Kelter P., Mosher M., Scott A. Chemistry. The Practical Science

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The Bottom Line

928 Chapter 21 Nuclear Chemistry

■

Each element is composed of atoms containing

the same number of protons. These may contain

isotopes with differing numbers of neutrons.

(Section 21.1)

■

Some nuclear conﬁgurations are unstable. They

decay in a stepwise progression toward stable nuclei.

(Section 21.2)

■

There are three main types of radioactive decay:

alpha-particle emission, beta-particle emission, and

gamma-ray emission. (Section 21.2)

■

Ionizing radiation can interact with living tissue and

cause damage to the DNA of a cell. This damage may

be repaired and cause no harm or, in some cases,

may lead to cancer. (Section 21.3)

■

Radioactive decay occurs via ﬁrst-order kinetics.

(Section 21.4)

■

Energy is released in radioactive decay processes as

a consequence of the mass defect in nuclei.

(Section 21.5)

■

Nuclei with a “magic” number of protons and/or

neutrons (2, 8, 20, 28, 50, or 82) are stable. Nuclei

with even numbers of protons and/or neutrons are

also more likely to be stable. (Section 21.6)

■

Nuclear ﬁssion is the splitting of heavier nuclei into

lighter ones. Nuclear fusion results when smaller nu-

clei combine into heavier nuclei. (Section 21.7)

■

Radioisotopes can be used in medicine for imaging

the body and for treating and eliminating cancerous

tissues. (Section 21.8)

alpha decay A type of radioactive decay wherein an

alpha particle is emitted from the nucleus of a

radioactive nuclide. Common for elements whose

nuclei are larger than bismuth, alpha decay is often

accompanied by the release of a gamma ray. (p. 905)

alpha particles (α particles) Particles emitted from the

nucleus of a radioactive element during the process

of alpha decay. They are helium nuclei (2 protons,

2 neutrons), with a +2 charge. (p. 905)

antimatter Particles that have the same mass as, but

charges opposite to, corresponding matter.

Antimatter particles such as the positron and anti-

neutrino are similar to the electron and neutrino,

respectively, but have opposite characteristics.

(p. 905)

antineutrino A subatomic particle produced in beta-

minus decay that has no charge, has essentially no

mass, and interacts only rarely with matter. (p. 904)

becquerel (Bq) An SI unit of activity equivalent to one

nuclear disintegration per second. (p. 911)

beta particles (β particles) Particles emitted from the

nucleus of a radioactive atom during the process of

beta decay. These particles are high-speed electrons.

(p. 904)

beta-particle emission A naturally occurring type of

radioactive decay wherein an electron is ejected at

high speed from the nucleus, typical of nuclei that

have too many neutrons to be energetically stable.

An antineutrino accompanies this emission, and

sometimes one or more gamma rays as well. Also

known as beta emission. (p. 904)

beta-plus emission A type of radioactive decay wherein a

positron is ejected at high speed from the nucleus,

typical of nuclides that have too few neutrons. A

neutrino accompanies this emission, and usually one

or more gamma rays as well. Also known as positron

emission. (p. 918)

binding energy The energy released when a nucleus is

formed from protons and neutrons. Binding energies

are expressed as a positive number. (p. 915)

chain reaction In nuclear chemistry, a reaction that is

self-sustaining as one event in turn causes more

events. (p. 923)

critical mass The amount of ﬁssionable fuel needed to

sustain a chain reaction. (p. 923)

curie (Ci) A larger unit of activity than the becquerel,

equivalent to 3.7 ×10

Bq. (p. 911)

daughter nuclide An isotope that is the product of a

nuclear reaction. (p. 904)

decay series A series of nuclear reactions that a large

nuclide undergoes as it changes from an unstable and

radioactive nucleus to a stable nucleus. (p. 907)

electron capture (EC) A type of radioactive decay that

occurs when an inner-core electron is captured by a

proton from the nucleus to form a neutron. The

process is usually accompanied by the emission of

X-rays. (p. 907)

ﬁssion (or nuclear ﬁssion) A type of nuclear reaction

wherein a large nucleus splits into two or three

smaller nuclei with the release of energy. (p. 917)

fusion (or nuclear fusion) A type of nuclear reaction

wherein small nuclei are joined to form a larger

Key Words

nucleus with the release of energy. Nuclear fusion

powers the stars. (p. 916)

gamma rays ( rays) A high-energy form of electro-

magnetic radiation that is emitted from the nucleus.

