812 CHAPTER 20 Nuclear Chemistry Figure 20.9 If a critical mass is present, many of the neutrons emitted during the fission process will be captured by other 23SU nuclei and a chai
Trang 1812 CHAPTER 20 Nuclear Chemistry
Figure 20.9 If a critical mass is
present, many of the neutrons emitted
during the fission process will be
captured by other 23SU nuclei and a
chain reaction will occur
Subcritical U-235 mass
Figure 20.10 Schematic
diagram of an atomic bomb The
TNT exp lo s ive s are set off fir s t
The explosion forces the sections of
fi ss ionable material together to form
an amount considerably larger than the
to the surro unding s Figure 20.9 s how s two types of fission reactions For a chain reaction to ' occur, enough uranium-235 must be present in the samp le to capture the neutron s Otherwise, many of the neutrons will escape from the sa mple and the chain reaction will not occur In this
s ituation the ma ss of the samp le is said to be subcr iti cal Figure 20.9 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
The first application of nuclear fission was in the development of the atomic bomb How is
s uch a bomb made and detonated ? The crucial factor in the bomb's design is the determination of the critical mass for the bomb A s mall atomic bomb is equivalent to 20,000 tons of TNT (trini- trotoluene) Because 1 ton of TNT releases about 4 X 109 J of energy, 20,000 tons would produce
8 X 1013 J Recall that 1 mole, or 235 g, of uranium-235 liberate s 2.0 X 1013 J of energy when it undergoe s fission Thus, the mass of the isotope present in a s mall bomb must be at lea s t
8 X 1013 J
= 1 kg
2.0 X 1013 J
(235 g)
An atomic bomb is never assembled with the critical ma ss already present Instead, the critical
ma ss i s formed by using a conventional explosive, s uch as TNT, to force the fissionable sec tions together, as s hown in Figure 20.10 Neutrons from a source at the center of the device trigger the nuclear chain reaction Uranium-235 was the fissionable material in the bomb dropped on Hiro-
s hima , Japan, on August 6, 1945 Plutonium-239 was u sed in the bomb exploded over Nagasaki
3 days later The fission reactions generated were similar in these two cases, as was the extent of the destruction
A peaceful but controversial application of nuclear fission i s the generation of electricity
u sing heat from a controlled c hain reaction in a nuclear reactor Currently, nuclear reactor s vide about 20 percent of the e lectri c 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 reac- tors are in operation; we will briefly discuss the main features of three of them , along with their advantages and disadvantage s
pro-Most of the nuclear reactors in the United States are li g ht water reactors Figure 20.11 is a
sc hematic diagram of s uch a reactor, and Figure 20.12 s how s the refueling process in the core of
a nuclear reactor
Trang 2SECTION 20.5 Nuclear Fission 8- 3
An important aspect of the fission process is the speed of the neutrons Slow neutron s split
uranium-235 nuclei more efficiently than do fa s t ones Because fission reaction s are highly
exo-thermic, the neutron s produced u s ually 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
neu-trons A good moderator must satisfy several requirement s : It should be nontoxic and inexpensive
(as very large quantities of it are neces sary), and it should resi st conversion into a radioactive
substance by neutron bombardment Furthermore , it i s advantageous for the moderator to be a
fluid so that it can also be used as a coolant No s ubstance fulfills all the se requireme nt s, although
water comes closer than many others that have been considered Nuclear reactors that use light
water (H20) as a moderator are called light water reactors because 1 H is the lightest isotope of
the element hydrogen
The nuclear fuel consists of uranium , usually in the form of its oxide, U30 S (Figure 20.13)
Naturally occurring uranium contains about 0.7 percent of the uranium-235 i so tope, which is
too Iowa concentration to sustain a small-scale chain reaction For effective operation of a light
Figure 20.11 Schematic di ag:r<illl
of a nuclear fis sio n reactor The fi i process is co ntrolled by cadmiu m or
-boron rods The heat generated by the
process is used to produce steam for
the generation of electricity via a hea;
exchange sys tem
Figure 20.12 Refueling the core of
Trang 3814 CHAPTER 20 Nuclear Chemistry
•
Figure 20.14 Radioactive
plutonium oxide (PU0 2 ) has a red glow
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 control rods between the fuel elements These rods capture neutrons according to the equations
