Nuclear Chemistry
Wade Baxter, Ph.D.
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Printed: April 27, 2014
AUTHOR
Wade Baxter, Ph.D.
EDITORS
Donald Calbreath, Ph.D.
Max Helix
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Chapter 1. Nuclear Chemistry
C HAPTER
1
Nuclear Chemistry
C HAPTER O UTLINE
1.1
Nuclear Radiation
1.2
Half-Life
1.3
Fission and Fusion
1.4
References
The world’s first nuclear power plants began operations in the 1950s in both the United States and what was then
the Soviet Union. There are now over 400 nuclear power plants operating throughout the world. Nuclear power
uses the energy released from a process called nuclear fission to turn water into steam, which turns a turbine and
generates electricity. Nuclear fission is also responsible for the destructive power of the atomic bomb. In the picture
above, the reactors are the dome-shaped structures on the far left and right of the image. The large towers releasing
steam are for cooling the heated water before it can be reused or returned to the environment. Nuclear power has
been very controversial throughout its history due to the risk of radiation release in the case of an accident and the
problems of dealing with the radioactive waste generated by the plant. In this chapter, you will learn about nuclear
chemistry. Nuclear reactions involve changes to the nuclei of atoms, whereas ordinary chemical reactions only
involve rearrangements of electrons and atoms.
Peretz Partensky (Flickr:peretzp). www. f lickr.com/photos/i f l/7238289476/. CC BY 2.0.
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1.1. Nuclear Radiation
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1.1 Nuclear Radiation
Lesson Objectives
• Explain how radioactivity involves a change in the nucleus of a radioisotope.
• Describe mass defect, and calculate the conversion of mass to energy according to Einstein’s equation.
• Describe the band of stability, the odd-even effect, and magic numbers in terms of their influence on the
stability of nuclei.
• Describe and write equations for the primary types of radioactive decay.
Lesson Vocabulary
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
alpha particle
band of stability
beta particle
gamma ray
mass defect
nuclear binding energy
nuclear reaction
nucleon
nuclide
positron
radiation
radioactive decay
radioactivity
radioisotope
transmutation
Check Your Understanding
Recalling Prior Knowledge
• What is the composition of an atomic nucleus?
• What quantities are represented by the terms atomic number and mass number?
Marie Curie (1867-1934) was a Polish scientist who pioneered research into nuclear radiation ( Figure 1.1). She
was awarded the Nobel Prize in physics in 1903 along with her husband Pierre and Antoine Henri Becquerel for
their work on radioactivity. She was awarded a second Nobel Prize in 1911, this time in chemistry, for her continued
research on radioactive elements. In this lesson, you will learn about radioactivity, the reasons why certain elements
and isotopes are radioactive, and the most common types of radioactive decay processes.
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Chapter 1. Nuclear Chemistry
FIGURE 1.1
Marie Curie was one of the leading scientists in the field of radioactivity.
She discovered two radioactive elements and was awarded two Nobel
Prizes for her work.
Radioactivity
Radioactivity was discovered quite by accident. In 1896, Henri Becquerel was studying the effect of certain uranium
salts on photographic film plates. He believed that the salts had an effect on the film only when they had been exposed
to sunlight. He accidentally found that uranium salts that had not been exposed to sunlight still had an effect on the
photographic plates. The Curies, associates of Becquerel at the time, showed that the uranium was emitting a type of
ray that interacted with the film. Marie Curie called this radioactivity. Radioactivity is the spontaneous breakdown
of an atom’s nucleus by the emission of particles and/or radiation. Radiation is the emission of energy through
space in the form of particles and/or waves.
Nuclear reactions are very different from chemical reactions. In chemical reactions, atoms become more stable by
participating in a transfer of electrons or by sharing electrons with other atoms. In nuclear reactions, it is the nucleus
of the atom that gains stability by undergoing a change of some kind. Some elements have no stable isotopes, which
means that any atom of that element is radioactive. For some other elements, only certain isotopes are radioactive.
A radioisotope is an isotope of an element that is unstable and undergoes radioactive decay. The energies that are
released in nuclear reactions are many orders of magnitude greater than the energies involved in chemical reactions.
Unlike chemical reactions, nuclear reactions are not noticeably affected by changes in environmental conditions,
such as temperature or pressure.
The discovery of radioactivity and its effects on the nuclei of elements disproved Dalton’s assumption that atoms
are indivisible. A nuclide is a term for an atom with a specific number of protons and neutrons in its nucleus. As
we will see, when nuclides of one type emit radiation, they are changed into different nuclides. Radioactive decay
is spontaneous and does not require an input of energy to occur. The stability of a particular nuclide depends on
the composition of its nucleus, including the number of protons, the number of neutrons, and the proton-to-neutron
ratio.
Nuclear Stability
Atoms are made of protons, neutrons, and electrons. According to the well-established Law of Conservation of
Mass, it would be reasonable to think that the mass of an atom should be exactly equal to the mass of all of its
isolated particles. This turns out to be untrue. An atom of the most common isotope of helium consists of two
3
1.1. Nuclear Radiation
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protons, two neutrons, and two electrons. The combined mass of those six particles is 4.03298 amu. The mass of
a helium atom, to the same number of decimal places, is 4.00260 amu. There has somehow been a loss of 0.03038
amu in going from the separated particles to the intact atom. The mass defect of an atom is the difference between
the mass of an atom and the sum of the masses of its protons, neutrons, and electrons.
Nuclear Binding Energy
If mass is lost as an atom forms from its particles, where does the mass go? Albert Einstein showed that mass and
energy are interconvertible according to his famous equation, E = mc2 . In other words, mass can be converted into
energy and energy can be converted into mass. The mass represented by the mass defect undergoes a conversion
to energy upon the formation of an atom from its particles. We can use Einstein’s equation to solve for the energy
produced when a helium atom is formed. First, the mass defect must be converted into the SI units of kilograms.
