This preview shows page 1. Sign up to view the full content.
Unformatted text preview: Back Forward Main Menu TOC Study Guide TOC Textbook Website MHHE Website CHAPTER 23 Nuclear Chemistry
NUCLEAR IN ATOMIC NUCLEI. THIS BRANCH OF CHEMISTRY BEGAN WITH THE DIS- COVERY OF NATURAL RADIOACTIVITY BY 23.2 NUCLEAR STABILITY ANTOINE BECQUEREL AND GREW AS A RESULT OF SUBSEQUENT INVESTIGATIONS BY CURIE 23.1 THE NATURE OF NUCLEAR REACTIONS CHEMISTRY IS THE STUDY OF REACTIONS INVOLVING CHANGES PIERRE AND 23.3 NATURAL RADIOACTIVITY MARIE 23.4 NUCLEAR TRANSMUTATION AND MANY OTHERS. NUCLEAR 23.5 NUCLEAR FISSION CHEMISTRY IS VERY MUCH IN THE NEWS TODAY. IN AD- 23.6 NUCLEAR FUSION DITION TO APPLICATIONS IN THE MANUFACTURE OF ATOMIC BOMBS, HY- 23.7 USES OF ISOTOPES DROGEN BOMBS, AND NEUTRON BOMBS, EVEN THE PEACEFUL USE OF 23.8 BIOLOGICAL EFFECTS OF RADIATION NUCLEAR ENERGY HAS BECOME CONTROVERSIAL, IN PART BECAUSE OF
SAFETY CONCERNS ABOUT NUCLEAR POWER PLANTS AND ALSO BECAUSE
OF PROBLEMS WITH DISPOSAL OF RADIOACTIVE WASTES. IN THIS CHAP- TER WE WILL STUDY NUCLEAR REACTIONS, THE STABILITY OF THE ATOMIC
NUCLEUS, RADIOACTIVITY, AND THE EFFECTS OF RADIATION ON BIOLOGICAL SYSTEMS. 903 Back Forward Main Menu TOC Study Guide TOC Textbook Website MHHE Website 904 NUCLEAR CHEMISTRY 23.1 THE NATURE OF NUCLEAR REACTIONS With the exception of hydrogen (1 H), all nuclei contain two kinds of fundamental par1
ticles, called protons and neutrons. Some nuclei are unstable; they emit particles and/or
electromagnetic radiation spontaneously (see Section 2.2). The name for this phenomenon is radioactivity. All elements having an atomic number greater than 83 are radioactive. For example, the isotope of polonium, polonium-210 (210 Po), decays spon84
taneously to 206 Pb by emitting an particle.
Another type of radioactivity, known as nuclear transmutation, results from the
bombardment of nuclei by neutrons, protons, or other nuclei. An example of a nuclear
transmutation is the conversion of atmospheric 14 N to 14 C and 1 H, which results when
the nitrogen isotope captures a neutron (from the sun). In some cases, heavier elements
are synthesized from lighter elements. This type of transmutation occurs naturally in
outer space, but it can also be achieved artificially, as we will see in Section 23.4.
Radioactive decay and nuclear transmutation are nuclear reactions, which differ
significantly from ordinary chemical reactions. Table 23.1 summarizes the differences.
BALANCING NUCLEAR EQUATIONS In order to discuss nuclear reactions in any depth, we need to understand how to write
and balance the equations. Writing a nuclear equation differs somewhat from writing
equations for chemical reactions. In addition to writing the symbols for various chemical elements, we must also explicitly indicate protons, neutrons, and electrons. In fact,
we must show the numbers of protons and neutrons present in every species in such
The symbols for elementary particles are as follows:
1p or 1H
0n neutron 0
1e or 0
1e or 0
2 He or 4
particle In accordance with the notation used in Section 2.3, the superscript in each case denotes the mass number (the total number of neutrons and protons present) and the subscript is the atomic number (the number of protons). Thus, the “atomic number” of a TABLE 23.1 Comparison of Chemical Reactions and Nuclear Reactions CHEMICAL REACTIONS 1. Atoms are rearranged by the breaking
and forming of chemical bonds.
2. Only electrons in atomic orbitals are
involved in the breaking and forming
3. Reactions are accompanied by absorption
or release of relatively small amounts
4. Rates of reaction are influenced by
temperature, pressure, concentration,
and catalysts. Back Forward NUCLEAR REACTIONS 1. Elements (or isotopes of the same elements)
are converted from one to another.
2. Protons, neutrons, electrons, and other
elementary particles may be involved. Main Menu TOC Study Guide TOC 3. Reactions are accompanied by absorption
or release of tremendous amounts of
4. Rates of reaction normally are not affected
by temperature, pressure, and catalysts. Textbook Website MHHE Website 23.1 THE NATURE OF NUCLEAR REACTIONS 905 proton is 1, because there is one proton present, and the “mass number” is also 1, because there is one proton but no neutrons present. On the other hand, the “mass number” of a neutron is 1, but its “atomic number” is zero, because there are no protons
present. For the electron, the “mass number” is zero (there are neither protons nor neutrons present), but the “atomic number” is 1, because the electron possesses a unit
The symbol 0e represents an electron in or from an atomic orbital. The symbol
represents an electron that, although physically identical to any other electron,
comes from a nucleus (in a decay process in which a neutron is converted to a proton
and an electron) and not from an atomic orbital. The positron has the same mass as
the electron, but bears a 1 charge. The particle has two protons and two neutrons,
so its atomic number is 2 and its mass number is 4.
In balancing any nuclear equation, we observe the following rules:
The total number of protons plus neutrons in the products and in the reactants must
be the same (conservation of mass number).
• The total number of nuclear charges in the products and in the reactants must be
the same (conservation of atomic number).
• If we know the atomic numbers and mass numbers of all the species but one in a nuclear equation, we can identify the unknown species by applying these rules, as shown
in the following example, which illustrates how to balance nuclear decay equations.
EXAMPLE 23.1 Balance the following nuclear equations (that is, identify the product X):
84 Po 88n 208 Pb
82 X (b) 137
55 Cs 137
56 Ba X 88n (a) The mass number and atomic number are 212 and 84, respectively, on
the left-hand side and 208 and 82, respectively, on the right-hand side. Thus, X must
have a mass number of 4 and an atomic number of 2, which means that it is an
particle. The balanced equation is
84 Po 88n 208 Pb
2 (b) In this case the mass number is the same on both sides of the equation, but the
atomic number of the product is 1 more than that of the reactant. The only way this
change can come about is to have a neutron in the Cs nucleus transformed into a
proton and an electron; that is, 1 n 88n 1 p
1 (note that this process does not
alter the mass number). Thus, the balanced equation is
We use the 0 notation here be1
cause the electron came from the
Similar problems: 23.5, 23.6. 137
55 Cs 88n 137 Ba
1 Note that the equation in (a) and (b) are balanced for nuclear particles
but not for electrical charges. To balance the charges, we would need to add two
electrons on the right-hand side of (a) and express barium as a cation (Ba ) in (b). Comment PRACTICE EXERCISE Identify X in the following nuclear equation:
33 As Back Forward Main Menu TOC 88n Study Guide TOC 0
1 X Textbook Website MHHE Website 906 NUCLEAR CHEMISTRY 23.2 NUCLEAR STABILITY The nucleus occupies a very small portion of the total volume of an atom, but it contains most of the atom’s mass because both the protons and the neutrons reside there.
In studying the stability of the atomic nucleus, it is helpful to know something about
its density, because it tells us how tightly the particles are packed together. As a sample calculation, let us assume that a nucleus has a radius of 5 10 3 pm and a mass
of 1 10 22 g. These figures correspond roughly to a nucleus containing 30 protons
and 30 neutrons. Density is mass/volume, and we can calculate the volume from the
known radius (the volume of a sphere is 4 r3, where r is the radius of the sphere).
First we convert the pm units to cm. Then we calculate the density in g/cm3:
density 5 mass
2 To dramatize the almost incomprehensibly high density, it has
been suggested that it is equivalent to packing the mass of all the
world’s automobiles into one
thimble. 10 3 pm
1 1 10 12 m
1 pm 10 22 g
3 100 cm
13 5 10 13 cm g
cm)3 1014 g/cm3 This is an exceedingly high density. The highest density known for an element is
22.6 g/cm3, for osmium (Os). Thus the average atomic nucleus is roughly 9 1012
(or 9 trillion) times more dense than the densest element known!
The enormously high density of the nucleus prompts us to wonder what holds the
particles together so tightly. From Coulomb’s law we know that like charges repel and
unlike charges attract one another. We would thus expect the protons to repel one another strongly, particularly when we consider how close they must be to each other.
This indeed is so. However, in addition to the repulsion, there are also short-range attractions between proton and proton, proton and neutron, and neutron and neutron. The
stability of any nucleus is determined by the difference between coulombic repulsion
and the short-range attraction. If repulsion outweighs attraction, the nucleus disintegrates, emitting particles and/or radiation. If attractive forces prevail, the nucleus is stable.
The principal factor that determines whether a nucleus is stable is the neutron-toproton ratio (n/p). For stable atoms of elements having low atomic number, the n/p
value is close to 1. As the atomic number increases, the neutron-to-proton ratios of the
stable nuclei become greater than 1. This deviation at higher atomic numbers arises
because a larger number of neutrons is needed to counteract the strong repulsion among
the protons and stabilize the nucleus. The following rules are useful in predicting nuclear stability:
Nuclei that contain 2, 8, 20, 50, 82, or 126 protons or neutrons are generally more
stable than nuclei that do not possess these numbers. For example, there are ten stable isotopes of tin (Sn) with the atomic number 50 and only two stable isotopes of
antimony (Sb) with the atomic number 51. The numbers 2, 8, 20, 50, 82, and 126
are called magic numbers. The significance of these numbers for nuclear stability
is similar to the numbers of electrons associated with the very stable noble gases
(that is, 2, 10, 18, 36, 54, and 86 electrons).
• Nuclei with even numbers of both protons and neutrons are generally more stable
than those with odd numbers of these particles (Table 23.2).
• All isotopes of the elements with atomic numbers higher than 83 are radioactive.
All isotopes of technetium (Tc, Z 43) and promethium (Pm, Z 61) are radioactive.