Gamma rays sometimes accompany alpha and beta

decays. (p. 904)

gray (Gy) A measure of absorbed radiation equal to

100 rad. (p. 911)

ionizing radiation Radiation such as alpha and beta

particles, gamma rays, or X-rays that is capable of

removing an electron from an atom or a bond when

it interacts with matter. (p. 910)

mass defect (Section 21.5) The loss in mass that occurs

when a nucleus is formed from its protons and

neutrons. (p. 915)

metastable state An energetically unstable arrangement

of protons and neutrons in a nucleus after a neutron

has become a proton. (p. 906)

neutrino A subatomic particle that is produced in beta-

plus decay and that has no charge, has essentially no

mass, and interacts only rarely with matter. (p. 905)

nuclear equation An equation showing a nuclear trans-

formation, where the atomic and mass numbers are

provided. (p. 904)

nuclear radiation The particles and/or energy emitted

during radioactive decay. (p. 901)

nucleon The name given to a particle (proton or

neutron) that is part of a nucleus. For example,

contains 13 nucleons, 6 protons, and 7 neutrons.

(p. 918)

positron The antimatter equivalent of an electron.

Positrons have a positive charge and the same mass as

an electron. (p. 907)

positron emission See beta-plus emission.(p. 907)

positron emission tomography A medical imaging tech-

nique that images metabolic processes within the

body. Also known as a PET scan. (p. 926)

rad A unit of energy absorbed by irradiated material

equal to 0.01 J/kg of exposed material. (p. 911)

radioactive decay The process by which an unstable

nucleus becomes more stable via the emission or

absorption of particles and energy. (p. 904)

radioactivity The emission of radioactive particles

and/or energy. (p. 901)

radiopharmaceuticals Compounds containing radioac-

tive nuclides that are used for imaging studies in

nuclear medicine. (p. 924)

rem A unit that measures the “equivalent dose” of

radiation; that is, it takes into account the interaction

of radiation with human tissue. The word stands for

“roentgen equivalent in man.” The rem is not an SI

unit but is related to the SI unit, the sievert. (p. 911)

roentgen A unit used to measure exposure to radiation.

One roentgen is equal to 2.58 ×10

−4

C/kg of dry air

at STP. (p. 911)

sievert (Sv) A unit that measures the “equivalent dose”

of radiation; that is, it takes into account the inter-

action of radiation with human tissue. One sievert

equals 100 rem. (p. 911)

subcritical mass A mass of a radioactive isotope that is

too small to sustain a chain reaction. (p. 923)

supercritical mass A mass of a radioactive isotope that

not only supports a chain reaction but causes the

majority of the nuclei to undergo unfettered radio-

active decay within a very short period of time,

releasing huge amounts of energy. (p. 923)

Focus Your Learning 929

The answers to the odd-numbered problems and some selected

problems appear at the back of the book, as represented by the

blue numbering.

Section 21.1 Isotopes and More Isotopes

Skill Review

1. What is the atom with smallest atomic number? The smallest

mass number?

2. Can different elements both have the same number of pro-

tons? The same number of neutrons? Explain.

3. Can an atom have no neutrons? Explain.

4. Can an atom have no protons? No electrons? Explain.

5. Can a helium nuclide have a smaller mass number than a hy-

drogen nuclide? Explain.

6. Can a carbon nuclide have a smaller mass number than a ni-

trogen nuclide? Explain.

7. Identify the number of protons, neutrons, and electrons in

each of the following isotopes.

Nb.

124

Sb c.

152

Eu d.

8. Identify the number of protons, neutrons, and electrons in

each of the following isotopes.

Li b.

122

Cs c.

Od.

Chemical Applications and Practices

9. A person who weighs 60 kg (132 lb) has about 120 g of potas-

sium in his or her body. How many grams of K-40 does this

include? K-40 has a natural abundance of 0.0118%.

10. No isotopes of potassium are chemically stable in the pres-

ence of water and/or oxygen.Potassium-39 and potassium-41

are stable isotopes. Explain these different meanings of the

word stable.

11. Fallout from a nuclear weapon includes the radioactive

nuclide Sr-90.

a. In the biosphere, which chemical form for Sr-90 is more

likely, Sr

or Sr?

b. Once Sr-90 lands downwind, it is extremely difﬁcult to

separate from the plants and soils. Suggest reasons why.

12. Strontium-90 from fallout gets into the food chain and even-

tually can end up in cows’ milk. Can you remove the radioac-

tivity from cows’ milk by boiling it? Why or why not?