0.03038 amu ×
1g
1 kg
×
= 5.0446 × 10−29 kg
6.0223 × 1023 amu 1000 g
The energy is then calculated by multiplying the mass in kilograms by the speed of light in a vacuum (c) squared.
The resulting unit of kg•m2 /s2 is equal to the energy unit of a joule (J).
E = mc2 = (5.0446 × 10−29 kg)(3.00 × 108 m/s)2 = 4.54 × 10−12 J
This quantity is called the nuclear binding energy, which is the energy released when a nucleus is formed from its
nucleons. A nucleon is a nuclear subatomic particle (either a proton or a neutron). The nuclear binding energy can
also be thought of as the energy required to break apart a nucleus into its individual nucleons.
Overall, larger nuclei tend to have larger nuclear binding energies, because the inclusion of each additional nucleon
generally results in an additional release of energy. When comparing the stabilities of different nuclei, a more
relevant value than the absolute nuclear binding energy is the nuclear binding energy per nucleon. For example, the
nuclear binding energy per nucleon for helium is 4.54 × 10−12 J / 4 nucleons = 1.14 × 10−12 J/nucleon. Nuclear
binding energies per nucleon are shown as a function of mass number in the figure below ( Figure 1.2).
FIGURE 1.2
Nuclear binding energy as a function of
the number of nucleons in an atom. A
higher nuclear binding energy per nucleon corresponds to a more stable nucleus.
Notice that elements of intermediate mass, such as iron, have the highest nuclear binding energies per nucleon and
are thus the most stable.
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Chapter 1. Nuclear Chemistry
The Band of Stability
Carbon-12, with six protons and six neutrons, is a stable nucleus, meaning that it does not spontaneously emit
radioactivity. Carbon-14, with six protons and eight neutrons, is unstable and naturally radioactive. Among atoms
with lower atomic numbers, the ideal ratio of neutrons to protons is approximately 1:1. As the atomic number
increases, the stable neutron-proton ratio gradually increases to about 1.5:1 for the heaviest known elements. For
example, lead-206 is a stable nucleus that contains 124 neutrons and 82 protons, a ratio of 1.51 to 1.
This observation is shown in the figure below ( Figure 1.3). The band of stability is the range of stable nuclei on
a graph that plots the number of neutrons in a nuclide against the number of protons. Known stable nuclides are
shown with individual black dots, while the 1:1 and 1.5:1 ratios are shown with a solid red line and a dotted red line,
respectively.
FIGURE 1.3
A graph of the number of neutrons in a
nucleus as a function of the number of
protons. Each known stable nucleus is
represented by a blue dot.
The ideal
neutron to proton ratio changes from 1:1
for light nuclei to 1.5:1 for the heaviest
nuclei.
The band of stability can be explained by a consideration of the forces involved inside the nucleus. Protons within
the nucleus repel one another due to electrostatic repulsion. However, another force called the strong nuclear force
exists over very short distances within the nucleus. This strong nuclear force allows the nucleus to stay together
despite the repulsive electrostatic force. However, as the number of protons increases, so does the electrostatic force.
More and more neutrons are necessary so that the attractive nuclear force can overcome the electrostatic repulsion.
Beyond the element lead (atomic number 82), the repulsive force is so great that no stable nuclides exist.
It should be noted that just because a nucleus is "unstable" (able to undergo spontaneous radioactive decay) does
not mean that it will rapidly decompose. For example, uranium-238 is unstable because it spontaneously decays
over time, but if a sample of uranium-238 is allowed to sit for 1000 years, only 0.0000155% of the sample will
have decayed. However, other unstable nuclei, such as berkelium-243, will be almost completely gone (>99.9999%
decayed) in less than a day.
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1.1. Nuclear Radiation
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There are several other characteristics common to stable nuclei. For example, stable nuclei with even numbers of
both protons and neutrons are far more common than stable nuclei with odd numbers of one or both of those particles
( Table 1.1).
TABLE 1.1: Isotopes
Protons
Odd
Odd
Even
Even
Neutrons
Odd
Even
Odd
Even
Number of Stable Isotopes
4
50
53
157
Nuclei that contain 2, 8, 20, 50, 82, or 126 protons or neutrons are unusually stable for a nucleus of that particular
size. For example, there are ten stable isotopes of the element tin (atomic number 50), but only two stable isotopes
of antimony (atomic number 51). These numbers are often referred to as magic numbers, and their existence lends
evidence to the theory that nucleons, like electrons, exist in different energy levels within the nucleus. These numbers
are thought to represent filled energy levels, and their stability is analogous to the stable electron configurations of
noble gases.
Radioactive Decay
Unstable nuclei spontaneously emit radiation in the form of particles and energy. This generally changes the number
of protons and/or neutrons in the nucleus, resulting in a more stable nuclide. A nuclear reaction is a reaction that
affects the nucleus of an atom. One type of a nuclear reaction is radioactive decay, a reaction in which a nucleus
spontaneously disintegrates into a slightly lighter nucleus, accompanied by the emission of particles, energy, or both.
An example is shown below, in which the nucleus of a polonium atom radioactively decays into a lead nucleus.
210 Po
84
4
→206
82 Pb +2 He
Note that in a balanced nuclear equation, the sum of the atomic numbers and the sum of the mass numbers must be
equal on both sides of the equation. Recall the notation system for isotopes, which shows both the atomic number
and mass number along with the chemical symbol.
Because the number of protons changes as a result of this nuclear reaction, the identity of the element changes.
Transmutation is a change in the identity of a nucleus as a result of a change in the number of protons. There are
several different types of naturally occurring radioactive decay, and we will examine each separately.
Alpha Decay
An alpha particle (α) is a helium nucleus with two protons and two neutrons. Alpha particles are emitted during
some types of radioactive decay. The net charge of an alpha particle is 2+, and its mass is approximately 4 amu.