• Back Forward Main Menu TOC Study Guide TOC Textbook Website MHHE Website 23.2 NUCLEAR STABILITY 907 TABLE 23.2 Number of Stable Isotopes with Even and Odd
Numbers of Protons and Neutrons
PROTONS NEUTRONS Odd
Even NUMBER OF STABLE ISOTOPES 4
157 Figure 23.1 shows a plot of the number of neutrons versus the number of protons
in various isotopes. The stable nuclei are located in an area of the graph known as the
belt of stability. Most radioactive nuclei lie outside this belt. Above the stability belt,
the nuclei have higher neutron-to-proton ratios than those within the belt (for the same
number of protons). To lower this ratio (and hence move down toward the belt of stability), these nuclei undergo the following process, called -particle emission:
1 88n 1 p
1 Beta-particle emission leads to an increase in the number of protons in the nucleus and
a simultaneous decrease in the number of neutrons. Some examples are
6 C 88n 7 N
19 K 88n 20 Ca
40 Zr 88n 41 Nb FIGURE 23.1 Plot of neutrons
versus protons for various stable
isotopes, represented by dots.
The straight line represents the
points at which the neutron-toproton ratio equals 1. The
shaded area represents the belt
of stability. 0
1 120 100 Number of neutrons 80
Belt of stability 60 Neutrons/Protons = 1 40 20 0 Back Forward Main Menu 20 40
Numbers of protons TOC Study Guide TOC 80 Textbook Website MHHE Website 908 NUCLEAR CHEMISTRY Below the stability belt the nuclei have lower neutron-to-proton ratios than those in
the belt (for the same number of protons). To increase this ratio (and hence move up
toward the belt of stability), these nuclei either emit a positron
1p 88n 1 n
1 or undergo electron capture. An example of positron emission is
19 K We use 0e rather than 0 here
because the electron came from
an atomic orbital and not from the
1 88n 38 Ar
18 Electron capture is the capture of an electron — usually a 1s electron — by the nucleus.
The captured electron combines with a proton to form a neutron so that the atomic
number decreases by one while the mass number remains the same. This process has
the same net effect as positron emission:
1 e 88n 17 Cl
1 e 88n 25 Mn 37
26 Fe NUCLEAR BINDING ENERGY A quantitative measure of nuclear stability is the nuclear binding energy, which is the
energy required to break up a nucleus into its component protons and neutrons. This
quantity represents the conversion of mass to energy that occurs during an exothermic
The concept of nuclear binding energy evolved from studies of nuclear properties
showing that the masses of nuclei are always less than the sum of the masses of the
nucleons, which is a general term for the protons and neutrons in a nucleus. For example, the 19 F isotope has an atomic mass of 18.9984 amu. The nucleus has 9 protons
and 10 neutrons and therefore a total of 19 nucleons. Using the known masses of the
1 H atom (1.007825 amu) and the neutron (1.008665 amu), we can carry out the following analysis. The mass of 9 1 H atoms (that is, the mass of 9 protons and 9 elec1
9 1.007825 amu 9.070425 amu and the mass of 10 neutrons is
10 1.008665 amu Therefore, the atomic mass of a
trons, protons, and neutrons is 19
9F 9.070425 amu
There is no change in the electron’s mass since it is not a
nucleon. 10.08665 amu atom calculated from the known numbers of elec10.08665 amu 19.15708 amu which is larger than 18.9984 amu (the measured mass of 19 F) by 0.1587 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. Relativity theory tells us that
the loss in mass shows up as energy (heat) given off to the surroundings. Thus the formation of 19 F is exothermic. According to Einstein’s mass-energy equivalence rela9
tionship (E mc2, where E is energy, m is mass, and c is the velocity of light), we can
calculate the amount of energy released. We start by writing
E ( m)c2 (23.1) where E and m are defined as follows: Back Forward Main Menu TOC Study Guide TOC Textbook Website MHHE Website 23.2 E energy of product m mass of product NUCLEAR STABILITY 909 energy of reactants
mass of reactants Thus we have for the change in mass
m 18.9984 amu 19.15708 amu 0.1587 amu Because 19 F has a mass that is less than the mass calculated from the number of elec9
trons and nucleons present, m is a negative quantity. Consequently, E is also a negative quantity; that is, energy is released to the surroundings as a result of the formation of the fluorine-19 nucleus. So we calculate E as follows:
E 108 m/s)2 ( 0.1587 amu)(3.00
16 1.43 10 22 amu m /s With the conversion factors
1 kg 6.022 1026 amu
22 1 kg m /s 1J we obtain
10 11 amu m2
s2 1.00 kg
6.022 1026 amu 1J
1 kg m2/s2 J This is the amount of energy released when one fluorine-19 nucleus is formed from 9
protons and 10 neutrons. The nuclear binding energy of the nucleus is 2.37 10 11 J,
which is the amount of energy needed to decompose the nucleus into separate protons
and neutrons. In the formation of 1 mole of fluorine nuclei, for instance, the energy
E ( 2.37 10 11 J)(6.022 1.43 1013 J/mol 1.43 1023/mol) 1010 kJ/mol The nuclear binding energy, therefore, is 1.43 1010 kJ for 1 mole of fluorine-19 nuclei, which is a tremendously large quantity when we consider that the enthalpies of
ordinary chemical reactions are of the order of only 200 kJ. The procedure we have
followed can be used to calculate the nuclear binding energy of any nucleus.
As we have noted, nuclear binding energy is an indication of the stability of a nucleus. However, in comparing the stability of any two nuclei we must account for the
fact that they have different numbers of nucleons. For this reason it is more meaningful to use the nuclear binding energy per nucleon, defined as
nuclear binding energy per nucleon nuclear binding energy
number of nucleons For the fluorine-19 nucleus,
nuclear binding energy per nucleon 2.37 10 11 J
1.25 Back Forward Main Menu TOC Study Guide TOC 10 12 J/nucleon Textbook Website MHHE Website NUCLEAR CHEMISTRY FIGURE 23.2 Plot of nuclear
binding energy per nucleon versus mass number. Nuclear binding energy per nucleon (J) 910 56Fe
4 1.5 × 10–12 He 238U 1.2 × 10–12
9 × 10–13
6 × 10–13
3 × 10–13 2H 0 20 40 60 80 100 120 140 160 180 200 220 240 260 Mass number The nuclear binding energy per nucleon allows us to compare the stability of all
nuclei on a common basis. Figure 23.2 shows the variation of nuclear binding energy
per nucleon plotted against mass number. As you can see, the curve rises rather steeply.
The highest binding energies per nucleon belong to elements with intermediate mass
numbers—between 40 and 100—and are greatest for elements in the iron, cobalt, and
nickel region (the Group 8B elements) of the periodic table. This means that the net
attractive forces among the particles (protons and neutrons) are greatest for the nuclei
of these elements.
Nuclear binding energy and nuclear binding energy per nucleon are calculated for
an iodine nucleus in the following example.
EXAMPLE 23.2 The atomic mass of 127 I is 126.9004 amu. Calculate the nuclear binding energy of
this nucleus and the corresponding nuclear binding energy per nucleon.
Answer There are 53 protons and 74 neutrons in the nucleus. The mass of 53 1 H
1 atoms is
53 1.007825 amu 53.41473 amu and the mass of 74 neutrons is
74 1.008665 amu Therefore, the predicted mass for
and the mass defect is
53 I 74.64121 amu is 53.41473 126.9004 amu 74.64121 128.05594 amu, 128.05594 amu 1.1555 amu The energy released is
E ( m)c2
( 1.1555 amu)(3.00
1.04 Back Forward Main Menu TOC 108 m/s)2 1017 amu m2/s2 Study Guide TOC Textbook Website MHHE Website 23.3 E 1.04
1.73 The neutron-to-proton ratio is 1.4,
which places iodine-127 in the
belt of stability. 1017
10 10 amu m2
s2 1.00 kg
6.022 1026 amu 911 1J
1 kg m2/s2 J Thus the nuclear binding energy is 1.73
nucleon is obtained as follows:
1.73 10 10 J
127 nucleons Similar problems: 23.19, 23.20. NATURAL RADIOACTIVITY 10 10 1.36 10 J. The nuclear binding energy per
12 J/nucleon PRACTICE EXERCISE Calculate the nuclear binding energy (in J) and the binding energy per nucleon of
83 Bi (208.9804 amu). 23.3 NATURAL RADIOACTIVITY Nuclei outside the belt of stability, as well as nuclei with more than 83 protons, tend
to be unstable. The spontaneous emission by unstable nuclei of particles or electromagnetic radiation, or both, is known as radioactivity. The main types of radiation are:
particles (or doubly charged helium nuclei, He2 ); particles (or electrons); rays,
which are very-short-wavelength (0.1 nm to 10 4 nm) electromagnetic waves; positron
emission; and electron capture.
The disintegration of a radioactive nucleus is often the beginning of a radioactive
decay series, which is a sequence of nuclear reactions that ultimately result in the formation of a stable isotope. Table 23.3 shows the decay series of naturally occurring
uranium-238, which involves 14 steps. This decay scheme, known as the uranium decay series, also shows the half-lives of all the products.
It is important to be able to balance the nuclear reaction for each of the steps in
a radioactive decay series. For example, the first step in the uranium decay series is
the decay of uranium-238 to thorium-234, with the emission of an particle. Hence,
the reaction is
92 U 88n 234 Th
2 The next step is represented by
90 Th 88n 234 Pa
1 and so on. In a discussion of radioactive decay steps, the beginning radioactive isotope is called the parent and the product, the daughter.
KINETICS OF RADIOACTIVE DECAY All radioactive decays obey first-order kinetics. Therefore the rate of radioactive decay at any time t is given by
rate of decay at time t N where is the first-order rate constant and N is the number of radioactive nuclei present at time t. (We use instead of k for rate constant in accord with the notation used
by nuclear scientists.) According to Equation (13.3), the number of radioactive nuclei
at time zero (N0) and time t (Nt) is Back Forward Main Menu TOC Study Guide TOC Textbook Website MHHE Website 912 NUCLEAR CHEMISTRY TABLE 23.3 The Uranium Decay Series
109 yr 4.51
24.1 days 234
1.17 min 234
86 104 yr 1.60 230
90 105 yr 103 yr Th Ra Rn
3.82 days 218
3.05 min 0.04%
82 At Pb 2s 26.8 min
83 Bi 99.96%
1.6 10 4 214
84 19.7 min
81 Po Tl s 1.32 min
20.4 yr 210
83 Bi 100%
84 5.01 days
81 Po Tl
4.20 min 138 days
82 Pb ln N0
Nt t and the corresponding half-life of the reaction is given by Equation (13.5):
We do not have to wait 4.51
109 yr to make a half-life measurement of uranium-238. Its
value can be calculated from the
rate constant using Equation
(13.5). Back Forward 0.693 The half-lives (hence the rate constants) of radioactive isotopes vary greatly from nucleus to nucleus. For example, looking at Table 23.3, we find two extreme cases: Main Menu 238
84 Po TOC 88n 234 Th
82 Pb Study Guide TOC 4
2 4.51 t1
2 1.6 Textbook Website 109 yr
10 4 s MHHE Website 23.3 NATURAL RADIOACTIVITY 913 The ratio of these two rate constants after conversion to the same time unit is about
1 1021, an enormously large number. Furthermore, the rate constants are unaffected
by changes in environmental conditions such as temperature and pressure. These highly
unusual features are not seen in ordinary chemical reactions (see Table 23.1). DATING BASED ON RADIOACTIVE DECAY The half-lives of radioactive isotopes have been used as “atomic clocks” to determine
the ages of certain objects. Some examples of dating by radioactive decay measurements will be described here.