1! ~ Cd + bn + I l~ Cd + 'Y
I~B + 6 n • jLi + ~a
where 'Y 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 bums fossil fuel In both cases, large quantitie s 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
Another type of nuclear reactor uses D2 0, or heavy water, as the moderator, rather than H2 0
Deuterium absorbs neutrons much less efficiently than does ordinary hydrogen Because fewer neutrons are absorbed, the reactor is more efficient and does not require enriched uranium More neutrons leak out of the reactor, too, though 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, D20 must be prepared by either fractional tillation or electrolysis of ordinary water, which can be very expensive considering the amount of
dis-water used in a nuclear reactor In countries where hydroelectric power is abundant, the cost of
producing D 2 0 by electrolysis can be reasonably low At present, Canada is the only nation cessfully 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
suc-A breeder reactor uses uranium fuel, but unlike a conventional nuclear reactor, it produces more fissionable materials than it uses
When uranium-238 is bombarded with fast neutrons, the following reactions take place:
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 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 2 § 6Th Upon capturing slow neutrons, thorium is transmuted to uranium-233, which, like uranium-235, is a fissionable isotope:
2§ 6 Th + bn + 2§~Th
2~~Th +' 2§ ~ Pa + -~f3 2§I Pa • 2 §i u+ -~f3
t 1 /2 = 22 min
t 1/ 2 = 27.4 days
Uranium-233 (t l /2 = 1.6 X 105 years) is stable enough for long-term storage
Although the amounts of uranium-238 and thorium-232 in Earth's crust are relatively
plenti-ful (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 dif-
Trang 4SECTION 20.6 Nuclear Fusion 8' :::
ficulties associated with the construction of such reactors As a result, the future of breeder
reac-tors, in the United States at least, is rather uncertain
Many people, including environmentalists, regard nuclear fission as a highly undesirable method of energy production Many fission products such as strontium-90 are dangerous radioac-
tive 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 a-emitter with a half-life of 24,400
years
Accidents, too, present many dangers An accident at the Three Mile Island reactor in sylvania in 197 9 first brought the potential hazards of nuclear plants to public attention In this
Penn-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 Ukraine 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 space technology that is
100 percent reliable
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 being debated and
questioned by both the scientific community and the general public It seems likely that the
con-troversy will continue for some time
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 20.2 showed that for the lighte st elements, nuclear stability increases with ing mass number This behavior suggests that if two light nuclei combine or fuse together to form
increas-a lincreas-arger, more stincreas-able nucleus, increas-an increas-appreciincreas-able increas-amount of energy will be releincreas-ased in the process
This is the basis for ongoing research into the harnessing of nuclear fusion for the production of
3.6 X 10 - 12 J
These reactions must 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
•
Trang 5816 CHAPTER 20 Nuclear Chemistry
Figure 20.15 A magnetic plasma
confinement design called a tokamak
Figure 20.16 This sm all- scale
fusion reaction was carried out a t
the Lawrence Livermore National
Laboratory using the world's most
attractive because the world's s upply of deuterium is virtually inexhaustible The total volume of
water on Earth is about 1.5 X 1021 L Becau se the natural abundance of deuterium is 0.015
per-cen t , the total amount of deuterium present i s roughly 4.5 X 1021 g, or 5.0 X 1015 tons Although
it is expensive to prepare deuterium, the cost i s minimal compared to 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 lea st on paper Although thermal pollution would be a problem, fusion has the following tages: (1) the fuels are cheap and almost inexhaustible and (2) the process produces little radio-
advan-active waste If a fusion machine were turned off, it would s hut down completely and instantly, without any danger of a meltd ow n
If nuclear fusion i s so great, why isn't there even one fusion reactor producing energy? Although we po ssess the scientific knowledge to design such a reactor, the technical difficulties