Focus Your Learning

Section 21.2 Types of Radioactive Decay

Skill Review

13. Both gamma rays and infrared radiation are forms of electro-

magnetic radiation. How do they differ?

14. Beta decay involves the emission of a high-speed electron

from an atom, yet the overall charge on the atom does not

become less negative. Explain why.

15. Write nuclear equations for the following processes:

a. An alpha particle (along with a gamma ray) is emitted

from plutonium-239.

b. Carbon-14 undergoes beta decay.

c. Cesium-137 emits a beta particle with an accompanying

gamma ray.

16. Write nuclear equations for the following processes:

a. Plutonium-238 emits an alpha particle with an accompa-

nying gamma ray.

b. Radon-222 is produced from the decay of a radium

isotope with the emission of a gamma ray.

c. Radium-225 emits a gamma ray followed by an alpha

particle.

17. Write nuclear equations for the following processes:

a. Polonium-215 (with a gamma ray) is produced by an

alpha emission.

b. Strontium-90 decays by beta emission. Little or no gamma

radiation is released.

c. Tc-99 decays by beta-minus emission.

18. Write nuclear equations for the following processes:

a. Nitrogen-14 is formed from a radioisotope of carbon.

b. Cadmium-110 is formed from a radioisotope of silver.

c. Technetium-99 is formed from Technetium-99m.

Chemical Applications and Practices

19. Samarium-146 is the lightest element found naturally on our

planet to undergo alpha emission. Write the equation for this

alpha decay. There is no accompanying gamma ray.

20. Iodine-131, used in medical imaging, undergoes beta decay.

Write the nuclear equation for this reaction.

21. Darlene Hoffman, an award-winning nuclear chemist, pos-

tulated the existence of Pu-244 before it was discovered. Into

which of the four natural decay series does it ﬁt?

22. A chemistry source states that radon-219 is produced in the

actinium-227 radioactive decay series. Into which of the four

decay series mentioned in Section 21.2 does actinium-227 ﬁt?

Section 21.3 Interaction of Radiation with Matter

Skill Review

23. Classify the following as ionizing or nonionizing radiation:

cosmic rays, infrared radiation, gamma rays, visible light,

microwaves, X-rays.

24. You can cook food using microwaves, and you can sterilize

food using gamma rays. Why do these two types of radiation

produce such different results?

25. Suppose that you had administered a gamma emitter to a pa-

tient in order to diagnose how well his or her heart was func-

tioning. Name three things you could do to minimize your

exposure to the radiation.

930 Chapter 21 Nuclear Chemistry

26. Which cells in your body are most susceptible to radiation?

Why?

27. Explain the similarities and differences between:

a. a curie and a becquerel

b. a rad and a rem

28. Explain the similarities and differences between:

a. a rem and a sievert

b. a curie and a rem

29. Smoke detectors use only a small quantity of americium, less

than 35 kBq. How many disintegrations per second is this?

30. Using the information in Problem 29, show that the result is

comparable to 1 microcurie.

Chemical Applications and Practices

31. In the mid-1990s, a watch was advertised that glowed in the

dark. The source of the glow was the radioisotope tritium in-

teracting with a luminous paint. The annual dose for a per-

son wearing the watch as estimated at 4.0 microsieverts.

a. How many rem is this?

b. Do you think this amount of radiation warrants concern?

Note the radiation symbol

between the hour and

minute hands and the H3

(

H, or tritium) on the face

of this watch.

32. In the previous problem, we noted that tritium, a radioiso-

tope of hydrogen, was used in some watches.

a. What mode of decay would you predict for tritium?

b. Given that radiation escapes from the watch case, does this

evidence support your prediction?

33. Three metals—aluminum, iron, and cadmium—are pro-

posed as materials that could be used to shield nuclear

radiation.

a. What property of these materials would you need to look

up to determine which one would have to be used in the

greatest thickness?

b. What else might you need to know about a substance

before you use it in shielding?

34. If radon-222 gas decays in your lungs to produce solid polo-

nium, which is then trapped there, how many decays does the

polonium progress through before reaching the stable iso-

tope of lead-206? How many alpha and beta particles are

emitted in the process?

Section 21.4 The Kinetics of Radioactive Decay

Skill Review

35. Here are the decay plots for two different hypothetical radio-

active nuclei. The plot denoted by the red line is A; that de-

noted by the blue line is B. From these graphs, determine:

a. Which nuclide has the longer half-life?

b. What is the half-life of the nuclide indicated by the red

line?

c. Which one has the higher activity?

d. Which one would be more dangerous if swallowed?