Radiocarbon Dating The carbon-14 isotope is produced when atmospheric nitrogen is bombarded by cosmic rays:
0n 88n 14 C
1H The radioactive carbon-14 isotope decays according to the equation
1 88n 14 N
7 This decay series is the basis of the radiocarbon dating technique described on p. 527.
Dating Using Uranium-238 Isotopes
We can think of the first step as
the rate-determining step in the
overall process. Because some of the intermediate products in the uranium series have very long halflives (see Table 23.3), this series is particularly suitable for estimating the age of rocks
in the earth and of extraterrestrial objects. The half-life for the first step (238 U to 234 Th)
is 4.51 109 yr. This is about 20,000 times the second largest value (that is, 2.47
105 yr), which is the half-life for 234 U to 230 Th. Therefore, as a good approximation
we can assume that the half-life for the overall process (that is, from 238 U to 206 Pb) is
governed solely by the first step:
92 U 238U t1
_ 238U 206Pb 206 g/2
238 g/2 4.51 × 109 yr FIGURE 23.3 After one halflife, half of the original uranium238 is converted to lead-206. Forward Main Menu 8 4
2 6 0
2 4.51 109 yr In naturally occurring uranium minerals we should and do find some lead-206
isotopes formed by radioactive decay. Assuming that no lead was present when the
mineral was formed and that the mineral has not undergone chemical changes that
would allow the lead-206 isotope to be separated from the parent uranium-238, it is
possible to estimate the age of the rocks from the mass ratio of 206 Pb to 238 U. The
above equation tells us that for every mole, or 238 g, of uranium that undergoes complete decay, 1 mole, or 206 g, of lead is formed. If only half a mole of uranium-238
has undergone decay, the mass ratio Pb-206/U-238 becomes 2 Back 88n 206 Pb
82 0.866 and the process would have taken a half-life of 4.51 109 yr to complete (Figure 23.3).
Ratios lower than 0.866 mean that the rocks are less than 4.51 109 yr old, and higher
ratios suggest a greater age. Interestingly, studies based on the uranium series as well
as other decay series put the age of the oldest rocks and, therefore, probably the age
of Earth itself at 4.5 109, or 4.5 billion, years. TOC Study Guide TOC Textbook Website MHHE Website 914 NUCLEAR CHEMISTRY Dating Using Potassium-40 Isotopes This is one of the most important techniques in geochemistry. The radioactive
potassium-40 isotope decays by several different modes, but the relevant one as far as
dating is concerned is that of electron capture:
19 K 88n 40 Ar
2 1.2 109 yr The accumulation of gaseous argon-40 is used to gauge the age of a specimen. When
a potassium-40 atom in a mineral decays, argon-40 is trapped in the lattice of the mineral and can escape only if the material is melted. Melting, therefore, is the procedure
for analyzing a mineral sample in the laboratory. The amount of argon-40 present can
be conveniently measured with a mass spectrometer (see p. 76). Knowing the ratio of
argon-40 to potassium-40 in the mineral and the half-life of decay makes it possible
to establish the ages of rocks ranging from millions to billions of years old. 23.4 NUCLEAR TRANSMUTATION The scope of nuclear chemistry would be rather narrow if study were limited to natural radioactive elements. An experiment performed by Rutherford in 1919, however,
suggested the possibility of producing radioactivity artificially. When he bombarded a
sample of nitrogen with particles, the following reaction took place:
2 88n 17O
1p An oxygen-17 isotope was produced with the emission of a proton. This reaction
demonstrated for the first time the feasibility of converting one element into another,
by the process of nuclear transmutation. Nuclear transmutation differs from radioactive decay in that the former is brought about by the collision of two particles.
The above reaction can be abbreviated as 17N( ,p)17O. Note that in the parenthe8
ses the bombarding particle is written first, followed by the ejected particle. The following example illustrates the use of this notation to represent nuclear transmutations.
EXAMPLE 23.3 Write the balanced equation for the nuclear reaction
resents the deuterium nucleus (that is, 2H).
26Fe(d, )54Mn, where d rep25 The abbreviation tells us that when iron-56 is bombarded with a deuterium
nucleus, it produces the manganese-54 nucleus plus an particle, 4He. Thus, the
equation for this reaction is
26 Fe Similar problems: 23.33, 23.34. 2
1H 88n 4
25 Mn PRACTICE EXERCISE Write a balanced equation for 106
46 Pd( ,p)109 Ag.
47 Although light elements are generally not radioactive, they can be made so by
bombarding their nuclei with appropriate particles. As we saw earlier, the radioactive
carbon-14 isotope can be prepared by bombarding nitrogen-14 with neutrons. Tritium,
1H, is prepared according to the following bombardment: Back Forward Main Menu TOC Study Guide TOC Textbook Website MHHE Website 23.4 6
3 Li Alternating voltage
± ± Magnetic field Tritium decays with the emission of
1H Target 88n 3 H
88n 3 He
2 12.5 yr Many synthetic isotopes are prepared by using neutrons as projectiles. This approach is particularly convenient because neutrons carry no charges and therefore are
not repelled by the targets — the nuclei. In contrast, when the projectiles are positively
charged particles (for example, protons or particles), they must have considerable
kinetic energy in order to overcome the electrostatic repulsion between themselves and
the target atoms. The synthesis of phosphorus from aluminum is one example: Dees
FIGURE 23.4 Schematic diagram of a cyclotron particle accelerator. The particle (an ion) to
be accelerated starts at the center
and is forced to move in a spiral
path through the influence of
electric and magnetic fields until
it emerges at a high velocity. The
magnetic fields are perpendicular
to the plane of the dees (socalled because of their shape),
which are hollow and serve as
0n 915 NUCLEAR TRANSMUTATION 27
13 Al 4
2 88n 30 P
0n A particle accelerator uses electric and magnetic fields to increase the kinetic energy
of charged species so that a reaction will occur (Figure 23.4). Alternating the polarity
and ) on specially constructed plates causes the particles to accelerate
along a spiral path. When they have the energy necessary to initiate the desired nuclear reaction, they are guided out of the accelerator into a collision with a target substance.
Various designs have been developed for particle accelerators, one of which accelerates particles along a linear path of about 3 km (Figure 23.5). It is now possible
to accelerate particles to a speed well above 90 percent of the speed of light. (According
to Einstein’s theory of relativity, it is impossible for a particle to move at the speed of
light. The only exception is the photon, which has a zero rest mass.) The extremely
energetic particles produced in accelerators are employed by physicists to smash atomic
nuclei to fragments. Studying the debris from such disintegrations provides valuable
information about nuclear structure and binding forces. FIGURE 23.5 A section of a
linear particle accelerator. Back Forward Main Menu TOC Study Guide TOC Textbook Website MHHE Website 916 NUCLEAR CHEMISTRY TABLE 23.4 The Transuranium Elements
ATOMIC NUMBER NAME 93
Meitnerium SYMBOL Np
0n 88n 93Np
93Np 88n 94Pu
0n 88n 95Am
2 88n 96Cm
2 88n 97Bk
2 88n 98Cf
U 15 0n 88n 253Es
170n 88n 100Fm
2 88n 101Md
6C 88n 102No
5B 88n 103Lr
Cf 12C 88n 257Rf
7N 88n 105Ha
8O 88n 106Sg
24Cr 88n 107Ns
26Fe 88n 108Hs
26Fe 88n 109Mt 0
0n THE TRANSURANIUM ELEMENTS Particle accelerators made it possible to synthesize the so-called transuranium elements, elements with atomic numbers greater than 92. Neptunium (Z 93) was first
prepared in 1940. Since then, 20 other transuranium elements have been synthesized.
All isotopes of these elements are radioactive. Table 23.4 lists the transuranium elements and the reactions through which they are formed. 23.5 NUCLEAR FISSION Nuclear fission is the process in which a heavy nucleus (mass number > 200) divides
to form smaller nuclei of intermediate mass and one or more neutrons. Because the
heavy nucleus is less stable than its products (see Figure 23.2), this process releases a
large amount of energy.
The first nuclear fission reaction to be studied was that of uranium-235 bombarded
with slow neutrons, whose speed is comparable to that of air molecules at room temperature. Under these conditions, uranium-235 undergoes fission, as shown in Figure
23.6. Actually, this reaction is very complex: More than 30 different elements have
92 143 Xe
54 FIGURE 23.6 Nuclear fission of U-235. When a U-235 nucleus captures a neutron (red dot), it
undergoes fission to yield two smaller nuclei. On the average, 2.4 neutrons are emitted for every
U-235 nucleus that divides. Back Forward Main Menu TOC Study Guide TOC Textbook Website MHHE Website 23.5 NUCLEAR FISSION 917 Relative amounts of fission product been found among the fission products (Figure 23.7). A representative reaction is
92U 80 100 120 140 160 Mass number
FIGURE 23.7 Relative yields of
the products resulting from the fission of U-235, as a function of
mass number. TABLE 23.5 Nuclear
Binding Energies of 235U
and Its Fission Products
0n 88n 90 Sr
54 Xe 3 1n
0 Although many heavy nuclei can be made to undergo fission, only the fission of naturally occurring uranium-235 and of the artificial isotope plutonium-239 has any practical importance. Table 23.5 shows the nuclear binding energies of uranium-235 and
its fission products. As the table shows, the binding energy per nucleon for uranium235 is less than the sum of the binding energies for strontium-90 and xenon-143.
Therefore, when a uranium-235 nucleus is split into two smaller nuclei, a certain amount
of energy is released. Let us estimate the magnitude of this energy. The difference between the binding energies of the reactants and products is (1.23 10 10 1.92
10 10) J (2.82 10 10) J, or 3.3 10 11 J per uranium-235 nucleus. For 1 mole
of uranium-235, the energy released would be (3.3 10 11)(6.02 1023), or 2.0
1013 J. This is an extremely exothermic reaction, considering that the heat of combustion of 1 ton of coal is only about 8 107 J.