The symbol for an alpha particle in a nuclear equation is usually 42 He , though sometimes 42 ff is used. Alpha decay
typically occurs for very heavy nuclei in which the nuclei are unstable due to large numbers of nucleons. For nuclei
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Chapter 1. Nuclear Chemistry
that undergo alpha decay, their stability is increased by the subtraction of two protons and two neutrons. For example,
uranium-238 decays into thorium-234 by the emission of an alpha particle ( Figure 1.4).
238 U
92
4
→234
90 Th +2 He
FIGURE 1.4
The unstable uranium-238 nucleus spontaneously decays into a thorium-234 nucleus by emitting an alpha particle.
Beta Decay
Nuclei above the band of stability are unstable because their neutron to proton ratio is too high. To decrease that
ratio, a neutron in the nucleus is capable of turning into a proton and an electron. The electron is immediately ejected
at a high speed from the nucleus. A beta particle (β) is a high-speed electron emitted from the nucleus of an atom
0 fi or
during some kinds of radioactive decay ( Figure 1.5). The symbol for a beta particle in an equation is either −1
0 e. Carbon-14 undergoes beta decay, transmutating into a nitrogen-14 nucleus.
−1
14 C
6
0
→14
7 N +−1 fi
Note that beta decay increases the atomic number by one, but the mass number remains the same.
FIGURE 1.5
The beta decay of a carbon-14 nuclide
involves the conversion of a neutron to a
proton and an electron, with the electron
being emitted from the nucleus.
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1.1. Nuclear Radiation
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Positron Emission
Nuclei below the band of stability are unstable because their neutron to proton ratio is too low. One way to increase
that ratio is for a proton in the nucleus to turn into a neutron and another particle called a positron. A positron
is a particle with the same mass as an electron, but with a positive charge. Like the beta particle, a positron is
0 fi or
immediately ejected from the nucleus upon its formation. The symbol for a positron in an equation is either +1
0 e. For example, potassium-38 emits a positron, becoming argon-38.
+1
38 K
19
0
→38
18 Ar ++1 fi
Positron emission decreases the atomic number by one, but the mass number remains the same.
Electron Capture
An alternate way for a nuclide to increase its neutron to proton ratio is by a phenomenon called electron capture.
In electron capture, an electron from an inner orbital is captured by the nucleus of the atom and combined with a
proton to form a neutron. For example, silver-106 undergoes electron capture to become palladium-106.
106 Ag + 0 e
47
−1
→106
46 Pd
Note that the overall result of electron capture is identical to positron emission. The atomic number decreases by
one while the mass number remains the same.
Gamma Ray Emission
Gamma rays (γ) are very high energy electromagnetic waves emitted from a nucleus. Gamma rays are emitted
by a nucleus when nuclear particles undergo transitions between nuclear energy levels. This is analogous to the
electromagnetic radiation emitted when excited electrons drop from higher to lower energy levels; the only difference
is that nuclear transitions release much more energetic radiation. Gamma ray emission often accompanies the decay
of a nuclide by other means.
230 Th
90
4
→226
88 Ra +2 He + γ
The emission of gamma radiation has no effect on the atomic number or mass number of the products, but it reduces
their energy.
Summary of Nuclear Radiation
The table below ( Table 1.2) summarizes the main types of nuclear radiation, including charge, mass, symbol, and
penetrating power. Penetrating power refers to the relative ability of the radiation to pass through common materials.
Radiation with high penetrating power is potentially more dangerous because it can pass through skin and do cellular
damage.
TABLE 1.2: Types of Radiation
Type
Alpha particle
Beta particle
Positron
Gamma ray
8
Mass
4 amu
~0
~0
0
Charge
2+
1−
1+
0
Penetration Power
Low
Moderate
Moderate
Very high
Shielding
Paper, skin
Metal foil
Metal foil
Lead, concrete (incomplete)
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Chapter 1. Nuclear Chemistry
Lesson Summary
• Radioactive nuclides become more stable by emitting radiation in the form of small particles and energy.
• Mass defect is the difference between the mass of an atom and the sum of the masses of its particles. Upon
the formation of a nucleus, that mass is converted to energy. The nuclear binding energy released during this
process is an indicator of the stability of a nucleus.
• Nuclei tend to be unstable when they are either very large or have a proton to neutron ratio that does not fall
within a band of stability. Greater stability comes from an even number of protons and/or neutrons, especially
when one or both of these values is equal to certain magic numbers.
• Radioactive decay results in the formation of more stable nuclei. Types of radioactive decay include alpha
decay, beta decay, positron emission, electron capture, and gamma ray emission.
Lesson Review Questions
Reviewing Concepts
1. How does an unstable nucleus release energy?
2. Answer the following questions.
a. What is the relationship between mass defect and nuclear binding energy?
b. How does nuclear binding energy per nucleon relate to nuclear stability?
3. What changes in atomic number and mass number occur in each of the following types of radioactive decay?
a.
b.
c.
d.
positron emission
alpha decay
beta decay
electron capture
4. What kind of radiation is the most penetrating? The least penetrating?
Problems
5. Given the following masses for isolated subatomic particles and for a neon-20 atom:
•
•
•
•
proton = 1.00728 amu
neutron = 1.00866 amu
electron = 5.486 × 10−4 amu
neon-20 = 19.99244 amu
a. Calculate the mass defect of a neon-20 atom in amu.
b. Calculate the nuclear binding energy in J. (Use the conversion 6.0223 × 1026 amu = 1 kg)
c. Calculate the nuclear binding energy per nucleon in J/nucleon.
6. Calculate the neutron-proton ratio for the following nuclides. Then, use the figure above ( Figure 1.3) to
determine whether each nuclide is on, above, or below the band of stability.
a.
b.
c.
d.
12 C
6
13 N
7
134 Sn
50
59
27 Co
7. For each pair of nuclides listed below, indicate which one you would expect to be radioactive. Explain.
9
1.1. Nuclear Radiation
a.
b.
c.
d.