45. A 0.1-microcurie sample of polonium-210 is used to demon-

strate alpha decay in the lecture hall, because there is little ac-

companying gamma radiation. The half-life of this isotope is

138 days. About how often does an instructor need to buy a

new source? State any assumptions you made in arriving at

your answer.

46. The world uranium reserves are currently estimated at

3.4 million tonnes, where a tonne is a metric ton, or 1000 kg.

Current knowledge places Earth at 4.5 billion years old. How

much uranium was present at the time our world formed?

47. The half-life of plutonium-239 is about 24,000 years. After

approximately how much time will over 99% of a sample of

plutonium-239 have decayed? How can this element exist on

our planet if its half-life is so short?

48. Plutonium oxide, used to power the Cassini space-probe to

Saturn, was stored in corrosion-resistant materials designed

to contain the fuel for 10 half-lives, or 870 years.

a. What is the half-life of Pu-238?

b. Why do you think the ﬁgure 10 half-lives was selected?

c. The Cassini batteries contained 72 lb (33 kg) of Pu-238 in

the form of plutonium dioxide. How much Pu-238 (in

kilograms and in grams) will remain after 870 years?

Section 21.5 Mass and Binding Energy

Skill Review

49. a. Why don’t the masses of the neutrons and protons that

make up an oxygen-16 atom add up to the mass of the

oxygen nucleus?

b. Is this true for all isotopes of oxygen?

50. a. Is an atom of carbon likely ever to fall apart into its com-

ponent protons, electrons, and neutrons?

b. Explain why or why not.

51. Explain how the mass defect and the binding energy are

related.

52. Is mass conserved in nuclear reactions, such as alpha decay,

that proceed spontaneously? Why or why not?

53. Calculate the mass defect and the resulting energy for the

following nuclear reaction:

(

n = 1.008665 g/mol;

He = 4.002603 g/mol)

170

Ir (169.974970 g/mol) decays to

166

Re (165.965740 g/mol)

54. Calculate the mass defect and the resulting energy for the fol-

lowing nuclear reaction:

K (39.963999 g/mol) n

Ca (39.962591 g/mol) +

−



Chemical Applications and Practices

55. The masses of the neutral atoms involved for an alpha emis-

sion from U-238 are shown below. These values include the

masses of the electrons.

a. Would you expect the U-238 or its decay products to have

more mass?

b. What is the source/result of any difference in these masses?

He 4.002603 u

238

U 238.050784 u

234

Th 234.040945 u

–

0.005485799 u

Focus Your Learning 931

20,000

40,000

60,000

80,000

100,000

120,000

0 102030405060

Time

(

minutes

)

Number of counts

36. Using the plot in Problem 35, estimate the half-life of each

nucleus. What does a shorter half-life imply?

37. If 75% of a radioactive sample is gone after 30 days, what is

the half-life of the nuclide?

38. If 50% of a radioactive sample remains after 30 days, what is

the half-life of the nuclide?

39. Tritium has a half-life of 12.3 years. What fraction of a sam-

ple of tritium will remain after 12.3 years? After 24.6 years?

40. What percent of the original sample of strontium-90 (half-

life = 28.9 years) will remain

a. after 14 years? b. after 49.6 years? c. after 1000 years?

41. The half-life of strontium-90 is 28.9 years. How long would it

take for the activity of a Sr-90 sample to diminish by 87.5%?

42. The half-life of strontium-85 (about 64 days) is considerably

shorter than that of strontium-90. How long would it take for

the activity of a sample of Sr-85 to diminish by 93.75%? Why

do you think that Sr-85 is used diagnostically for bone scans,

but Sr-90 is not?

Chemical Applications and Practices

43. A 10-mCi sample of a tracer isotope is used to diagnose

blood ﬂow from the heart. If it is desirable that nearly all of

the radioactivity (99%) be gone after 3 days, approximately

what half-life should the isotope have?

44. It is estimated that the nuclear accident at Chernobyl released

1.85 × 10

becquerels, a large amount of radiation. How

many curies is this? Why is it not easy to translate either of

these values into the number of radioactive atoms present?

Chernobyl nuclear power plant.

56. Use the data in Problem 55 to calculate the mass defect for

the process. What is the source/result of any difference in

these masses?

57. Describe the similarities and differences between beta-minus

and beta-plus emission.