The significant feature of uranium-235 fission is not just the enormous amount of
energy released, but the fact that more neutrons are produced than are originally captured in the process. This property makes possible a nuclear chain reaction, which is
a self-sustaining sequence of nuclear fission reactions. The neutrons generated during
the initial stages of fission can induce fission in other uranium-235 nuclei, which in
turn produce more neutrons, and so on. In less than a second, the reaction can become
uncontrollable, liberating a tremendous amount of heat to the surroundings.
Figure 23.8 shows two types of fission reactions. For a chain reaction to occur,
enough uranium-235 must be present in the sample to capture the neutrons. Otherwise,
many of the neutrons will escape from the sample and the chain reaction will not occur, as depicted in Figure 23.8(a). In this situation the mass of the sample is said to be
subcritical. Figure 23.8(b) shows what happens when the amount of the fissionable
material is equal to or greater than the critical mass, the minimum mass of fissionable
material required to generate a self-sustaining nuclear chain reaction. In this case most
of the neutrons will be captured by uranium-235 nuclei, and a chain reaction will occur. FIGURE 23.8 Two types of nuclear fission. (a) If the mass of
U-235 is subcritical, no chain reaction will result. Many of the
neutrons produced will escape to
the surroundings. (b) If a critical
mass is present, many of the neutrons emitted during the fission
process will be captured by other
U-235 nuclei and a chain reaction will occur. (a) Back Forward Main Menu TOC Study Guide TOC (b) Textbook Website MHHE Website 918 NUCLEAR CHEMISTRY TNT explosive THE ATOMIC BOMB The first application of nuclear fission was in the development of the atomic bomb.
How is such a bomb made and detonated? The crucial factor in the bomb’s design is
the determination of the critical mass for the bomb. A small atomic bomb is equivalent to 20,000 tons of TNT (trinitrotoluene). Since 1 ton of TNT releases about 4
109 J of energy, 20,000 tons would produce 8 1013 J. Earlier we saw that 1 mole, or
235 g, of uranium-235 liberates 2.0 1013 J of energy when it undergoes fission. Thus
the mass of the isotope present in a small bomb must be at least
Subcritical U-235 wedge
FIGURE 23.9 Schematic cross
section of an atomic bomb. The
TNT explosives are set off first.
The explosion forces the sections
of fissionable material together to
form an amount considerably
larger than the critical mass. 8 1013 J
2.0 1013 J 1 kg For obvious reasons, an atomic bomb is never assembled with the critical mass already
present. Instead, the critical mass is formed by using a conventional explosive, such
as TNT, to force the fissionable sections together, as shown in Figure 23.9. Neutrons
from a source at the center of the device trigger the nuclear chain reaction. Uranium235 was the fissionable material in the bomb dropped on Hiroshima, Japan, on August
6, 1945. Plutonium-239 was used in the bomb exploded over Nagasaki three days later.
The fission reactions generated were similar in these two cases, as was the extent of
the destruction. NUCLEAR REACTORS In Europe, nuclear reactors
provide about 40 percent of the
electrical energy consumed. A peaceful but controversial application of nuclear fission is the generation of electricity using heat from a controlled chain reaction in a nuclear reactor. Currently, nuclear reactors provide about 20 percent of the electrical energy in the United States.
This is a small but by no means negligible contribution to the nation’s energy production. Several different types of nuclear reactors are in operation; we will briefly discuss the main features of three of them, along with their advantages and disadvantages. Light Water Reactors Most of the nuclear reactors in the United States are light water reactors. Figure 23.10
is a schematic diagram of such a reactor, and Figure 23.11 shows the refueling process
in the core of a nuclear reactor.
An important aspect of the fission process is the speed of the neutrons. Slow neutrons split uranium-235 nuclei more efficiently than do fast ones. Because fission reactions are highly exothermic, the neutrons produced usually move at high velocities.
For greater efficiency they must be slowed down before they can be used to induce
nuclear disintegration. To accomplish this goal, scientists use moderators, which are
substances that can reduce the kinetic energy of neutrons. A good moderator must satisfy several requirements: It should be nontoxic and inexpensive (as very large quantities of it are necessary); and it should resist conversion into a radioactive substance
by neutron bombardment. Furthermore, it is advantageous for the moderator to be a
fluid so that it can also be used as a coolant. No substance fulfills all these requirements, although water comes closer than many others that have been considered.
Nuclear reactors that use light water (H2O) as a moderator are called light water reactors because 1 H is the lightest isotope of the element hydrogen.
1 Back Forward Main Menu TOC Study Guide TOC Textbook Website MHHE Website 23.5 NUCLEAR FISSION 919 Shield Steam
To steam turbine Shield Water Pump
FIGURE 23.10 Schematic diagram of a nuclear fission reactor. The fission process is controlled
by cadmium or boron rods. The heat generated by the process is used to produce steam for the
generation of electricity via a heat exchange system. The nuclear fuel consists of uranium, usually in the form of its oxide, U3O8 (Figure
23.12). Naturally occurring uranium contains about 0.7 percent of the uranium-235 isotope, which is too low a concentration to sustain a small-scale chain reaction. For effective operation of a light water reactor, uranium-235 must be enriched to a concentration of 3 or 4 percent. In principle, the main difference between an atomic bomb
and a nuclear reactor is that the chain reaction that takes place in a nuclear reactor is
kept under control at all times. The factor limiting the rate of the reaction is the number of neutrons present. This can be controlled by lowering cadmium or boron rods
between the fuel elements. These rods capture neutrons according to the equations
FIGURE 23.12 Uranium oxide, U3O8. FIGURE 23.11 Refueling the
core of a nuclear reactor. Back Forward Main Menu TOC Study Guide TOC Textbook Website MHHE Website 920 NUCLEAR CHEMISTRY 113
0n 88n 114 Cd
88n 7 Li
2 where denotes gamma rays. Without the control rods the reactor core would melt
from the heat generated and release radioactive materials into the environment.
Nuclear reactors have rather elaborate cooling systems that absorb the heat given
off by the nuclear reaction and transfer it outside the reactor core, where it is used to
produce enough steam to drive an electric generator. In this respect, a nuclear power
plant is similar to a conventional power plant that burns fossil fuel. In both cases, large
quantities of cooling water are needed to condense steam for reuse. Thus, most nuclear
power plants are built near a river or a lake. Unfortunately this method of cooling
causes thermal pollution (see Section 12.4). Heavy Water Reactors Another type of nuclear reactor uses D2O, or heavy water, as the moderator, rather than
H2O. Deuterium absorbs neutrons much less efficiently than does ordinary hydrogen.
Since fewer neutrons are absorbed, the reactor is more efficient and does not require
enriched uranium. The fact that deuterium is a less efficient moderator has a negative
impact on the operation of the reactor, because more neutrons leak out of the reactor.
However, this is not a serious disadvantage.
The main advantage of a heavy water reactor is that it eliminates the need for
building expensive uranium enrichment facilities. However, D2O must be prepared by
either fractional distillation or electrolysis of ordinary water, which can be very expensive considering the amount of water used in a nuclear reactor. In countries where
hydroelectric power is abundant, the cost of producing D2O by electrolysis can be reasonably low. At present, Canada is the only nation successfully using heavy water nuclear reactors. The fact that no enriched uranium is required in a heavy water reactor
allows a country to enjoy the benefits of nuclear power without undertaking work that
is closely associated with weapons technology. Breeder Reactors A breeder reactor uses uranium fuel, but unlike a conventional nuclear reactor, it produces more fissionable materials than it uses.
We know that when uranium-238 is bombarded with fast neutrons, the following
reactions take place:
92 U 1
93 Np Plutonium-239 forms plutonium
oxide, which can be readily
separated from uranium. 88n 239 U
88n 239 Np
94 Pu 0
2 23.4 min t1
2 2.35 days In this manner the nonfissionable uranium-238 is transmuted into the fissionable isotope plutonium-239 (Figure 23.13).
In a typical breeder reactor, nuclear fuel containing uranium-235 or plutonium239 is mixed with uranium-238 so that breeding takes place within the core. For every
uranium-235 (or plutonium-239) nucleus undergoing fission, more than one neutron is
captured by uranium-238 to generate plutonium-239. Thus, the stockpile of fissionable
material can be steadily increased as the starting nuclear fuels are consumed. It takes Back Forward Main Menu TOC Study Guide TOC Textbook Website MHHE Website 23.5 NUCLEAR FISSION 921 about 7 to 10 years to regenerate the sizable amount of material needed to refuel the
original reactor and to fuel another reactor of comparable size. This interval is called
the doubling time.
Another fertile isotope is 232 Th. Upon capturing slow neutrons, thorium is trans90
muted to uranium-233, which, like uranium-235, is a fissionable isotope:
0 n 88n 90 Th
90 Th 88n 91 Pa
91Pa 88n 92 U FIGURE 23.13 The red glow
of the radioactive plutonium-239
isotope. The orange color is due
to the presence of its oxide. 0
2 22 min t1
2 27.4 days Uranium-233 is stable enough for long-term storage.
Although the amounts of uranium-238 and thorium-232 in Earth’s crust are relatively plentiful (4 ppm and 12 ppm by mass, respectively), the development of breeder
reactors has been very slow. To date, the United States does not have a single operating breeder reactor, and only a few have been built in other countries, such as France
and Russia. One problem is economics; breeder reactors are more expensive to build
than conventional reactors. There are also more technical difficulties associated with
the construction of such reactors. As a result, the future of breeder reactors, in the
United States at least, is rather uncertain.
Hazards of Nuclear Energy Molten glass is poured over nuclear waste before burial. Back Forward Main Menu Many people, including environmentalists, regard nuclear fission as a highly undesirable method of energy production. Many fission products such as strontium-90 are dangerous radioactive isotopes with long half-lives. Plutonium-239, used as a nuclear fuel
and produced in breeder reactors, is one of the most toxic substances known. It is an
alpha emitter with a half-life of 24,400 yr.
Accidents, too, present many dangers. An accident at the Three Mile Island reactor in Pennsylvania in 1979 first brought the potential hazards of nuclear plants to public attention. In this instance very little radiation escaped the reactor, but the plant remained closed for more than a decade while repairs were made and safety issues
addressed. Only a few years later, on April 26, 1986, a reactor at the Chernobyl nuclear plant in Belarus surged out of control. The fire and explosion that followed released much radioactive material into the environment. People working near the plant
died within weeks as a result of the exposure to the intense radiation. The long-term
effect of the radioactive fallout from this incident has not yet been clearly assessed, although agriculture and dairy farming were affected by the fallout. The number of potential cancer deaths attributable to the radiation contamination is estimated to be between a few thousand and more than 100,000.