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20 Ne and 17 Ne
10
10
96
96
42 Mo and 43 Tc
45 Ca and 40 Ca
20
20
196
195
80 Hg and 80 Hg
8. For each pair of elements listed, predict which one has more stable isotopes. Explain.
a.
b.
c.
d.
Co or Ni
Hg or Pb
Ag or Am
Sr or Y
9. Fill in the blanks for each of the nuclear reactions below. State the type of decay in each case.
a.
b.
c.
d.
e.
f.
198
0
79 Au → _______ +−1 e
57 Co + 0 e → _______
27
−1
230 U → _______ +4 He
2
92
128 Ba → _______ + 0 e
+1
56
131 I →131 Xe+ _______
53
54
239
235
94 Pu →92 U+ _______
10. Write balanced nuclear reactions for each of the following.
a.
b.
c.
d.
Francium-220 undergoes alpha decay.
Arsenic-76 undergoes beta decay.
Uranium-231 captures an electron.
Promethium-143 emits a positron.
Further Reading / Supplemental Links
• The Discovery of Radioactivity, http://www.kentchemistry.com/links/Nuclear/radioactivity.htm
• Alpha, Beta, and Gamma Radiation, http://www.kentchemistry.com/links/Nuclear/AlphaBetaGamma.htm
• Penetrating Power of Ionizing Radiation, http://www.kentchemistry.com/links/Nuclear/penetration.htm
Points to Consider
The stability of a radioisotope is indicated by its half-life, which is the amount of time required for half of the nuclei
in a given sample to undergo a decay process.
• What is the range of half-lives for known isotopes?
• How can half-life be used to determine the age of objects, such as fossils or certain rocks and minerals?
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Chapter 1. Nuclear Chemistry
1.2 Half-Life
Lesson Objectives
•
•
•
•
Define half-life as it relates to radioactive nuclides and solve half-life problems.
Describe the general process by which radioactive dating is used to determine the age of various objects.
Explain the mechanism of a decay series.
Define and write equations for artificial transmutation processes.
Lesson Vocabulary
•
•
•
•
artificial transmutation
decay series
half-life
radioactive dating
Check Your Understanding
Recalling Prior Knowledge
• Kinetics is the study of what aspect of chemical reactions?
• What must be balanced in an equation for a nuclear reaction?
The rate of radioactive decay is different for every radioisotope. Less stable nuclei decay at a faster rate than more
stable nuclei. In this lesson, you will learn about the half-lives of radioactive nuclei.
Half-Life
The rate of radioactive decay is often characterized by the half-life of a radioisotope. Half-life (t 1 ) is the time
2
required for one half of the nuclei in a sample of radioactive material to decay. After each half-life has passed,
one half of the radioactive nuclei will have transformed into a new nuclide ( Table 1.3). The rate of decay and the
half-life does not depend on the original size of the sample. It also does not depend upon environmental factors such
as temperature and pressure.
TABLE 1.3: Half life
Number of Half-Lives Passed
1
Percentage of Radioisotope Remaining
50
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1.2. Half-Life
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TABLE 1.3: (continued)
Number of Half-Lives Passed
2
3
4
5
Percentage of Radioisotope Remaining
25
12.5
6.25
3.125
As an example, iodine-131 is a radioisotope with a half-life of 8 days. It decays by beta particle emission into
xenon-131.
131 I
53
0
→131
54 Xe +−1 e
After eight days have passed, half of the atoms of any sample of iodine-131 will have decayed, and the sample will
now be 50% iodine-131 and 50% xenon-131. After another eight days pass (a total of 16 days), the sample will be
25% iodine-131 and 75% xenon-131. This continues until the entire sample of iodine-131 has completely decayed (
Figure 1.6).
FIGURE 1.6
The half-life of iodine-131 is eight days.
Half of a given sample of iodine-131
decays after each eight-day time period
elapses.
Half-lives have a very wide range, from billions of years to fractions of a second. Listed below ( Table 1.4) are the
half-lives of some common and important radioisotopes.
TABLE 1.4: half life
Nuclide
Half-Life (t 1 )
Decay mode
Carbon-14
Cobalt-60
Francium-220
Hydrogen-3
5730 years
5.27 years
27.5 seconds
12.26 years
β−
β−
α
β−
12
2
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Chapter 1. Nuclear Chemistry
TABLE 1.4: (continued)
Nuclide
Half-Life (t 1 )
Decay mode
Cobalt-60
Iodine-131
Nitrogen-16
Phosphorus-32
Plutonium-239
Potassium-40
Radium-226
Radon-222
Strontium-90
Technetium-99
Thorium-234
Uranium-235
Uranium-238
5.27 years
8.07 days
7.2 seconds
14.3 days
24,100 years
1.28 × 109 years
1600 years
3.82 days
28.1 days
2.13 × 105 years
24.1 days
7.04 × 108 years
4.47 × 109 years
β−
β−
β−
β−
α
β− and e− capture
α
α
β−
β−
β−
α
α
2
Sample Problem 24.1 illustrates how to use the half-life of a sample to determine the amount of radioisotope that
remains after a certain period of time has passed.
Sample Problem 24.1: Half-Life Calculation
Strontium-90 has a half-life of 28.1 days. If you start with a 5.00 mg sample of the isotope, how much remains after
140.5 days have passed?
Step 1: List the known values and plan the problem.
Known
• original mass = 5.00 mg
• t 1 = 28.1 days
2
• time elapsed = 140.5 days
Unknown
• final mass of Sr-90 = ? mg
First, find the number of half-lives that have passed by dividing the time elapsed by the half-life. Then, reduce the
amount of Sr-90 by half, once for each half-life.
Step 2: Solve.
1 half − life
= 5 half-lives
28.1 days
× 12 × 21 × 12 = 0.156 mg
number of half lives = 140.5 days ×
mass of Sr-90 = 5.00 mg ×
1
2
×
1
2
Step 3: Think about your result.
According to the table above ( Table 1.3), the passage of 5 half-lives means that 3.125% of the original Sr-90
remains, and 5.00 mg × 0.03125 = 0.156 mg. The remaining 4.844 mg has decayed by beta particle emission to
yttrium-90.
