Chemistry part 33, Julia Burdge,2e (2009) ppsx

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 chain reaction will occur. • . _ I _ : Multimedia Nuclear Chemistry- nuclear chain reaction. .""""''''''''''': M u It i m e d i a Nuclear Chemistry- nuclear fission . TNT- explosive Subcritical U-235 mass 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 critical mass. • 23 5U 92 143Xe 54 ~ Sr • • • 235U 92 23 5U 92 , -1 -° •. • • ~ ~ '",. '"t ••• - . ~ , ~ - . . " • 141 Ba 56 • 92Kr 36 • • • 235U 92 235U 92 235U 92 235U 92 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 make s 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 20.9 shows 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 situation the ma ss of the sample is said to be subcritical. 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 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 (trini- trotoluene). Because 1 ton of TNT releases about 4 X 10 9 J of energy, 20,000 tons would produce 8 X 10 13 J. Recall that 1 mole, or 235 g, of uranium-235 liberates 2.0 X 1013 J of energy when it undergoes fission. Thus, the mass of the isotope present in a small bomb must be at lea st 8 X 10 13 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 is formed by using a conventional explosive, such as TNT, to force the fissionable sections together, as shown 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- shima, Japan, on August 6, 1945. Plutonium-239 was used 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 is the generation of electricity using heat from a controlled chain reaction in a nuclear reactor. Currently, nuclear reactors pro- vide about 20 percent of the electric 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. Most of the nuclear reactors in the United States are light water reactors. Figure 20.11 is a schematic diagram of such a reactor, and Figure 20.12 shows the refueling process in the core of a nuclear reactor. SECTION 20.5 Nuclear Fission 8- 3 Shield To steam turbine • Steam Shield- Control rod- Water • Uranium fuel- Pump An important aspect of the fission process is the s peed of the neutrons. Slow neutron s split uranium-235 nuclei more efficiently than do fast ones. Because fission reactions are highly exo- thermic, 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 neu- trons. A good moderator must satisfy several requirement s: 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 the se requir eme nt s, although water comes closer than many others that have been considered. Nuclear reactors that use light water (H 2 0) 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, U 3 0 S (Figure 20.13). Naturally occurring uranium contains about 0.7 percent of the uranium-235 isotope, 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 sion reactor. The fi i- 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 hea; exchange sys tem. Figure 20.12 Refueling the core of a nuclear reactor. ._- , Multimedia Nuclear Chemistry-Dperation of a nu clea r pow er plant. Figure 20.13 Uranium oxi de (U 3 0 8 )· 814 CHAPTER 20 Nuclear Chemistry • Figure 20.14 Radioactive plutonium oxide (PU02) 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 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. Another type of nuclear reactor uses D 2 0, or heavy water, as the moderator, rather than H 2 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, D 2 0 must be prepared by either fractional dis- tillation 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 D 2 0 by electrolysis can be reasonably low. At present, Canada is the only nation suc- 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. 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: ?~~ U + bn +. 2 ~ ~U 2~~U +. 2 ~~ Np + - ~f3 ? ~~Np +. 2 ~ ~PU + - ~f3 t 1/2 = 23.4 min t 1/2 = 2.35 days In this manner, the nonfissionable uranium-238 is transmuted into the fissionable isotope plutonium-239 (Figure 20.14). In a typical breeder reactor, nuclear fuel containing uranium-235 or plutonium-239 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 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 10 5 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- SECTION 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 Penn- sylvania 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 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 lightest elements, nuclear stability increases with increas- ing 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. 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: lH + iH -_. ~He ~He + ~He • iHe + 21H lH + lH -_. ~H + - ?f3 Because fusion reactions take place only at very high temperatures, they are often called thermo- nuclear reactions. 