In addition to the risk of accidents, the problem of radioactive waste disposal has
not been satisfactorily resolved even for safely operated nuclear plants. Many suggestions have been made as to where to store or dispose of nuclear waste, including burial underground, burial beneath the ocean floor, and storage in deep geologic formations. But none of these sites has proved absolutely safe in the long run. Leakage of
radioactive wastes into underground water, for example, can endanger nearby communities. The ideal disposal site would seem to be the sun, where a bit more radiation
would make little difference, but this kind of operation requires 100 percent reliability in space technology.
Because of the hazards, the future of nuclear reactors is clouded. What was once
hailed as the ultimate solution to our energy needs in the twenty-first century is now TOC Study Guide TOC Textbook Website MHHE Website 922 NUCLEAR CHEMISTRY Chemistry in Action Chemistry in Action Chemistry in Action Chemistry in Action Chemistry in Action Chemistry in Action Chemistry in Action Chemistry in Action Chemistry in Action Chemistry in Action Chemistry in Action Chemistry in Action Chemistry Back Nature’s Own Fission Reactor
It all started with a routine analysis in May 1972 at
the nuclear fuel processing plant in Pierrelatte, France.
A staff member was checking the isotope ratio of
U-235 to U-238 in a uranium ore and obtained a puzzling result. It had long been known that the relative
natural occurrence of U-235 and U-238 is 0.7202
percent and 99.2798 percent, respectively. In this
case, however, the amount of U-235 present was only
0.7171 percent. This may seem like a very small deviation, but the measurements were so precise that this
difference was considered highly significant. The ore
had come from the Oklo mine in the Gabon Republic,
a small country on the west coast of Africa. Subsequent
analyses of other samples showed that some contained
even less U-235, in some cases as little as 0.44 percent.
The logical explanation for the low percentages
of U-235 was that a nuclear fission reaction at the
mine must have consumed some of the U-235 isotopes.
But how did this happen? There are several conditions
under which such a nuclear fission reaction could take
place. In the presence of heavy water, for example,
a chain reaction is possible with unenriched uranium.
Without heavy water, such a fission reaction could still
occur if the uranium ore and the moderator were
arranged according to some specific geometric constraints at the site of the reaction. Both of the possibilities seem rather farfetched. The most plausible explanation is that the uranium ore originally present in
the mine was enriched with U-235 and that a nuclear
fission reaction took place with light water, as in a
conventional nuclear reactor.
As mentioned earlier, the natural abundance of
U-235 is 0.7202 percent, but it has not always been
that low. The half-lives of U-235 and U-238 are 700
million and 4.51 billion years, respectively. This means
that U-235 must have been more abundant in the past,
because it has a shorter half-life. In fact, at the time
Earth was formed, the natural abundance of U-235
was as high as 17 percent! Since the lowest concentration of U-235 required for the operation of a fission reactor is 1 percent, a nuclear chain reaction
could have taken place as recently as 400 million
years ago. By analyzing the amounts of radioactive Forward Main Menu TOC fission products left in the ore, scientists concluded that
the Gabon “reactor” operated about 2 billion years
Having an enriched uranium sample is only one
of the requirements for starting a controlled chain reaction. There must also have been a sufficient amount
of the ore and an appropriate moderator present. It
appears that as a result of a geological transformation, uranium ore was continually being washed into
the Oklo region to yield concentrated deposits. The
moderator needed for the fission process was largely
water, present as water of crystallization in the sedimentary ore.
Thus, in a series of extraordinary events, a natural nuclear fission reactor operated at the time when
the first life forms appeared on Earth. As is often the
case in scientific endeavors, humans are not necessarily the innovators but merely the imitators of nature. Photo showing the natural nuclear reactor site (lower righthand corner) at Oklo, Gabon Republic. Study Guide TOC Textbook Website MHHE Website 23.6 NUCLEAR FUSION 923 being debated and questioned by both the scientific community and laypeople. It seems
likely that the controversy will continue for some time. 23.6 NUCLEAR FUSION In contrast to the nuclear fission process, nuclear fusion, the combining of small nuclei into larger ones, is largely exempt from the waste disposal problem.
Figure 23.2 showed that for the lightest elements, nuclear stability increases with
increasing mass number. This behavior suggests that if two light nuclei combine or
fuse together to form a larger, more stable nucleus, an appreciable amount of energy
will be released in the process. This is the basis for ongoing research into the harnessing of nuclear fusion for the production of energy.
Nuclear fusion occurs constantly in the sun (Figure 23.14). The sun is made up
mostly of hydrogen and helium. In its interior, where temperatures reach about 15 million degrees Celsius, the following fusion reactions are believed to take place: FIGURE 23.14 Nuclear fusion
keeps the temperature in the interior of the sun at about 15 million °C. 1
1H 88n 3He
1 88n 2H
1 Because fusion reactions take place only at very high temperatures, they are often
called thermonuclear reactions.
FUSION REACTORS A major concern in choosing the proper nuclear fusion process for energy production
is the temperature necessary to carry out the process. Some promising reactions are
3 Li 2
1H 88n 1H
H 88n 2He 1 n
1H 88n 2 2He ENERGY RELEASED 6.3
10 13 J
J 12 These reactions take place at extremely high temperatures, on the order of 100 million
degrees Celsius, to overcome the repulsive forces between the nuclei. The first reaction
is particularly attractive because the world’s supply of deuterium is virtually
inexhaustible. The total volume of water on Earth is about 1.5 1021 L. Since the
natural abundance of deuterium is 1.5 10 2 percent, the total amount of deuterium
present is roughly 4.5 1021 g, or 5.0 1015 tons. The cost of preparing deuterium
is minimal compared with the value of the energy released by the reaction.
In contrast to the fission process, nuclear fusion looks like a very promising energy source, at least “on paper.” Although thermal pollution would be a problem, fusion has the following advantages: (1) The fuels are cheap and almost inexhaustible
and (2) the process produces little radioactive waste. If a fusion machine were turned
off, it would shut down completely and instantly, without any danger of a meltdown.
If nuclear fusion is so great, why isn’t there even one fusion reactor producing
energy? Although we command the scientific knowledge to design such a reactor, the
technical difficulties have not yet been solved. The basic problem is finding a way to
hold the nuclei together long enough, and at the appropriate temperature, for fusion to Back Forward Main Menu TOC Study Guide TOC Textbook Website MHHE Website 924 NUCLEAR CHEMISTRY FIGURE 23.15 A magnetic
plasma confinement design called
tokamak. Plasma Magnet occur. At temperatures of about 100 million degrees Celsius, molecules cannot exist,
and most or all of the atoms are stripped of their electrons. This state of matter, a
gaseous mixture of positive ions and electrons, is called plasma. The problem of containing this plasma is a formidable one. What solid container can exist at such temperatures? None, unless the amount of plasma is small; but then the solid surface would
immediately cool the sample and quench the fusion reaction. One approach to solving
this problem is to use magnetic confinement. Since a plasma consists of charged particles moving at high speeds, a magnetic field will exert force on it. As Figure 23.15
shows, the plasma moves through a doughnut-shaped tunnel, confined by a complex
magnetic field. Thus the plasma never comes in contact with the walls of the container.
Another promising design employs high-power lasers to initiate the fusion reaction. In test runs a number of laser beams transfer energy to a small fuel pellet, heating it and causing it to implode, that is, to collapse inward from all sides and compress
into a small volume (Figure 23.16). Consequently, fusion occurs. Like the magnetic
confinement approach, laser fusion presents a number of technical difficulties that still
need to be overcome before it can be put to practical use on a large scale.
THE HYDROGEN BOMB The technical problems inherent in the design of a nuclear fusion reactor do not affect
the production of a hydrogen bomb, also called a thermonuclear bomb. In this case the
objective is all power and no control. Hydrogen bombs do not contain gaseous hydrogen or gaseous deuterium; they contain solid lithium deuteride (LiD), which can be
packed very tightly. The detonation of a hydrogen bomb occurs in two stages—first a
fission reaction and then a fusion reaction. The required temperature for fusion is
achieved with an atomic bomb. Immediately after the atomic bomb explodes, the following fusion reactions occur, releasing vast amounts of energy (Figure 23.17):
1H Back Forward Main Menu TOC 2
1H 88n 2 4
88n 3 H
1 Study Guide TOC 1
1H Textbook Website MHHE Website 23.6 NUCLEAR FUSION 925 FIGURE 23.16 This small-scale
fusion reaction was created at
the Lawrence Livermore National
Laboratory using the world’s most
powerful laser, Nova. There is no critical mass in a fusion bomb, and the force of the explosion is limited only by the quantity of reactants present. Thermonuclear bombs are described as
being “cleaner” than atomic bombs because the only radioactive isotopes they produce
are tritium, which is a weak -particle emitter (t 1 12.5 yr), and the products of the
fission starter. Their damaging effects on the environment can be aggravated, however,
by incorporating in the construction some nonfissionable material such as cobalt. Upon
bombardment by neutrons, cobalt-59 is converted to cobalt-60, which is a very strong
-ray emitter with a half-life of 5.2 yr. The presence of radioactive cobalt isotopes in
the debris or fallout from a thermonuclear explosion would be fatal to those who survived the initial blast. FIGURE 23.17 Explosion of a
thermonuclear bomb. Back Forward Main Menu TOC Study Guide TOC Textbook Website MHHE Website 926 NUCLEAR CHEMISTRY 23.7 USES OF ISOTOPES Radioactive and stable isotopes alike have many applications in science and medicine.
We have previously described the use of isotopes in the study of reaction mechanisms
(see Section 13.5) and in dating artifacts (p. 527 and Section 23.3). In this section we
will discuss a few more examples.