ha ve not yet been solved The basic problem i s finding a way to hold the nuclei together long
enough, and at the appropriate temperature, for fusion to occur At temperature s 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, i s called plasma
The problem of containing thi s plasma is a formidable one No solid container can exist at such temperatures, unle ss the amount of plasma is s mall, but then the solid surface would immedi-
ately cool the sample and quench the fusion reaction One approach to solving this problem is to
u se magnetic confinement Because plasma consists of charged particle s moving at high speeds,
a magnetic field will exert a force on it As Figure 20.15 shows, the plasma moves through a doughnut- s haped tunnel, confined by a complex magnetic field Thus, the plasma never comes in
contact with the walls of the container
Another promi s ing design employs high-power laser s to initiate the fusion reaction In test runs , a number of laser beam s transfer energy to a small fuel pellet, heating it and causing it
to impl ode that is, to collapse inward from all sides and compress into a small volume
(Fig-ure 20.16) Consequently, fusion occurs Like the magnetic confinement approach, laser fusion presents a number of technical difficulties that sti ll need to be overcome before it can be put to practical u se on a large scale
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 rium; they contain so lid lithium deuteride (LiD), which can be packed very tightly The detona-
deute-tion 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, relea s ing vast amounts of energy
(F igure 20.17):
~ Li + T H - _ I 2in:
TH + i H - - IH + :H
There is no critical ma ss in a fusion bomb, and the force of the explo s ion is limited only by the
quantity of reactants present Thermonuclear bomb s are described as being "cleaner" than atomic bombs because the only radioactive isotopes they produce are tritium, which is a weak ,B-particle
emitter (t1 / 2 = 12.5 years), and the products of the fission starte r Their damaging effects on the
environment can be aggravated, however, by incorporating in the construction some
Trang 6nonfission-SECTION 20.7 Uses of Isotopes 8 17
able material such as cobalt Upon bombardment by neutrons, cobalt-59 is converted to cobalt-60,
which is a very strong y-ray emitter with a half-life of 5.2 years 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
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 [ ~~ Section 14.5 ]
and in dating artifacts (page 806) In this section we will discuss a few more examples
Chemical Analysis
The formula of the thiosulfate ion is S20 ~- For some years, chemists were unceltain as to whether
the two sulfur atoms occupied equivalent positions in the ion The thiosulfate ion is prepared by
treating the sulfite ion with elemental sulfur:
When thiosulfate is treated with dilute acid, the reaction is reversed The sulfite ion is re-formed,
and elemental sulfur precipitates:
If this sequence is started with elemental sulfur enriched with the radioactive s ulfur-35 isotope , the
isotope acts as a "label" for S atoms All the labels are found in the sulfur precipitate; none of them
appears in the final sulfite ions As a result, the two atoms of sulfur in S20 5 - are not structurally
equivalent, as would be the case if the structure were
[Q-~-Q-~-QJ2
-If the sulfur atoms were equivalent, 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
struc-ture of the thiosulfate ion is
The study of photosynthesis is also rich with isotope applications The overall photosynthesis
reaction can be represented a s
In Section 14.5 we learned that the 18 0 isotope was used to determine the source of O 2 The
radio-active 14C isotope helped to determine the path of carbon in photosynthesis Starting with 14C02,
it was possible to isolate the intermediate products during photosynthesis and measure the amount
of radioactivity of each carbon-containing compound In thi s manner the path from CO ? 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
Isotopes in Medicine
Tracers are also used for diagnosis in medicine Sodium-24 (a f3-emitter with a half-life 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 f3-emitter
with a half-life of 8 days) has been used to test the activity of the thyroid gland A
malfunction-ing thyroid can be detected by giving the patient a drink of a so lution containing a known amount
of Nal3II and measuring the radioactivity just above the thyroid to see if the iodine is absorbed
at the normal rate Another radioactive isotope of iodine, iodine-123 (a y-ray emitter), is used to