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 listed here: Reaction iH + iH -~. ~H + lH TH + iH -_. ~He + 26 n ~Li + iH • 2iHe Energy Released 6.3 X 10 13 J 2.8 X 10- 12 J 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 • 816 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 at the Lawrence Livermore National Laboratory using the world's most powerful la ser, Nova. Figure 20.17 Exp l os ion of a thermonuclear bomb. I Plasma Magnet attractive because the world's supply of deuterium is virtually inexhaustible. The total volume of water on Earth is about 1.5 X 10 21 L. Becau se the natural abundance of deuterium is 0.015 per- cent, the total amount of deuterium present is roughly 4.5 X 10 21 g, or 5.0 X 10 15 tons. Although it is expensive to prepare deuterium, the cost is 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 least on paper. Although thermal pollution would be a problem, fusion has the following advan- tages: (1) the fuels are cheap and almost inexhaustible and (2) the process produces little radio- active waste. If a fusion machine were turned off, it would shut down completely and instantly, without any danger of a meltdow n. If nuclear fusion is so great, why isn't there even one fusion reactor producing energy? Although we possess the scientific knowledge to design such a reactor, the technical difficulties ha ve 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 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. No solid container can exist at such temperatures, unless the amount of plasma is small, but then the solid surface would immedi- ately cool the sample and quench the fusion reaction. One approach to solving this problem is to use magnetic confinement. Because plasma consists of charged particles moving at high speeds, a magnetic field will exert a force on it. As Figure 20.15 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 (Fig- ure 20.16). 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 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 deute- rium; they contain solid lithium deuteride (LiD), which can be packed very tightly. The detona- 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, releasing vast amounts of energy (Figure 20.17): ~ Li + TH -_I 2in: TH + iH IH + :H There is no critical ma ss in a fusion bomb, and the force of the explosion 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 nonfission- SECTION 20.7 Uses of Isotopes 817 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: SO ~-( aq) + S(s) -_. S ? O ~- (aq) 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 sulfur-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 S205- 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 • • ·S· 2- . . II :O-S-O: . . II . 0. • • The study of photosynthesis is also rich with isotope applications. The overall photosynthesis reaction can be represented as 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 this 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 solution containing a known amount of N a l3II 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 these cases, though, the amount of radioisotope used must be kept small to prevent the patient from suffering permanent damage from the high-energy radiation. . ,1 •• ~ . . Multimedia Nuclear Chemistry- nuclear medical techniques. ; ) . Figure 20.18 1 23 1 image of a normal brain (top) and the brain of an Alzheimer's victim (bottom ). 818 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 electrode s. This current is amplified and is used to flash a light or operate a counter with a clicking sou nd . . _- __ Multimedia Nuclear Chemistry- alpha, beta, and gamma emission (interactive). • Insulator Amplifier and counter Cathode Anode Argon gas Window -High voltage Technetium, the first artificially prepared element, is one of the most useful elements in nuclear medicine. Although technetium is a transition metal, all its isotopes are radioactive. In the laboratory it is prepared by the nuclear reactions where the superscript "m" denotes that the technetium-99 isotope 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 valuable diagnostic tool. The patient either drinks or is injected with a solution containing 99 mTc . By detecting the y rays emitted by 99m Tc, 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 20.19 is a diagram of a Geiger counter, an instrument widely used in scientific work and medical laboratories to detect radiation. Biological Effects of Radiation In this section we will examine briefly the effects of radiation on