reactor is a device that permits
a controlled chain reaction of nuclear fission.
Fuel rods contain the fissionable material. They
alternate with control rods that absorb neutrons.


20 | 106


20 | 107

Nuclear Fusion
Energy is released when light nuclei combine into
a heavier nucleus in a fusion reaction. These
reactions have been observed in the laboratory
using particle accelerators.
For the nuclei to react, the bombarding nuclei must
have enough kinetic energy to overcome the
repulsion between positive nuclei.
The energy required is not practically available at
this time.

20 | 108

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