image the brain (Figure 20.18) In each of the se cases, though, the amount of radioisotope u se d
Trang 7818 CHAPTER 20 Nuclear Chemistry
Figure 20.19 Schematic diagram
of a Geiger counter Radiation (a, {3,
or 'Y rays) entering through the window
ionize the argon gas to generate a small
current flow between the electrodes
This current is amplified and is used to
clicking sound
_- Multimedia
Nuclear Chemistry - alpha, beta, and
gamma emission (interacti v e )
•
Insulator
Amplifier and counter
where the superscript "m" denotes that the technetium-99 i sotope is produced in its excited nuclear state This isotope ha s a half-life of about 6 hours , decaying by y radiation to technetium-99 in its nuclear ground state Thus, it is a va luable diagno stic tool The patient either drinks or is injected with a sol ution containing 99 mTc By detecting the y ray s emitted by 99m Tc, doctors can obtain images of organs s uch as the heart , liver , and lungs
A major advantage of u s ing radioactive isotopes as tracers is that they are easy to detect
Their presence even in very s mall amounts can be detected by photographic techniques or by
devices known as counters Figure 20.19 is a diagram of a Geiger counter, an instrument widely
u se d in sc ientific work and medical laboratorie s to detect radiation
Biological Effects of Radiation
In thi s sect ion we will examine briefly the effects of radiation on biological systems But first we
mu s t define the quantitative measures of radiation The fundamental unit of radioactivity is the
curie (Ci); 1 Ci corresponds to exactly 3.70 X 1010 nuclear di s integrations per second This decay
rate i s equivalent to that of 1 g of radium A millicuri e (mCi) i s one-thousandth of a curie Thus,
10 mCi of a carbon-14 samp le i s the quantity that undergoe s (10 X 10 - 3)( 3.70 X 1010) = 3.70 X
108 disintegrations per seco nd
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 do se of radiation is the rad (radia-
ti on absorbed dose), which is the amount of radiation that results in the absorption of 1 X 10 - 5 J
p er gram of irradiated material The biological effect of radiation depend s on the part of the body
irradiated and the type of radiation For this reason, the rad i s often multiplied by a factor ' called the
RBE (relative biological effec tivene ss) The product i s called a rem (roentgen equivalent for man):
number of rems = number of rad s X 1 RBE
Of the three types of nuclear radiation, a particles u s ually have the least penetrating power Beta par t icle s are more penetrating than a particles, but le ss so than y rays
Gamma ra ys have very s hort wavelengths and high energies Furthermore , becau se they
carry no charge, they cannot be stoppe d by shielding materials as easily as a and f3 particles If
a- o r f3-emitters are ing ested or inhaled , however , their damaging effects are greatly aggravated
becau se the organs will b e constantly s ubject to damaging radiation at close range For example,
stront ium-90 , a f3-emitter , can replace calcium in bone s, where it does the greatest damage
Table 20.5 lists the average amounts of radiation an American receives every year For term exposures to radiation, a do sage of 50 to 200 rems will cause a decrease in white blood cell counts and other complications, while a dosage of 500 rem s or greater may result in death within
short-weeks Current safety s tandard s pelIllit nuclear workers to be exposed to no more than 5 rems per yea r and s pecify a maximum of 0.5 rem of human-made radiation per year for the general pUblic
Trang 8SECTION 20.8 Biological Effects of Radiation
Source
Cosmic rays Ground and surroundings
*1 mrem = millirem = 1 X 10 - 3 rem
tThe radi ' oactivity in the body comes from food and air
The chemical basis of radiation damage is that of ionizing radiation Radiation (of either
particles or 'Y rays) can remove electrons from atoms and molecules in its path, leading to the
for-mation of ions and radicals Radicals (also called free radicals) are molecular fragments having
one or more unpaired electrons; they are usually short lived and highly reacti ve When water i s
irradiated with 'Y rays, for example, the following reactions take place:
The electron (in the hydrated fonn) can subsequently react with water or with a hydrogen ion to
form atomic hydrogen, and with oxygen to produce the superoxide ion (0 2) (a radical):
In the tissues the superoxide ions and other free radical s 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 human s
and other animals Cancer is characterized by uncontrolled cellular growth On the other hand, it i s