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1.2. Half-Life
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Practice Problems
1. The half-life of polonium-218 is 3.0 min. How much of a 0.540 mg sample would remain after 9.0 minutes
have passed?
2. The half-life of hydrogen-3, commonly known as tritium, is 12.26 years. If 4.48 mg of tritium has decayed to
0.280 mg, how much time has passed?
Radioactive Dating
Radioactive dating is a process by which the approximate age of an object is determined through the use of certain
radioactive nuclides. For example, carbon-14 has a half-life of 5,730 years and is used to measure the age of organic
material. The ratio of carbon-14 to carbon-12 in living things remains constant while the organism is alive because
fresh carbon-14 is entering the organism whenever it consumes nutrients. When the organism dies, this consumption
stops, and no new carbon-14 is added to the organism. As time goes by, the ratio of carbon-14 to carbon-12 in the
organism gradually declines, because carbon-14 radioactively decays while carbon-12 is stable. Analysis of this
ratio allows archaeologists to estimate the age of organisms that were alive many thousands of years ago. The ages
of many rocks and minerals are far greater than the ages of fossils. Uranium-containing minerals that have been
analyzed in a similar way have allowed scientists to determine that the Earth is over 4 billion years old.
Decay Series
In many instances, the decay of an unstable radioactive nuclide simply produces another radioactive nuclide. It may
take several successive steps to reach a nuclide that is stable. A decay series is a sequence of successive radioactive
decays that proceeds until a stable nuclide is reached. The terms reactant and product are generally not used for
nuclear reactions. Instead, the terms parent nuclide and daughter nuclide are used to refer to the starting and ending
isotopes in a decay process. The figure below ( Figure 1.7) shows the decay series for uranium-238.
FIGURE 1.7
The decay of uranium-238 proceeds
along many steps until a stable nuclide,
lead-206, is reached. Each decay has its
own characteristic half-life.
14
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Chapter 1. Nuclear Chemistry
In the first step, uranium-238 decays by alpha emission to thorium-234 with a half-life of 4.5 × 109 years. This
decreases its atomic number by two. The thorium-234 rapidly decays by beta emission to protactinium-234 (t 1 =
2
24.1 days). The atomic number increases by one. This continues for many more steps until eventually the series
ends with the formation of the stable isotope, lead-206.
Artificial Transmutation
As we have seen, transmutation occurs when atoms of one element spontaneously decay and are converted to
atoms of another element. Artificial transmutation is the bombardment of stable nuclei with charged or uncharged
particles in order to cause a nuclear reaction. The bombarding particles can be protons, neutrons, alpha particles,
or larger atoms. Ernest Rutherford performed some of the earliest bombardments, including the bombardment of
nitrogen gas with alpha particles to produce the unstable fluorine-18 isotope.
14 N +4 He
7
2
→18
9 F
Fluorine-18 quickly decays to the stable nuclide oxygen-17 by releasing a proton.
18 F
9
1
→17
8 O +1 H
When beryllium-9 is bombarded with alpha particles, carbon-12 is produced with the release of a neutron.
9
4
4 Be +2 He
1
→12
6 C +0 n
This nuclear reaction contributed to the discovery of the neutron in 1932 by James Chadwick. A shorthand notation
for artificial transmutations can be used. The above reaction would be written as:
9
12
4 Be(ff, n)6 C
The parent isotope is written first. In the parentheses is the bombarding particle followed by the ejected particle.
The daughter isotope is written after the parentheses.
Positively charged particles need to be accelerated to high speeds before colliding with a nucleus in order to
overcome the electrostatic repulsion. The necessary acceleration is provided by a combination of electric and
magnetic fields. Shown below ( Figure 1.8) is an aerial view of the Fermi National Accelerator Laboratory in
Illinois.
Transuranium Elements
Many, many radioisotopes that do not occur naturally have been generated by artificial transmutation. The elements
technetium and promethium have been produced, since these elements no longer occur in nature. All of their isotopes
are radioactive and have half-lives short enough that any amount of the elements that once existed have long since
disappeared through natural decay. The transuranium elements are elements with atomic numbers greater than
92. All isotopes of these elements are radioactive and none occur naturally. Listed below ( Table 1.5) are the
transuranium elements up through meitnerium, and the reactions by which they were formed.
TABLE 1.5: The Transuranium Elements
Atomic Number
93
Name
Neptunium
Symbol
Np
Preparation
238 U +1 n →239 Np + 0 fi
0
92
−1
93
15
1.2. Half-Life
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TABLE 1.5: (continued)
Atomic Number
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
16
Name
Plutonium
Americium
Curium
Berkelium
Californium
Einsteinium
Fermium
Mendelevium
Nobelium
Lawrencium
Rutherfordium
Dubnium
Seaborgium
Bohrium
Hassium
Meitnerium
Symbol
Pu
Am
Cm
Bk
Cf
Es
Fm
Md
No
Lr
Rf
Db
Sg
Bh
Hs
Mt
Preparation
239
239
0
93 Np →94 Pu +−1 fi
239
1
240
0
94 Pu +0 n →95 Am +−1 fi
239
4
242
1
94 Pu +2 ff →96 Cm +0 n
241 Am +4 ff →243 Bk + 21 n
94
2
0
97
242 Cm +4 ff →245 Cf +1 n
2
0
96
98
238 U + 151 n →253 Es + 7 0 fi
0
92
99
−1
238 U + 171 n →253 Fm + 8 0 fi
0
92
100
−1
253 Es +4 ff →256 Md +1 n
2
0
99
101
246 Cm +12 C →254 No + 41 n
0
6
102
96
252 Cm +10 B →257 Lr + 51 n
0
98
103
5
249
12 C →257 Rf + 41 n
Cf
+
0
6
104
98
249
15
260
1
98 Cf +7 N →105 Db + 40 n
249
18
263
1
98 Cf +8 O →106 Sg + 40 n
209
54
262
1
83 Bi +24 Cr →107 Bh +0 n
208 Pb +58 Fe →265 Hs +1 n
0
82
108
26
209
58 Fe →266 Mt +1 n
Bi
+
0
109
83
26
www.ck12.org
Chapter 1. Nuclear Chemistry
FIGURE 1.8
The Fermi National Accelerator Laboratory in Illinois.