biological systems. But first we must define the quantitative measures of radiation. The fundamental unit of radioactivity is the curie (Ci); 1 Ci corresponds to exactly 3.70 X 10 10 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 X 10- 3 )(3.70 X 10 10 ) = 3.70 X 10 8 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 (radia- tion absorbed dose), which is the amount of radiation that results in the absorption of 1 X 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 the RBE (relative biological effectiveness). The product is called a rem (roentgen equivalent for man): number of rems = number of rads X 1 RBE Of the three types of nuclear radiation, a particles usually have the least penetrating power. Beta particles are more penetrating than a particles, but le ss so than y rays. Gamma rays have very short wavelengths and high energies. Furthermore, because they carry no charge, they cannot be stopped by shielding materials as easily as a and f3 particles. If a- or f3-emitters are ingested or inhaled, however, their damaging effects are greatly aggravated because the organs will be constantly subject to damaging radiation at close range. For example, strontium-90, a f3-emitter, can replace calcium in bones, where it does the greatest damage. Table 20.5 lists the average amounts of radiation an American receives every year. For short- term exposures to radiation, a dosage of 50 to 200 rems will cause a decrease in white blood cell counts and other complications, while a dosage of 500 rems or greater may result in death within weeks. Current safety standards pelIllit nuclear workers to be exposed to no more than 5 rems per year and specify a maximum of 0.5 rem of human-made radiation per year for the general pUblic. SECTION 20.8 Biological Effects of Radiation Source Cosmic rays Ground and surroundings Human bodyt Medical and dental X rays Air travel Fallout from weapons tests Nuclear waste Total *1 mrem = millirem = 1 X 10 - 3 rem. tThe radi'oactivity in the body comes from food and air. Dose (mrem/yr)* 20 - 50 25 26 50-75 5 5 2 133-188 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 reactive. When water is irradiated with 'Y rays, for example, the following reactions take place: H 2 0 radiatiof H 2 0 + + e- H 2 0+ + H 2 0 + H30 + + 'OH hydroxyl radical 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 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. Bringing Chemistry to Life Radioactivity in Tobacco "SURGEON GENERAL'S WARNING: Smoking Is Ha zardous to Your Health." Warning labels such as this appear on every package of cigarettes sold in the United States. 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 leaves from which cigarettes are made. The soil in which tobacco is grown is heavily treated with phosphate fertilizers, which are rich in uranium and its decay products. Consider a particularly important step in the uranium-238 decay series: 22868Ra __ 222R + 4 • 86 n 20' • - - 820 CHAPTER 20 Nuclear Chemistry • The product formed, radon-222, is an unreactive gas. (Radon is the only gaseous species in the uranium-238 decay series.) Radon-222 emanates from radium-226 and is present at high concentrations in soil gas and in the surface air layer under the vegetation canopy provided by the field of growing tobacco. In this layer some of the daughters of radon-222, such as polonium- 2I8 and lead-214, become firmly attached to the surface and interior of tobacco leaves. As Figure 20.3 shows, the next few decay reactions leading to the formation of lead- 210 proceed rapidly. Gradually, the concentration of radioactive lead-2I0 can build to quite a high level. During the combustion of a cigarette, tiny insoluble smoke particles are inhaled and deposited in the respiratory tract of the smoker and are eventually transported and stored at si tes in the liver, spleen, and bone marrow. Measurements indicate a high lead-21O content in these particles. The lead-2I0 content is not high enough to be hazardous chemically (it is insufficient to cause lead poisoning) , but it is hazardous because it is radioactive. Because of its long half-life (20A years), lead-21O and its radioactive daughters bismuth-21O and polonium-210-continue to build up in the body of a smoker over the years. Constant expo- sure of the organs and bone marrow to a- and J3-particle radiation increases the probability the smoker will develop cancer. The overall impact on health is similar to that caused by indoor radon gas. • i f i j i , ! • APPLYING WHAT YOU'VE LEARNED Applying What You've Learned In addition to BNCT, another promising treatment for brain tumors is brachytherapy using iodine-l2s. In brachytherapy, "seeds" containing 125 1 are implanted directly into the tumor. As the radioisotope decays, 'Y rays destroy the tumor cells. Careful implanta- tion prevents the radiation from harming nearby healthy cell s. Problems: a) 125 1 is produced by a two-step 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 125 1 nucleus 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) I 92 Ir. [ ~~ Sample Problem 20.5] • Brachytherapy seeds (shown with a penny to illustrate their size). [...]