also well established that cancer cells can be de s troyed by proper radiation treatment In radiation
therapy, a compromise is sought The radiation to which the patient i s exposed must be s ufficient
to destroy cancer cells without killing too many normal cells and , it is hoped, without inducing
another form of cancer
Radiation damage to living sys tems is generally classified as somatic or ge n etic Somatic
injuries are those that affect the organism during its own lifetime Sunburn, ski n rash, cancer, and
cataracts are examples of somatic damage Genetic damage mean s inheritable changes or gene
mutations For example, a person whose chromosomes have been damaged or altered by radiation
may have deformed offspring
Bringing Chemistry to Life
Radioactivity in Tobacco
"SURGEON GENERAL'S WARNING: Smoking Is Ha zar dou s to Your Health " Warning labels s uch as
this appear on every package of cigarettes sold in the United State s The link between cigarette
smoke and cancer has long been established There is , however, another cancer-causing
mecha-nism in smokers The culprit in this case is a radioactive environmental pollutant present in the
tobacco leave s from which cigarettes are made
The soil in which tobacco is grown i s heavily treated with phosphate fertilizers, which are
rich in uranium and its decay products Consider a particularly important step in the uranium-238
Trang 9-820 CHAPTER 20 Nuclear Chemistry
•
concentrations in so il gas and in the s urface air layer under the vegetation canopy provided
b y the field of growing tobacco In this layer some of the daughters of radon-222, such as
a high le v el
During the combustion of a cigarette, tiny insoluble smoke particles are inhaled and depo s ited in the re s pirator y tract of the smok er and are eventually transported and stored at
si te s in the li ve r , spleen , and bone marrow Measurements indicate a high lead-21O content
in the se particle s The lead-2I0 content i s not high enough to be hazardous chemically (it is
expo-s ure of th e organs and bone marrow to a- and J3-particle radiation increases the probability the
•
Trang 10APPLYING WHAT YOU'VE LEARNED
Applying What You've Learned
In addition to BNCT, another promising treatment for brain tumor s i s brachytherapy using iodine-l2s In brachytherapy, "seeds" containing 1 2 5 1 are implanted directly into the tumor As the radioisotope decays, 'Y rays destroy the tumor cell s Careful implanta- tion prevents the radiation from harming nearby healthy cell s
Problems:
a) 125 1 is produced by a two- s tep process in which I24 Xe nuclei are bombarded with
neutrons to produce 125Xe a process called neutron activation 125Xe then decays
by electron capture to produce 125 1, which also decays by electron capture Write nuclear equations for the two steps that produce 12 5 1 from 1 24 Xe , and identify the product of the electron-capture decay of 125I [ ~~ Sample Problem 20.1]
b) The mass of an 1 2 5 1 nucleu s is 124.904624 amu Calculate the nuclear binding
energy and the nuclear binding energy per nucleon [ ~~ Sample Problem 20.2]
c) The half-life of 125 1 is 59.4 days How long will it take for the activity of implanted
125 1 seeds to fall to 5.00 percent of its original value ? [ ~ Sample Problem 20.3]
d) Iridium-l92 is another isotope used in brachytherapy It is produced by a nuclear
transmutation Identify the target nucleus X, and write the balanced nuclear equation for the reaction represented by 19l X(n , 'Y) I92Ir [ ~~ Sample Problem 20.5]
•
Brachytherapy seeds (shown with a
penny to illustrate their size)
Trang 11822 CHAPTER 20 Nuclear Chemistry
CHAPTER SUMMARY
Section 20.1
• Spontaneous emission of particles or radiation from unstable nuclei is
known as radioactivity Unstable nuclei emit a particles, f3 particles,
positrons, or 'Y rays
• Nuclear transmutation is the conversion of one nucleus to another
Nuclear reactions are balanced b summing the mass numbers and the
atomic numbers
Section 20.2
• Stable nuclei with low atomic numbers have neutron-to-proton ratios
close to 1 Heavier stable nuclei have higher ratios Nuclear stability is
favored by certain numbers of nucleons including even numbers and
"magic" numbers
• The difference between the actual mass of a nucleus and the mass
calculated by summing the masses of the individual nucleons is the
mass defect
• Nuclear binding energy, determined by using Einstein's equation
E = mc 2 , is a measure of nuclear stability
Section 20.3
• Uranium-238 is the parent of a natural radioactive decay series that
can be used to determine the ages of rocks Radiocarbon dating is
done using carbon-14
Section 20.4