STRUCTURAL DETERMINATION The formula of the thiosulfate ion is S2O2 . For some years chemists were uncertain
as to whether the two sulfur atoms occupied equivalent positions in the ion. The thiosulfate ion is prepared by treatment of the sulfite ion with elemental sulfur:
3 S(s) 88n S2O2 (aq)
3 When thiosulfate is treated with dilute acid, the reaction is reversed. The sulfite ion is
reformed and elemental sulfur precipitates:
S2O2 (aq) 88n SO2 (aq)
3 S(s) (23.2) If this sequence is started with elemental sulfur enriched with the radioactive sulfur35 isotope, the isotope acts as a “label” for S atoms. All the labels are found in the sulfur precipitate in Equation (23.2); none of them appears in the final sulfite ions. Clearly,
then, the two atoms of sulfur in S2O2 are not structurally equivalent, as would be the
case if the structure were
QQQQQ 2 Otherwise, the radioactive isotope would be present in both the elemental sulfur precipitate and the sulfite ion. Based on spectroscopic studies, we now know that the structure of the thiosulfate ion is
SOO S OOS
O 2 STUDY OF PHOTOSYNTHESIS The study of photosynthesis is also rich with isotope applications. The overall photosynthesis reaction can be represented as
6CO2 6H2O 88n C6H12O6 6O2 18 In Section 13.5 we learned that the O isotope was used to determine the source of
O2. The radioactive 14C isotope helped to determine the path of carbon in photosynthesis. Starting with 14CO2, it was possible to isolate the intermediate products during
photosynthesis and measure the amount of radioactivity of each carbon-containing compound. In this manner the path from CO2 through various intermediate compounds to
carbohydrate could be clearly charted. Isotopes, especially radioactive isotopes that
are used to trace the path of the atoms of an element in a chemical or biological
process, are called tracers. Back Forward Main Menu TOC Study Guide TOC Textbook Website MHHE Website 23.7 USES OF ISOTOPES 927 ISOTOPES IN MEDICINE Technetium was the first artificially prepared element. Tracers are used also for diagnosis in medicine. Sodium-24 (a emitter with a halflife of 14.8 h) injected into the bloodstream as a salt solution can be monitored to trace
the flow of blood and detect possible constrictions or obstructions in the circulatory
system. Iodine-131 (a emitter with a half-life of 8 days) has been used to test the activity of the thyroid gland. A malfunctioning thyroid can be detected by giving the patient a drink of a solution containing a known amount of Na131I and measuring the radioactivity just above the thyroid to see if the iodine is absorbed at the normal rate. Of
course, the amounts of radioisotope used in the human body must always be kept small;
otherwise, the patient might suffer permanent damage from the high-energy radiation.
Another radioactive isotope of iodine, iodine-123 (a -ray emitter), is used to image
the brain (Figure 23.18).
Technetium is one of the most useful elements in nuclear medicine. Although technetium is a transition metal, all its isotopes are radioactive. Therefore, technetium does
not occur naturally on Earth. In the laboratory it is prepared by the nuclear reactions
42 Mo 1
0n 88n 99 Mo
42 Mo 88n 99m Tc
1 where the superscript m denotes that the technetium-99 isotope is produced in its excited nuclear state. This isotope has a half-life of about 6 hours, decaying by radiation to technetium-99 in its nuclear ground state. Thus it is a valuable diagnostic tool.
The patient either drinks or is injected with a solution containing 99mTc. By detecting
the rays emitted by 99mTc, doctors can obtain images of organs such as the heart,
liver, and lungs.
A major advantage of using radioactive isotopes as tracers is that they are easy to
detect. Their presence even in very small amounts can be detected by photographic
techniques or by devices known as counters. Figure 23.19 is a diagram of a Geiger
counter, an instrument widely used in scientific work and medical laboratories to detect radiation. FIGURE 23.18 A compound
labeled with iodine-123 is used
to image the brain. Left: A normal
brain. Right: The brain of a patient with Alzheimer’s disease. Back Forward Main Menu TOC Study Guide TOC Textbook Website MHHE Website 928 NUCLEAR CHEMISTRY FIGURE 23.19 Schematic diagram of a Geiger counter.
Radiation ( , , or rays) entering through the window ionized
the argon gas to generate a
small current flow between the
electrodes. This current is amplified and is used to flash a light
or operate a counter with a clicking sound. Cathode Anode Insulator Window Argon gas Amplifier and counter High voltage 23.8 BIOLOGICAL EFFECTS OF RADIATION In this section we will examine briefly the effects of radiation on biological systems.
But first let us define quantitative measures of radiation. The fundamental unit of radioactivity is the curie (Ci); 1 Ci corresponds to exactly 3.70 1010 nuclear disintegrations per second. This decay rate is equivalent to that of 1 g of radium. A millicurie
(mCi) is one-thousandth of a curie. Thus, 10 mCi of a carbon-14 sample is the quantity that undergoes
(10 10 3)(3.70 1010) 3.70 108 disintegrations per second. The intensity of radiation depends on the number of disintegrations as well as on the energy and type of radiation emitted. One common unit
for the absorbed dose of radiation is the rad (radiation absorbed dose), which is the
amount of radiation that results in the absorption of 1 10 5 J per gram of irradiated
material. The biological effect of radiation depends on the part of the body irradiated
and the type of radiation. For this reason the rad is often multiplied by a factor called
RBE (relative biological effectiveness). The product is called a rem (roentgen equivalent for man):
1 rem 1 rad 1 RBE Of the three types of nuclear radiation, particles usually have the least penetrating
power. Beta particles are more penetrating than particles, but less so than rays.
Gamma rays have very short wavelengths and high energies. Furthermore, since they
carry no charge, they cannot be stopped by shielding materials as easily as and
particles. However, if or emitters are ingested, their damaging effects are greatly
aggravated because the organs will be constantly subject to damaging radiation at close
range. For example, strontium-90, a emitter, can replace calcium in bones, where it
does the greatest damage.
Table 23.6 lists the average amounts of radiation an American receives every year.
It should be pointed out that for short-term exposures to radiation, a dosage of 50 –
200 rem will cause a decrease in white blood cell counts and other complications, while Back Forward Main Menu TOC Study Guide TOC Textbook Website MHHE Website 23.8 BIOLOGICAL EFFECTS OF RADIATION 929 TABLE 23.6 Average Yearly Radiation Doses
SOURCE DOSE (mrem/yr)* Cosmic rays
Ground and surroundings
Medical and dental X rays
Fallout from weapons tests
*1 mrem 1 millirem 1 10 3 20 – 50
50 – 75
133 – 188 rem. † The radioactivity in the body comes from food and air. a dosage of 500 rem or greater may result in death within weeks. Current safety standards permit nuclear workers to be exposed to no more than 5 rem per year and specify a maximum of 0.5 rem of human-made radiation per year for the general public.
The chemical basis of radiation damage is that of ionizing radiation. Radiation of
either particles or rays can remove electrons from atoms and molecules in its path,
leading to the formation of ions and radicals. Radicals (also called free radicals) are
molecular fragments having one or more unpaired electrons; they are usually shortlived and highly reactive. For example, when water is irradiated with rays, the following reactions take place:
H2O 88888n H2O H2O H2O 88n H3O e
hydroxyl radical The electron (in the hydrated form) can subsequently react with water or with a hydrogen ion to form atomic hydrogen, and with oxygen to produce the superoxide ion,
O2 (a radical):
e Chromosomes are the parts of the
cell that contain the genetic
material (DNA). Back Forward Main Menu O2 88n O2 In the tissues the superoxide ions and other free radicals attack cell membranes and a
host of organic compounds, such as enzymes and DNA molecules. Organic compounds
can themselves be directly ionized and destroyed by high-energy radiation.
It has long been known that exposure to high-energy radiation can induce cancer
in humans and other animals. Cancer is characterized by uncontrolled cellular growth.
On the other hand, it is also well established that cancer cells can be destroyed by
proper radiation treatment. In radiation therapy, a compromise is sought. The radiation
to which the patient is exposed must be sufficient to destroy cancer cells without killing
too many normal cells and, it is hoped, without inducing another form of cancer.
Radiation damage to living systems is generally classified as somatic or genetic.
Somatic injuries are those that affect the organism during its own lifetime. Sunburn,
skin rash, cancer, and cataracts are examples of somatic damage. Genetic damage means
inheritable changes or gene mutations. For example, a person whose chromosomes
have been damaged or altered by radiation may have deformed offspring. TOC Study Guide TOC Textbook Website MHHE Website 930 NUCLEAR CHEMISTRY Chemistry in Action Chemistry in Action Chemistry in Action Chemistry in Action Chemistry in Action Chemistry in Action Chemistry in Action Chemistry in Action Chemistry in Action Chemistry in Action Chemistry in Action Chemistry in Action Chemistry Back Food Irradiation
If you eat processed food, you have probably eaten
ingredients exposed to radioactive rays. In the United
States, up to 10 percent of herbs and spices are irradiated to control mold, zapped with X rays at a
dose equal to 60 million chest X rays. Although food
irradiation has been used in one way or another for
more than 40 years, it faces an uncertain future in this
Back in 1953 the U.S. Army started an experimental program of food irradiation so that deployed
troops could have fresh food without refrigeration. The
procedure is a simple one. Food is exposed to high
levels of radiation to kill insects and harmful bacteria.
It is then packaged in airtight containers, in which it
can be stored for months without deterioration. The
radiation sources for most food preservation are
cobalt-60 and cesium-137, both of which are emitters, although X rays and electron beams can also be
used to irradiate food.
The benefits of food irradiation are obvious — it
reduces energy demand by eliminating the need for
refrigeration, and it prolongs the shelf life of various
foods, which is of vital importance for poor countries.
Yet there is considerable opposition to this procedure.
First, there is a fear that irradiated food may itself become radioactive. No such evidence has been found.
A more serious objection is that irradiation can de- Strawberries irradiated at 200 kilorads (right) are still fresh
after 15 days’ storage at 4°C; those not irradiated are
moldy. stroy the nutrients such as vitamins and amino acids.
Furthermore, the ionizing radiation produces reactive
species, such as the hydroxyl radical, which then react with the organic molecules to produce potentially
harmful substances. Interestingly, the same effects are
produced when food is cooked by heat. Food Irradiation Dosages and Their Effects†
EFFECT Low dose (Up to 100 kilorad) Medium dose (100–1000 kilorads) High dose (1000 to 10,000 kilorads) Inhibits sprouting of potatoes, onions, garlics.
Inactivates trichinae in pork.
Kills or prevents insects from reproducing in grains, fruits, and vegetables
Delays spoilage of meat, poultry and fish by killing spoilage microorganism.
Reduces salmonella and other food-borne pathogens in meat, fish, and
Extends shelf life by delaying mold growth on strawberries and some other
Sterilizes meat, poultry, fish, and some other foods.
Kills microorganisms and insects in spices and seasoning. † Source: Chemical & Engineering News, May 5 (1986). Forward Main Menu TOC Study Guide TOC Textbook Website MHHE Website QUESTIONS AND PROBLEMS 931 KEY EQUATION • SUMMARY OF FACTS
AND CONCEPTS 1. For stable nuclei of low atomic number, the neutron-to-proton ratio is close to 1. For
heavier stable nuclei, the ratio becomes greater than 1. All nuclei with 84 or more protons
are unstable and radioactive. Nuclei with even atomic numbers tend to have a greater
number of stable isotopes than those with odd atomic numbers.
2. Nuclear binding energy is a quantitative measure of nuclear stability. Nuclear binding energy can be calculated from a knowledge of the mass defect of the nucleus.