...822 CHAPTER 20 Nuclear Chemistry CHAPTER SUMMARY Section 20.1 Section 20.5 • 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... total of six IX particles and four f3 particles in a lO-stage process What is the final isotope produced? 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 IX Time (days) 0 1 2 3 4 5 6 Problems 20.13 Fill in the blanks in the following radioactive decay series: 824 20.29 20.30 20.31 20.32 20.33 CHAPTER 20 Nuclear Chemistry Consider... 14N (14.00307 amu), (d) 56Fe (55.9349 amu) §~ Sr Write complete nuclear equations for the following processes: (a) tritium eH) undergoes f3 decay, (b) 242pu undergoes a-particle emission, (c) 1311 undergoes f3 decay, (d) 251Cf emits an a particle 20.70 , 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 preceding two decays... radioactivity corresponds (The half-life of I3 1 i 1 8.1 days.) 826 CHAPTER 20 Nuclear Chemistry 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 the same type of radiation ( Q' or (3 ) and comparable energetics per particle emitted.] 20.85 From the definition of curie, calculate Avogadro 's number,... U-23 8 isotope decays to Th-234 The atomic masses are as follows: U-238: 238.0508 amu; Th-234: 234.0366 amu; and He-4: 4.0026 amu (b) The energy released in part (a) is transformed into the kinetic energy of the recoiling Th-234 nucleus and the Q' particle Which of the two will move away faster? Explain A person received an anonymous gift of a decorative cube which he placed on his desk A few months... IN-CHAPTER MATERIALS r PRE-PROFESSIONAL PRACTICE EXAM PROBLEMS: PHYSICAL AND BIOLOGICAL SCIENCES The radioactive isotope 238 pu , used in pacemakers, decays by emitting an a particle with a half-life of 86 years The energy of the emitted a particle is 3 9.0 X 10- 1 J, which is the energy per decay After 10 years, the activity of the isotope decreases by 8.0 percent (Power is measured in watts or J/s.) I 3... Rowland and Molina, along with Dutch atmospheric chemist Paul Crutzen, were awarded the Nobel Prize in Chemistry for their elucidation of the role of human-made chemicals in the catalytic destruction of stratospheric ozone In This Chapter, You Will learn about some of the facets of environmental chemistry Before you begin, you should review • Bond enthalpy [111 Section 8.9] • Catalysis [ 111 Section... of the bombardment of molecular oxygen and nitrogen and atomic species by energetic particles, such as electrons and protons, from the sun Typical reactions are N 2 - - · 2N AW = 941.4 kJ/mol ARo = 1400 kJ/mol ARo = 1176 kJ/mol In reverse, these processes liberate the equivalent amount of energy, mostly as heat Ionized particles are responsible for the reflection of radio waves back toward Earth Figure... most pronounced on the parts of the shuttle facing its direction of travel This fact led scientists to postulate that collision between oxygen atoms in the atmosphere and the fast-moving shuttle somehow produced the orange light Spectroscopic measurements of the glow, as well as laboratory tests, strongly suggested that nitric oxide (NO) and nitrogen dioxide (N0 2) also played a part It is believed that... therefore can take part in many such reactions In fact, one CI atom can destroy up to 100,000 0 3 molecules Commercial refrigeration 17% Solvent cleaning 14% Aerosols 4% Equation 21.6 Auto air conditioning 21% Others (sterilization, household refrigeration) 11 % Figure 21 6 Uses ofCFCs Since 1978, the use of aerosol propellan - : been banned in the United State 836 CHAPTER 21 Environmental Chemistry O ~ . Nuclear Chemistry CHAPTER SUMMARY Section 20.1 • Spontaneous emission of particles or radiation from unstable nuclei is known as radioactivity. Un stable nuclei emit a particle s, f3 particle s,. Of the three types of nuclear radiation, a particles usually have the least penetrating power. Beta particles are more penetrating than a particles, but le ss so than y rays. Gamma rays. thorium-232 loses a total of six IX particles and four f3 particles in a lO-stage process. What is the final isotope produced? 824 CHAPTER 20 Nuclear Chemistry 20.29 Consider the decay