Lesson Summary
• A half-life is the time it takes for half of a given sample of a radioactive nuclide to decay. Scientists use the
half-lives of some naturally occurring radioisotopes to estimate the age of various objects.
• A decay series is a sequence of steps by which a radioactive nuclide decays to a stable nuclide.
• Artificial transmutation is used to produce other nuclides, including the transuranium elements.
Lesson Review Questions
Reviewing Concepts
1.
2.
3.
4.
What fraction of a radioactive isotope remains after one half-life? Two half-lives? Five half-lives?
When does a decay series end?
What is the difference between natural radioactive decay and artificial transmutation?
Why is an electric field unable to accelerate a neutron?
Problems
5. The half-life of protactinium-234 is 6.69 hours. If a 0.812 mg sample of Pa-239 decays for 40.1 hours, what
mass of the isotope remains?
6. 2.86 g of a certain radioisotope decays to 0.358 g over a period of 22.8 minutes. What is the half-life of the
radioisotope?
7. Use the table above ( Table 1.4) to determine the time it takes for 100 mg of carbon-14 to decay to 6.25 mg.
8. Fill in the blanks in the following radioactive decay series.
a.
b.
c.
β
α
232 Th →
→
90
β
α
235 U →
→
92
β
α
→ 233
91 Pa →
β 228
90 Th
α 227
→ 89 Ac
α
→
→
9. Fill in the blanks in the following artificial transmutation reactions.
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1.2. Half-Life
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2
a. 80
+11 H
34 Se +1 H →
b. 63 Li +10 n → 31 H +
1
c.
+42 ff → 30
15 P +0 n
10. Write balanced nuclear equations for the following reactions and identify X.
a. X(p, ff)12
6 C
b. 10
B(n,
ff)X
5
c. X(ff, p)109
47 Ag
Further Reading / Supplemental Links
• Half-Life, http://www.kentchemistry.com/links/Nuclear/halflife.htm
• Artificial Transmutation, http://www.kentchemistry.com/links/Nuclear/ArtificialTrans.htm
• Particle Accelerators, http://www.kentchemistry.com/links/Nuclear/accelerators.htm
Points to Consider
Nuclear fission and nuclear fusion are two processes that can occur naturally or through bombardment. Both release
tremendous amounts of energy.
• What are two modern applications of nuclear fission?
• Where does nuclear fusion occur naturally?
18
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Chapter 1. Nuclear Chemistry
1.3 Fission and Fusion
Lesson Objectives
•
•
•
•
•
Define fission and explain why energy is released during the fission process.
Describe a nuclear chain reaction and how it is applied in both a fission bomb and in a nuclear power plant.
Define fusion and explain the difficulty in using fusion as a controlled energy source.
Explain how ionizing radiation is measured and how it is detected.
Describe some uses of radiation in medicine and agriculture.
Vocabulary
•
•
•
•
•
•
•
•
•
•
•
•
•
nuclear fission
chain reaction
critical mass
nuclear fusion
control rod
moderator
ionizing radiation
roentgen
rem
Geiger counter
scintillation counter
film badge
radioactive tracer
Check Your Understanding
Recalling Prior Knowledge
• What is nuclear binding energy and how does it relate to the stability of nuclei?
• Why do alpha particles need to be accelerated in order to collide with other nuclei, but neutrons do not?
Common methods of energy production include the burning of fossil fuels, solar power, wind, and hydroelectric.
Each has advantages and disadvantages. In this lesson, you will learn about nuclear fission and nuclear fusion, two
processes that release tremendous amounts of energy but also present unique challenges.
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1.3. Fission and Fusion
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Nuclear Fission
According to the graph of nuclear binding energy per nucleon in the lesson “Nuclear Radiation,” the most stable
nuclei are of intermediate mass. To become more stable, the heaviest nuclei are capable of splitting into smaller
fragments. Nuclear fission is a process in which a very heavy nucleus (mass >200) splits into smaller nuclei of
intermediate mass. Because the smaller nuclei are more stable, the fission process releases tremendous amounts of
energy. Nuclear fission may occur spontaneously or may occur as result of bombardment. When uranium-235 is
hit with a slow-moving neutron, it absorbs it and temporarily becomes the very unstable uranium-236. This nucleus
splits into two medium-mass nuclei while also emitting more neutrons. The mass of the products is less than the
mass of the reactants, with the lost mass being converted to energy.
Nuclear Chain Reaction
Because the fission process produces more neutrons, a chain reaction can result. A chain reaction is a reaction in
which the material that starts the reaction is also one of the products and can start another reaction. Illustrated
below ( Figure 1.9) is a nuclear chain reaction for the fission of uranium-235.
FIGURE 1.9
The nuclear chain reaction is a series of
fission processes that sustains itself due
to the continuous production of neutrons
in each reaction.
The original uranium-235 nucleus absorbs a neutron, splits into a krypton-92 nucleus and a barium-141 nucleus, and
releases three more neutrons upon splitting.
235 U +1 n
0
92
141
1
→92
36 Kr +56 Ba + 3 0 n
Those three neutrons are then able to cause the fission of three more uranium-235 nuclei, each of which release more
neutrons, and so on. The chain reaction continues until all of the uranium-235 nuclei have been split, or until the
20
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Chapter 1. Nuclear Chemistry
released neutrons escape the sample without striking any more nuclei. If the size of the original sample of uranium235 is sufficiently small, too many neutrons escape without striking other nuclei, and the chain reaction quickly
ceases. The critical mass is the minimum amount of fissionable material needed to sustain a chain reaction. Atomic
bombs and nuclear reactors are two ways to harness the large energy released during nuclear fission.