• Transuranium elements are created by bombarding other elements
with accelerated neutrons, protons, a particles, or other nuclei
Breeder reactor, 814 Nuclear binding energy, 801
Critical mass, 812 Nuclear chain reaction, 812
Mass defect, 802 Nuclear fission, 811
Moderator, 813 Nuclear fusion, 815
KEY EQUATION
Section 20.5
• Nuclearfission 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 nuclear chain reaction can occur in
which the fission process is sustained The minimum amount of
fissionable material required to sustain the reaction is known as the
critical mass
• Nuclear reactors use the heat from a controlled nuclear fission reaction
to produce power Fission is controlled, in part, by
moderators-materials that limit the speed of liberated neutrons but that do not themselves undergo fission when bombarded with neutrons The three important types of reactors are light water reactors, heavy
water reactors, and breeder reactors Breeder reactors produce more
fissionable material than they consume
Section 20.6
• N uclear fusion, the type of reaction that occurs in the sun, is the
combination of two light nuclei to fonn one heavier nucleus Fusion reactions are sometimes referred to as thermonuclear reactions
because they take place only at very high temperatures
Section 20.7
• Radioactive isotopes are easy to detect and thus make excellent
tracers in chemical reactions and in medical procedures
Section 20.8
• High-energy radiation damages living systems by causing ionization
and the formation of radicals, or fre e radicals, which are chemical
species with unpaired electrons
•
Nuclear transmutation, 798 Radioactivity, 798
Positron, 798 Thermonuclear reaction, 815
Radioactive decay series, 804 Transuranium elements, 809
2 0.1 I1E = ( I1m ) c 2
Trang 12QUESTIONS AND PROBLEMS
What are the steps in balancing nuclear equations?
What is the difference between _ ~e and -~f3?
What is the difference between an electron and a positron?
20.7 State the general rules for predicting nuclear stability
20.8 What is the belt of stability?
20.9 Why is it impossible for the isotope i He to exist?
20.10 Define nuclear binding energy, mass defect, and nucleon
20.11 How does Einstein's equation, E = me 2 , enable us to calculate
nuclear binding energy?
20.12 Why is it preferable to use nuclear binding energy per nucleon
for a comparison of the stabilities of different nuclei?
Problems
20.13 The radius of a uranium-235 nucleus is about 7.0 X 10- 3 pm
Calculate the density of the nucleus in g/cm3 (Assume the atomic mass is 235 amu.)
20.14 For each pair of isotopes listed, predict which one is less stable:
(a) ~Li or ~Li, (b) nNa or TiNa, (c) i~Ca or i~sc
20.15 For each pair of elements listed, predict which one has more
stable isotopes: (a) Co or Ni, (b) ForSe, (c) Ag or Cd
20.16 In each pair of isotopes shown, indicate which one you would
expect to be radioactive: (a) f8Ne or lZNe, (b) i8Ca or igca, (c)
~~Mo or ~~Tc, (d) 1 ~6Hg or 1~8Hg, (e) 2~§Bi or 2~~ Cm
20.17 Given that
H( g) + H (g) + H 2 (g) i1H O = - 436.4 kJ/mol
calculate the change in mass (in kg) per mole of H2 formed
20.18 Estimates show that the total energy output of the sun is
5 X 1026 J/s What is the corresponding mass loss in kg/s
of the sun?
20.19 Calculate the nuclear binding energy (in joules) and the binding
energy per nucleon of the following isotopes: (a) ~ Li
(7.01600 amu) and (b) N Cl (34.96885 amu)
20.20 Calculate the nuclear binding energy (in joules) and the binding
energy per nucleon of the following isotopes: (a) i H e
(4.0026 amu) and (b) l~~W (183.9509 amu)
Review Questions
20.21 Discuss factors that lead to nuclear decay
20.22 Outline the principle for dating materials using radioactive
20.24 A radioactive substance undergoes decay as follows:
Time (days) Mass (g)
20.25 The radioactive decay of Tl-206 to Pb-206 has a half-life of
4.20 min Starting with 5.00 X 1022 atoms of Tl-206, calculate
the number of such atoms left after 42.0 min
20.26 A freshly isolated sample of 90y was found to have an activity of
9.8 X 105 disintegrations per minute at 1:00 P.M on December
3, 2006 At 2: 15 P M on December 17, 2006, its activity was measured again and found to be 2.6 X 104 disintegrations per minute Calculate the half-life of , 90y
20.27 A wooden artifact has a 14C activity of 18.9 disintegrations per
minute, compared to 27.5 disintegrations per minute for live wood Given that the half-life of 14C is 5715 years, determine the
age of the artifact
20.28 In the thorium decay series, thorium-232 loses a total of six IX
particles and four f3 particles in a lO-stage process What is the
final isotope produced?