3. Radioactive nuclei emit particles, particles, positrons, or rays. The equation for a
nuclear reaction includes the particles emitted, and both the mass numbers and the atomic
numbers must balance.
4. Uranium-238 is the parent of a natural radioactive decay series that can be used to determine the ages of rocks.
5. Artificial radioactive elements are created by bombarding other elements with accelerated
neutrons, protons, or particles.
6. Nuclear fission is the splitting of a large nucleus into two smaller nuclei and one or more
neutrons. When the free neutrons are captured efficiently by other nuclei, a chain reaction
7. Nuclear reactors use the heat from a controlled nuclear fission reaction to produce power.
The three important types of reactors are light water reactors, heavy water reactors, and
8. Nuclear fusion, the type of reaction that occurs in the sun, is the combination of two light
nuclei to form one heavy nucleus. Fusion takes place only at very high temperatures, so
high that controlled large-scale nuclear fusion has so far not been achieved.
9. Radioactive isotopes are easy to detect and thus make excellent tracers in chemical reactions and in medical practice.
10. High-energy radiation damages living systems by causing ionization and the formation of
free radicals. E ( m)c2 (23.1) Relation between mass defect and energy released. KEY WORDS
Breeder reactor, p. 920
Critical mass, p. 917
Mass defect, p. 908
Moderators, p. 918 Nuclear
Nuclear binding energy, p. 908
chain reaction, p. 917
fission, p. 916
fusion, p. 923 Nuclear transmutation, p. 904
Plasma, p. 924
Positron, p. 905
Radical, p. 929 Radioactive decay series, p. 911
Thermonuclear reaction, p. 923
Tracer, p. 926
Transuranium elements, p. 916 QUESTIONS AND PROBLEMS
Review Questions 23.1 How do nuclear reactions differ from ordinary chemical reactions?
23.2 What are the steps in balancing nuclear equations?
23.3 What is the difference between 0e and 0 ?
23.4 What is the difference between an electron and a
Problems 23.5 Complete the following nuclear equations and identify X in each case: Back Forward Main Menu TOC X
(a) 26 Mg 1p 88n 4
(b) 59 Co 2H 88n 60 Co X
(c) 235 U 1 n 88n 94 Kr 139 Ba 3X
(d) 53Cr 4 88n 1 n X
(e) 20 O 88n 20 F X
23.6 Complete the following nuclear equations and identify X in each case:
(a) 135 I 88n 135 Xe X
(b) 40 K 88n 0
(c) 59 Co 1 n 88n 56 Mn X
25 Study Guide TOC Textbook Website MHHE Website 932 NUCLEAR CHEMISTRY (d) 235
92 U 1
0n 88n 99 Sr
52 Te Problems 2X 23.23 Fill in the blanks in the following radioactive decay
series: NUCLEAR STABILITY
Review Questions (a) 23.7 State the general rules for predicting nuclear stability.
23.8 What is the belt of stability?
23.9 Why is it impossible for the isotope 2 He to exist?
23.10 Define nuclear binding energy, mass defect, and nucleon.
23.11 How does Einstein’s equation, E mc2, allow us to
calculate nuclear binding energy?
23.12 Why is it preferable to use nuclear binding energy
per nucleon for a comparison of the stabilities of different nuclei? (b) H(g) H(g) 88n H2(g) H° 436.4 kJ calculate the change in mass (in kg) per mole of H2
23.18 Estimates show that the total energy output of the sun
is 5 1026 J/s. What is the corresponding mass loss
in kg/s of the sun?
23.19 Calculate the nuclear binding energy (in J) and the
binding energy per nucleon of the following isotopes:
(a) 7 Li (7.01600 amu) and (b) 35 Cl (34.95952 amu).
23.20 Calculate the nuclear binding energy (in J) and the
binding energy per nucleon of the following isotopes:
(a) 4 He (4.0026 amu) and (b) 184 W (183.9510 amu).
74 Th 88n _____ 88n _____ 88n 228Th 235 U 88n _____ 88n _____ 88n 227Ac (c) _____ 88n 233Pa 88n _____ 88n _____
23.24 A radioactive substance undergoes decay as follows:
TIME (DAYS) 23.25 23.26 23.27
23.28 23.29 23.30 MASS (g) 0
6 Problems 23.13 The radius of a uranium-235 nucleus is about 7.0
10 3 pm. Calculate the density of the nucleus in
g/cm3. (Assume the atomic mass is 235 amu.)
23.14 For each pair of isotopes listed, predict which one is
less stable: (a) 6 Li or 9 Li, (b) 23 Na or 25 Na,
(c) 48 Ca or 48 Sc.
23.15 For each pair of elements listed, predict which one
has more stable isotopes: (a) Co or Ni, (b) F or Se,
(c) Ag or Cd.
23.16 In each pair of isotopes shown, indicate which one
you would expect to be radioactive: (a) 20 Ne and
10 Ne, (b) 20 Ca and 20 Ca, (c) 42 Mo and 43 Tc,
(d) 80 Hg and 80 Hg, (e) 83 Bi and 96 Cm.
23.17 Given that 232 500
112 Calculate the first-order decay constant and the halflife of the reaction.
The radioactive decay of T1-206 to Pb-206 has a halflife of 4.20 min. Starting with 5.00 1022 atoms of
T1-206, calculate the number of such atoms left after 42.0 min.
A freshly isolated sample of 90Y was found to have
an activity of 9.8 105 disintegrations per minute at
1:00 P.M. on December 3, 1992. At 2:15 P.M. on
December 17, 1992, its activity was redetermined and
found to be 2.6 104 disintegrations per minute.
Calculate the half-life of 90Y.
Why do radioactive decay series obey first-order kinetics?
In the thorium decay series, thorium-232 loses a total of 6 particles and 4 particles in a 10-stage
process. What is the final isotope produced?
Strontium-90 is one of the products of the fission of
uranium-235. This strontium isotope is radioactive,
with a half-life of 28.1 yr. Calculate how long (in yr)
it will take for 1.00 g of the isotope to be reduced to
0.200 g by decay.
Consider the decay series
A 88n B 88n C 88n D NATURAL RADIOACTIVITY
Review Questions 23.21 Discuss factors that lead to nuclear decay.
23.22 Outline the principle for dating materials using radioactive isotopes. Back Forward Main Menu TOC where A, B, and C are radioactive isotopes with halflives of 4.50 s, 15.0 days, and 1.00 s, respectively,
and D is nonradioactive. Starting with 1.00 mole of
A, and none of B, C, or D, calculate the number of
moles of A, B, C, and D left after 30 days. Study Guide TOC Textbook Website MHHE Website 933 QUESTIONS AND PROBLEMS NUCLEAR TRANSMUTATION USES OF ISOTOPES Review Questions Problems 23.31 What is the difference between radioactive decay and
23.32 How is nuclear transmutation achieved in practice? 23.47 Describe how you would use a radioactive iodine isotope to demonstrate that the following process is in
PbI2(s) 34 Pb2 (aq) Problems 23.33 Write balanced nuclear equations for the following
reactions and identify X:
(a) X(p, )12C, (b) 27Al(d, )X, (c) 55Mn(n, )X
23.34 Write balanced nuclear equations for the following
reactions and identify X:
(a) 80Se(d,p)X, (b) X(d,2p)9Li, (c) 10B(n, )X
23.35 Describe how you would prepare astatine-211, starting with bismuth-209.
23.36 A long-cherished dream of alchemists was to produce
gold from cheaper and more abundant elements. This
dream was finally realized when 198Hg was converted
into gold by neutron bombardment. Write a balanced
equation for this reaction.
Review Questions 23.37 Define nuclear fission, nuclear chain reaction, and
23.38 Which isotopes can undergo nuclear fission?
23.39 Explain how an atomic bomb works.
23.40 Explain the functions of a moderator and a control
rod in a nuclear reactor.
23.41 Discuss the differences between a light water and a
heavy water nuclear fission reactor. What are the advantages of a breeder reactor over a conventional nuclear fission reactor?
23.42 No form of energy production is without risk. Make
a list of the risks to society involved in fueling and
operating a conventional coal-fired electric power
plant, and compare them with the risks of fueling and
operating a nuclear fission-powered electric plant.
Review Questions 23.43 Define nuclear fusion, thermonuclear reaction, and
23.44 Why do heavy elements such as uranium undergo fission while light elements such as hydrogen and
lithium undergo fusion?
23.45 How does a hydrogen bomb work?
23.46 What are the advantages of a fusion reactor over a
fission reactor? What are the practical difficulties in
operating a large-scale fusion reactor? Back Forward Main Menu TOC 2I (aq) 23.48 Consider the following redox reaction:
IO4 (aq) 2I (aq) H2O(l) 88n
I2(s) IO3 (aq) 2OH (aq) When KIO4 is added to a solution containing iodide
ions labeled with radioactive iodine-128, all the radioactivity appears in I2 and none in the IO3 ion.
What can you deduce about the mechanism for the
23.49 Explain how you might use a radioactive tracer to
show that ions are not completely motionless in crystals.
23.50 Each molecule of hemoglobin, the oxygen carrier in
blood, contains four Fe atoms. Explain how you
would use the radioactive 59 Fe (t 1 46 days) to show
that the iron in a certain food is converted into hemoglobin.
ADDITIONAL PROBLEMS 23.51 How does a Geiger counter work?
23.52 Nuclei with an even number of protons and an even
number of neutrons are more stable than those with
an odd number of protons and/or an odd number of
neutrons. What is the significance of the even numbers of protons and neutrons in this case?
23.53 Tritium, 3H, is radioactive and decays by electron
emission. Its half-life is 12.5 yr. In ordinary water the
ratio of 1H to 3H atoms is 1.0 1017 to 1.
(a) Write a balanced nuclear equation for tritium decay. (b) How many disintegrations will be observed
per minute in a 1.00-kg sample of water?
23.54 (a) What is the activity, in millicuries, of a 0.500-g
sample of 237 Np? (This isotope decays by -particle
emission and has a half-life of 2.20 106 yr.) (b)
Write a balanced nuclear equation for the decay of
23.55 The following equations are for nuclear reactions that
are known to occur in the explosion of an atomic
bomb. Identify X.
(a) 235 U 1 n 88n 140 Ba 31 n X
(b) 235 U 1 n 88n 144 Cs 90 Rb 2X
(c) 235 U 1 n 88n 87 Br 31 n X
(d) 235 U 1 n 88n 160 Sm 72 Zn 4X
23.56 Calculate the nuclear binding energies, in J/nucleon,
for the following species: (a) 10B (10.0129 amu), Study Guide TOC Textbook Website MHHE Website 934 NUCLEAR CHEMISTRY 23.57 23.58 23.59
23.61 23.62 23.63 23.64 23.65 23.66 23.67 Back (b) 11B (11.00931 amu), (c) 14N (14.00307 amu), (d)
Fe (55.9349 amu).