Atomic Bomb
In an atomic bomb, or fission bomb, the nuclear chain reaction is designed to be uncontrolled, releasing huge amounts
of energy in a short amount of time. A schematic for one type of fission bomb is shown below ( Figure 1.10). A
critical mass of fissionable plutonium is contained within the bomb, but not at a sufficient density. Conventional
explosives are used to compress the plutonium, causing it to go critical and trigger a nuclear explosion.
FIGURE 1.10
An atomic bomb uses a conventional explosive to bring together a critical mass of
fissionable material, which then explodes
because of the chain reaction and releases a large amount of energy.
Nuclear Power Plant
A nuclear power plant ( Figure 1.11) uses a controlled fission reaction to produce large amounts of heat. The heat
is then used to generate electrical energy.
FIGURE 1.11
A nuclear reactor harnesses the energy of
nuclear fission to generate electricity.
Uranium-235, the usual fissionable material in a nuclear reactor, is first packaged into fuel rods. In order to keep
the chain reaction from proceeding unchecked, moveable control rods are placed in between the fuel rods. Control
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1.3. Fission and Fusion
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rods, limit the amount of available neutrons by absorbing some of them and preventing the reaction from proceeding
too rapidly. Common control rod materials include alloys with various amounts of silver, indium, cadmium, or
boron. A moderator is a material that slows down high-speed neutrons. This is beneficial because slow-moving
neutrons are more efficient at splitting nuclei. Water is often used as a moderator. The heat released by the fission
reaction is absorbed by constantly circulating coolant water. The coolant water releases its heat to a steam generator,
which turns a turbine and generates electricity. The core of the reactor is surrounded by a containment structure that
absorbs radiation.
Controversy abounds over the use of nuclear power. An advantage of nuclear power over the burning of fossil fuels is
that it does not emit carbon dioxide or various other conventional pollutants. However, the smaller nuclei produced
in the fission process are themselves radioactive and must be disposed of or contained in a safe manner. Containment
of this nuclear waste is a challenging problem because the half-lives of the waste products can often be thousands
of years. Spent fuel rods are typically stored temporarily on site in large pools of water to cool them before being
transported to permanent storage facilities where the waste will be kept forever.
Another risk of nuclear power is that an accident at a nuclear power plant is life-threatening and very harmful to the
environment. On April 26, 1986, an accident occurred at the Chernobyl Nuclear Power Plant in Ukraine. Thousands
were killed either from the initial effects of the explosion or from radiation-induced cancers in subsequent years.
Note that this was not a nuclear explosion, simply a conventional explosion that unfortunately spread radioactive
materials into the surrounding environment. Because the fuel rods used in nuclear reactors are not as enriched
with radioactive nuclei as the materials used in an atomic bomb, an accidental release of the massive destruction
associated with a runaway nuclear reaction is not a major risk factor at nuclear power plants.
Nuclear Fusion
The lightest nuclei are also not as stable as nuclei of intermediate mass. Nuclear fusion is a process in which lightmass nuclei combine to form a heavier and more stable nucleus. Fusion produces even more energy than fission.
In the sun and other stars, four hydrogen nuclei combine at extremely high temperatures and pressures to produce a
helium nucleus. The concurrent loss of mass is converted into extraordinary amounts of energy ( Figure 1.12).
Fusion is even more appealing than fission as an energy source because no radioactive waste is produced and the
only reactant needed is hydrogen. However, fusion reactions only occur at very high temperatures—in excess of
40,000,000°C. No known materials can withstand such temperatures, so there is currently no feasible way to harness
nuclear fusion for energy production, although research is ongoing.
Uses of Radiation
As we saw earlier, different types of radiation vary in their abilities to penetrate through matter. Alpha particles have
very low penetrating ability and are stopped by skin and clothing. Beta particles have a penetrating ability that is
about 100 times that of alpha particles. Gamma rays have very high penetrating ability, and great care must be taken
to avoid overexposure to gamma rays.
Exposure and Detection
Radiation emitted by radioisotopes is called ionizing radiation. Ionizing radiation is radiation that has enough
energy to knock electrons off the atoms of a bombarded substance and produce ions. The roentgen is a unit that
measures nuclear radiation and is equal to the amount of radiation that produces 2 × 109 ion pairs when it passes
through 1 cm3 of air. The primary concern is that ionizing radiation can do damage to living tissues. Radiation
22
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Chapter 1. Nuclear Chemistry
FIGURE 1.12
Nuclear fusion takes place when small
nuclei combine to make larger ones. The
enormous amounts of energy produced
by fusion, powers our sun and other
stars.
damage is measured in rems, which stands for roentgen equivalent man. A rem is the amount of ionizing radiation
that does as much damage to human tissue as is done by 1 roentgen of high-voltage x-rays. Tissue damage from
ionizing radiation can cause genetic mutations due to interactions between the radiation and DNA, which can lead
to cancer.
You are constantly being bombarded with background radiation from space and from geologic sources that vary
depending on where you live. Average exposure is estimated to be about 0.1 rem per year. The maximum permissible
dose of radiation exposure for people in the general population is 0.5 rem per year. Some people are naturally at
higher risk because of their occupations, so reliable instruments to detect radiation exposure have been developed.
A Geiger counter is a device that uses a gas-filled metal tube to detect radiation ( Figure 1.13). When the gas
is exposed to ionizing radiation, it conducts a current, and the Geiger counter registers this as audible clicks. The
frequency of the clicks corresponds to the intensity of the radiation.
A scintillation counter is a device that uses a phosphor-coated surface to detect radiation by the emission of bright
bursts of light. Workers who are at risk of exposure to radiation wear small portable film badges. A film badge
consists of several layers of photographic film that can measure the amount of radiation to which the wearer has
been exposed. Film badges are removed and analyzed at periodic intervals to ensure that the person does not become
overexposed to radiation on a cumulative basis.