Trang 13824 CHAPTER 20 Nuclear Chemistry
A, B, and C are radioactive isotopes with half-lives 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
20.30 The activity of radioactive carbon-14 decay of a piece of charcoal
found at a volcanic site is 11.2 disintegrations per second If the activity of carbon-14 decay in an equal mass of living matter is
18.3 disintegrations per second, what is the age of the charcoal?
•
(See Problem 20.27 for the half-life of carbon-14.)
n-14 dating to be 8.4 X 103 years old Calculate the activity of
the carbon-14 in the bones in disintegrations per minute per gram, given that the original activity was 15.3 disintegrations
per minute per gram (See Problem 20.27 for the half-life of carbon-14 )
20.32 Given that the half-life of 238U is 4.51 X 109 years, determine the
age of a rock found to contain 1.09 mg 238U and 0.08 mg 206Pb
20.33 Determine the ratio of 238U to 206Pb in a rock that is 1.7 X 108
years old (See Problem 20.32 for the half-life of 238U.)
Section 20.4: Nuclear Transmutation
20.36 Write balanced nuclear equations for the following reactions, and
identify X: (a) X(p,O') I~C, (b) gAI(d,O')X, (c) ~~Mn(n,'y)X
20.37 Write the abbreviated forms for the following reactions:
(a) IjN + ia I~O + lp
(b) ~Be + ia • I~C + t n
(c) 2§~U + T H • 2~~Np + 2bn
20.38 Write balanced nuclear equations for the following reactions, and
identify X: (a) ~~Se(d,p)X, (b) X(d,2p)~Li, (c) I~B(n,O')X
20.39 Write the abbreviated forms for the following reactions:
(a) i8Ca + T H • ibC a + lp (b) i~s + tn • f~P + l p
(c) 2~~Pu + iO' • 2~~Cm + bn
20.40 Describe how you would prepare astatine-211 , starting with
bismuth-209
from cheaper and more abundant elements This dream was
finally realized when l~gHg was converted into gold by neutron bombardment Write a balanced equation for this reaction
Section 20.5: Nuclear Fission
Review Questions
20.42 Define nuclear fission, nuclear chain reaction, and critical mass
20.43 Which isotopes can undergo nuclear fission?
20.44 Explain how an atomic bomb works
20.45 Explain the functions of a moderator and a control rod in a
nuclear reactor
20.46 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?
20.47 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
Section 20.6: Nuclear Fusion
Review Questions
20.48 Define nuclear fusion, thermonuclear reaction, and plasma
20.49 Why do heavy elements such as uranium undergo fission whereas
light elements such as hydrogen and lithium undergo fusion?
20.50 How does a hydrogen bomb work?
20.51 What are the advantages of a fusion reactor over a fission reactor?
What are the practical difficulties in operating a large-scale
fusion reactor?
Section 20.7: Uses of Isotopes
Problems
20.52 Describe how you would use a radioactive iodine isotope to
demonstrate that the following process is in dynamic equilibrium:
PbI2(s) • • Pb 2+ (aq) + 2I - (aq)
I0 4 (aq) + 2r(aq) + H 2 0(l ) - _
When KI04 is added to a solution containing iodide ions labeled
with radioactive iodine-128, all the radioactivity appears in 12 and
none in the 103 ion What can you deduce about the mechanism
for the redox process?
20.54 Explain how you might use a radioactive tracer to show that ions
are not completely motionless in crystals
contains four Fe atoms Explain how you would use the
radioactive ~~Fe (tI l, = 46 days) to show that the iron in a certain food is converted into hemoglobin
Section 20.8: Biological Effects of Radiation
Review Questions
20.56 List the factors that affect the intensity of radiation from a
radioactive element
20.57 What are rad and rem, and how are they related?
20.58 Explain, with examples, the difference between somatic and
genetic radiation damage
20.59 Compare the extent of radiation damage done by a, {3, and y
sources
Problems
20.60 The half-life of strontium-90 is 29.1 years Calculate the
radioactivity in millicuries of 15.6 mg of 90Sr