Write complete nuclear equations for the following
processes: (a) tritium, 3H, undergoes
(b) 242Pu undergoes -particle emission; (c) 131I undergoes decay; (d) 251Cf emits an particle.
The nucleus of nitrogen-18 lies above the stability
belt. Write an equation for a nuclear reaction by
which nitrogen-18 can achieve stability.
Why is strontium-90 a particularly dangerous isotope
How are scientists able to tell the age of a fossil?
After the Chernobyl accident, people living close to
the nuclear reactor site were urged to take large
amounts of potassium iodide as a safety precaution.
What is the chemical basis for this action?
Astatine, the last member of Group 7A, can be prepared by bombarding bismuth-209 with particles.
(a) Write an equation for the reaction. (b) Represent
the equation in the abbreviated form as discussed in
To detect bombs that may be smuggled onto airplanes, the Federal Aviation Administration (FAA)
will soon require all major airports in the United
States to install thermal neutron analyzers. The thermal neutron analyzer will bombard baggage with
low-energy neutrons, converting some of the nitrogen-14 nuclei to nitrogen-15, with simultaneous
emission of rays. Because nitrogen content is usually high in explosives, detection of a high dosage of
rays will suggest that a bomb may be present.
(a) Write an equation for the nuclear process.
(b) Compare this technique with the conventional Xray detection method.
Explain why achievement of nuclear fusion in the laboratory requires a temperature of about 100 million
degrees Celsius, which is much higher than that in
the interior of the sun (15 million degrees Celsius).
Tritium contains one proton and two neutrons. There
is no proton-proton repulsion present in the nucleus.
Why, then, is tritium radioactive?
The carbon-14 decay rate of a sample obtained from
a young tree is 0.260 disintegration per second per
gram of the sample. Another wood sample prepared
from an object recovered at an archaeological excavation gives a decay rate of 0.186 disintegration per
second per gram of the sample. What is the age of
The usefulness of radiocarbon dating is limited to objects no older than 50,000 years. What percent of the
carbon-14, originally present in the sample, remains
after this period of time? Forward Main Menu TOC 23.68 The radioactive potassium-40 isotope decays to argon-40 with a half-life of 1.2 109 yr. (a) Write a
balanced equation for the reaction. (b) A sample of
moon rock is found to contain 18 percent potassium40 and 82 percent argon by mass. Calculate the age
of the rock in years.
23.69 Both barium (Ba) and radium (Ra) are members of
Group 4A and are expected to exhibit similar chemical properties. However, Ra is not found in barium
ores. Instead, it is found in uranium ores. Explain.
23.70 Nuclear waste disposal is one of the major concerns
of the nuclear industry. In choosing a safe and stable
environment to store nuclear wastes, consideration
must be given to the heat released during nuclear dedecay of 90Sr
cay. As an example, consider the
38Sr The 90 88n 90Y
39 23.72 23.73 23.74 23.75 t1
2 28.1 yr Y (89.907152 amu) further decays as follows:
39Y 23.71 0
1 88n 90Zr
2 64 h Zirconium-90 (89.904703 amu) is a stable isotope.
(a) Use the mass defect to calculate the energy released (in joules) in each of the above two decays.
(The mass of the electron is 5.4857 10 4 amu.) (b)
Starting with one mole of 90Sr, calculate the number
of moles of 90Sr that will decay in a year. (c) Calculate
the amount of heat released (in kilojoules) corresponding to the number of moles of 90Sr decayed to
Zr in (b).
Which of the following poses a greater health hazard: A radioactive isotope with a short half-life or a
radioactive isotope with a long half-life? Explain.
[Assume same type of radiation ( or ) and comparable energetics per particle emitted.]
As a result of being exposed to the radiation released
during the Chernobyl nuclear accident, the dose of
iodine-131 in a person’s body is 7.4 mC (1 mC
N to cal1 10 3 Ci). Use the relationship rate
culate the number of atoms of iodine-131 this radioactivity corresponds. (The half-life of I-131 is
Referring to the Chemistry in Action essay on p. 930,
why is it highly unlikely that irradiated food would
From the definition of curie, calculate Avogadro’s
number. Given that the molar mass of Ra-226 is
226.03 g/mol and that it decays with a half life of
1.6 103 yr.
Since 1994, elements 110, 111, and 112 have been
synthesized. Element 110 was created by bombarding 208Pb with 62Ni; element 111 was created by bom- Study Guide TOC Textbook Website MHHE Website QUESTIONS AND PROBLEMS 23.76 23.77 23.78 23.79 23.80 Back barding 209Bi with 64Ni; and element 112 was created by bombarding 208Pb with 66Zn. Write an equation for each synthesis. Predict the chemical properties of these elements. Use X for element 110, Y for
element 111, and Z for element 112.
Sources of energy on Earth include fossil fuels, geothermal, gravitational, hydroelectric, nuclear fission,
nuclear fusion, solar, wind. Which of these have a
“nuclear origin,” either directly or indirectly?
A person received an anonymous gift of a decorative
cube which he placed on his desk. A few months later
he became ill and died shortly afterward. After investigation, the cause of his death was linked to the
box. The box was air-tight and had no toxic chemicals on it. What might have killed the man?
Identify two of the most abundant radioactive elements that exist on Earth. Explain why they are still
present? (You may need to consult a handbook of
(a) Calculate the energy released when an U-238 isotope decays to Th-234. The atomic masses are given
by: U-238: 238.0508 amu; Th-234: 234.0436 amu;
He-4: 4.0026 amu. (b) The energy released in (a) is
transformed into the kinetic energy of the recoiling
Th-234 nucleus and the particle. Which of the two
will move away faster? Explain.
Cobalt-60 is an isotope used in diagnostic medicine
and cancer treatment. It decays with ray emission.
Calculate the wavelength of the radiation in nanometers if the energy of the ray is 2.4 10 13 J/photon. Forward Main Menu TOC 935 23.81 Am-241 is used in smoke detectors because it has a
long half-life (458 yr) and its emitted particles are
energetic enough to ionize air molecules. Given the
schematic diagram of a smoke detector below, explain how it works.
Current 241Am Battery 23.82 The constituents of wine contain, among others, carbon, hydrogen, and oxygen atoms. A bottle of wine
was sealed about 6 years ago. To confirm its age,
which of the isotopes would you choose in a radioactive dating study? The half-lives of the isotopes
are: 13C: 5730 yr; 15O: 124 s; 3H: 12.5 yr. Assume
that the activities of the isotopes were known at the
time the bottle was sealed.
23.83 Name two advantages of a nuclear-powered submarine over a conventional submarine.
23.1 78 Se. 23.2 2.63
J/nucleon. 23.3 106 Pd 4 88n
2 Answers to Practice Exercises: 10 10 J; 1.26
1 p. Study Guide TOC 10 12 Textbook Website MHHE Website C HEMICAL M YSTERY The Art Forgery of the Century H an van Meegeren must be one of the few forgers ever to welcome technical analysis of his work. In 1945 he was captured by the Dutch police and accused of selling a painting by the Dutch artist Jan Vermeer
(1632 – 1675) to Nazi Germany. This was a crime punishable by death.
Van Meegeren claimed that not only was the painting in question, entitled The Woman Taken in Adultery, a forgery, but he had also produced other
To prove his innocence, van Meegeren created another Vermeer to demonstrate
his skill at imitating the Dutch master. He was acquitted of charges of collaboration
with the enemy, but was convicted of forgery. He died of a heart attack before he could
serve the one-year sentence. For twenty years after van Meegeren’s death art scholars
debated whether at least one of his alleged works, Christ and His Disciples at Emmaus,
was a fake or a real Vermeer. The mystery was solved in 1968 using a radiochemical
White lead — lead hydroxy carbonate [Pb3(OH)2(CO3)2] — is a pigment used by
artists for centuries. The metal in the compound is extracted from its ore, galena (PbS),
which contains uranium and its daughter products in radioactive equilibrium with it.
By radioactive equilibrium we mean that a particular isotope along the decay series is
formed from its precursor as fast as it breaks down by decay, and so its concentration
(and its radioactivity) remains constant with time. This radioactive equilibrium is disturbed in the chemical extraction of lead from its ore. Two isotopes in the uranium decay series are of particular importance in this process: 226Ra (t 1 1600 yr) and 210Pb
(t 1 21 yr). (See Table 23.3.) Most 226Ra is removed during the extraction of lead
from its ore, but 210Pb eventually ends up in the white lead, along with the stable isotope of lead (206Pb). No longer supported by its relatively long-lived ancestor, 226Ra,
Pb begins to decay without replenishment. This process continues until the 210Pb
activity is once more in equilibrium with the much smaller quantity of 226Ra that survived the separation process. Assuming the concentration ratio of 210Pb to 226Ra is
100:1 in the sample after extraction, it would take 270 years to reestablish radioactive
equilibrium for 210Pb.
If Vermeer did paint Emmaus around the mid-seventeenth century, the radioactive
equilibrium would have been restored in the white lead pigment by 1960. But this was
not the case. Radiochemical analysis showed that the paint used was less than one hundred years old. Therefore, the painting could not have been the work of Vermeer. 936 Back Forward Main Menu TOC Study Guide TOC Textbook Website MHHE Website “Christ and His Disciples at Emmaus,” a
painting attributed to Han van Meegeren. CHEMICAL CLUES 1.
2. Write equations for the decay of 226Ra and 210Pb.
Consider the following consecutive decay series:
A 88n B 88n C where A and B are radioactive isotopes and C is a stable isotope. Given that the
half-life of A is 100 times that of B, plot the concentrations of all three species versus time on the same graph. If only A was present initially, which species would
reach radioactive equilibrium?
3. The radioactive decay rates for 210Pb and 226Ra in white lead paint taken from
Emmaus in 1968 were 8.5 and 0.8 disintegrations per minute per gram of lead
(dpm/g), respectively. (a) How many half lives of 210Pb had elapsed between 1660
and 1968? (b) If Vermeer had painted Emmaus, what would have been the decay
rate of 210Pb in 1660? Comment on the reasonableness of this rate value.
4. To make his forgeries look authentic, van Meegeren re-used canvases of old paintings. He rolled one of his paintings to create cracks in the paint to resemble old
works. X-ray examination of this painting showed not only the underlying painting, but also the cracks in it. How did this discovery reveal to the scientists that
the painting on top was of a more recent origin? 937 Back Forward Main Menu TOC Study Guide TOC Textbook Website MHHE Website ...
View Full Document