Medicine and Agriculture
Radioactive nuclides, such as cobalt-60, are frequently used in medicine to treat certain types of cancers. The faster
growing cancer cells are exposed to the radiation and are more susceptible to damage than healthy cells. Thus, the
cells in the cancerous area are killed by the exposure to high-energy radiation. Radiation treatment is risky because
some healthy cells are also killed, and cells at the center of a cancerous tumor can become resistant to the radiation.
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1.3. Fission and Fusion
www.ck12.org
FIGURE 1.13
A Geiger counter is used to detect radiation.
Radioactive tracers are radioactive atoms that are incorporated into substances so that the movement of these
substances can be tracked by a radiation detector. Tracers are used in the diagnosis of cancer and other diseases.
For example, iodine-131 is used to detect problems with a person’s thyroid. A patient first ingests a small amount of
iodine-131. About two hours later, the iodine uptake by the thyroid is determined by a radiation scan of the patient’s
throat. In a similar way, technetium-99 is used to detect brain tumors and liver disorders, and phosphorus-32 is used
to detect skin cancer.
Radioactive tracers can be used in agriculture to test the effectiveness of various fertilizers. The fertilizer is enriched
with a radioisotope, and the uptake of the fertilizer by the plant can be monitored by measuring the emitted radiation
levels. Nuclear radiation is also used to prolong the shelf life of produce by killing bacteria and insects that would
otherwise cause the food to spoil faster.
Lesson Summary
• Nuclear fission involves the splitting of large nuclei into nuclei of intermediate size. A chain reaction is
self-sustaining and is used in an uncontrolled fashion in an atomic bomb.
• Nuclear power plants use controlled fission to generate electricity from the large amounts of heat produced.
• Nuclear fusion involves combining of small nuclei into larger ones, a process that releases considerably more
energy than fission. Fusion powers our sun and other stars but cannot currently be harnessed directly to
generate electricity.
• Ionizing radiation is capable of doing cellular damage and is detected by a Geiger counter, scintillation counter,
or film badge.
• Radiation is used for various cancer treatments, as a tracer to study other processes, and as a way to prolong
the shelf life of food.
24
www.ck12.org
Chapter 1. Nuclear Chemistry
Lesson Review Questions
Reviewing Concepts
1.
2.
3.
4.
5.
6.
7.
8.
Explain the difference between nuclear fission and nuclear fusion.
What is a nuclear chain reaction, and how is it related to the concept of a critical mass?
What is the function of the conventional explosive in an atomic bomb?
Describe the purposes of control rods and moderators in a nuclear power plant.
Why is fusion not used to generate electrical power?
Why are x-rays and the radiation emitted by radioisotopes called ionizing radiation?
Describe two applications of radioisotopes in medicine.
Explain why irradiation of food helps it to last longer.
Problems
9. Explain why fusion reactions release more energy than fission reactions. (Hint: Use the nuclear binding energy
per nucleon graph from the lesson “Nuclear Radiation.”)
10. Fill in the unknown nuclides for the fission and fusion reactions shown below.
1
144
a. 235
92 U +0 n →55 Cs +
2
3
b. 1 H +1 H → +
+10 n
+ 210 n
Further Reading / Supplemental Links
•
•
•
•
Nuclear Fission, http://www.kentchemistry.com/links/Nuclear/fission.htm
Nuclear Fusion, http://www.kentchemistry.com/links/Nuclear/fusion.htm
Nuclear Power Plants, http://www.kentchemistry.com/links/Nuclear/NuclearPP.htm
Uses of Radioisotopes, http://www.kentchemistry.com/links/Nuclear/radioisotopes.htm
Points to Consider
Fission and fusion are promising as energy sources, but they are not without difficulties and controversy.
• Do the advantages of nuclear power justify the risk of accidents and the problems associated with the disposal
of nuclear waste?
• What methods are being investigated as a way to use controlled nuclear fusion as an energy source?
25
1.4. References
www.ck12.org
1.4 References
1. . http://commons.wikimedia.org/wiki/File:Marie_Curie_c1920.png . Public Domain
2. User:Fastfission/Wikimedia Commons. http://commons.wikimedia.org/wiki/File:Binding_energy_curve_-_common_isotopes.svg . Public Domain
3. Joy Sheng, using data from http://en.wikipedia.org/wiki/List_of_elements_by_stability_of_isotopes. CK-12
Foundation . CC BY-NC 3.0
4. User:Inductiveload/Wikimedia Commons, modified by CK-12 Foundation. http://commons.wikimedia.org
/wiki/File:Alpha_Decay.svg . Public Domain
5. User:Inductiveload/Wikimedia Commons, modified by CK-12 Foundation. http://commons.wikimedia.org
/wiki/File:Beta-minus_Decay.svg . Public Domain
6. Joy Sheng. CK-12 Foundation . CC BY-NC 3.0
7. Courtesy of the US Geological Survey. http://pubs.usgs.gov/of/2004/1050/uranium.htm . Public Domain
8. Courtesy of US Department of Energy. http://commons.wikimedia.org/wiki/File:Fermilab.jpg . Public Domain
9. Christopher Auyeung. . CC BY-NC 3.0
10. User:Dake and User:Malyszkz/Wikimedia Commons. http://commons.wikimedia.org/wiki/File:Fatman_in
ner1.svg . Public Domain
11. Christopher Auyeung. CK-12 Foundation . CC BY-NC 3.0
12. CK-12 Foundation, using sun photograph courtesy of NASA photo-by=. Sun: http://commons.wikimedia.or
g/wiki/File:The_Sun_by_the_Atmospheric_Imaging_Assembly_of_NASA%27s_Solar_Dynamics_Observato
ry_-_20100819.jpg . CC BY-NC 3.0 (photograph of sun available in public domain)
13. Tim Vickers. http://commons.wikimedia.org/wiki/File:Geiger_counter_2.jpg . Public Domain
26