58. The stable isotopes of carbon are

C and

C. What decay

mode would you predict for

Section 21.6 Nuclear Stability

and Human-made Radioactive Nuclides

Skill Review

59. a. What does “doubly magic” mean, in reference to nuclides?

b. Give two examples of nuclei that are doubly magic and

two examples of nuclides that have no magic numbers at

all. How would you expect these nuclei to differ?

60. Suggest a reason why the heavier elements have proportion-

ately more neutrons than the lighter ones.

Chemical Applications and Practices

61. Element 114 was recently discovered. Why was this element

sought, but not the neighboring elements 115 and 113?

62. Some claims to the discovery of element 118 have been made.

Give reasons why this element may be considered to have

both nuclear and chemical stability.

Section 21.7 Splitting the Atom: Nuclear Fission

Skill Review

63. Why is alpha emission a better prediction for the mode of ra-

dioactive decay for uranium or plutonium than it is for iron,

carbon, or hydrogen?

64. Answer the following questions for the ﬁssion of plutonium:

239

Pu +

n n [

240

Pu] n

136

Sb +

100

Tc + 4

n + energy

a. What is the signiﬁcance of the fact that neutrons are

produced?

b. What is the source of the energy in this equation?

65. Iron and cobalt are not expected to undergo nuclear ﬁssion.

Why?

66. Would you expect helium to undergo nuclear ﬁssion? Will

hydrogen-1 undergo ﬁssion?

67. Write nuclear reactions for the ﬁssion of

235

U to form:

Kr and

139

Ba and neutrons

Sr and

153

Xe and neutrons

68. An isotope of the element technetium can be produced by

bombarding molybdenum-97 with deuterium nuclei. Two

neutrons are also formed. Write the nuclear equation.

Chemical Applications and Practices

69. On our planet, both U-235 and U-238 occur naturally.

a. How do these isotopes differ?

b. What is the natural abundance of each?

c. Propose a reason why it is very difﬁcult to separate these

two isotopes.

932 Chapter 21 Nuclear Chemistry

70. Using “conventional” explosives such as TNT, you can make

tiny explosive devices as well as huge ones. Is it possible to

make a similarly tiny nuclear bomb? Why or why not?

71. When ﬁssion of U-235 or Pu-239 occurs, elements such as

americium, californium, and berkelium are not found in the

fallout. Explain why.

72. One particularly nasty component of nuclear fallout is stron-

tium-90. Explore the reactivity of this particular isotope and

explain why it may be harmful to living creatures.

Section 21.8 Medical Uses of Radioisotopes

Skill Review

73. What questions should you ask about a radionuclide to be in-

jected for diagnostic purposes?

74. Why aren’t alpha emitters useful for diagnostic scans, such as

a scan of the heart or of the thyroid gland?

75. Technetium-99m samples should not be stored overnight

but, rather, should be freshly prepared each day for diagnos-

tic scans. Why?

76. Why is molybdenum-99 not used directly as a component of

a radiopharmaceutical?

Chemical Applications and Practices

77. A patient was injected with 10 mg of ﬂuorine-18–labeled glu-

cose for a PET scan. Fluorine-18 has a half-life of 110 min-

utes and disintegrates by positron emission. Write the nu-

clear equation for the decay.

78. Using the information in Problem 77, determine the amount

of time needed to reduce the radioactivity of ﬂuorine-18 to

1/16 of its original activity.

79. In the chapter, it was mentioned that it takes about two and a

half weeks for Tc-99 to be eliminated from the body. The

Department of Energy reports that it takes approximately

60 hours for the body to eliminate half of the technetium. Are

these two ﬁgures consistent with each other?

80. Gallium-67 citrate is used as a radiopharmaceutical for diag-

nosing tumors and infections.

a. What type of radioactive decay would you predict for this

nuclide?

b. A typical activity of a radionuclide used in the treatment

of an adult lymphoma is on the order of 10 mCi. After how

many days would radiation levels drop to less than a

millicurie?

Comprehensive Problems

81. Use the Internet to research the connection between smoking

and exposure to the nuclides polonium-210 and lead-210.

82. For the same dose of radiation, which has a higher dose

equivalent, strontium-90 or radon-222?

83. How do you know whether or not a gamma ray accompanies

an alpha or a beta emission?

84. Write nuclear equations for the following processes:

a. A positron is emitted by oxygen-15.

b. Boron-11 is formed by positron emission.

c. A positron is emitted by chlorine-35.

d. Oxygen-18 is formed by positron emission.

85. One of the radioactive decay series is shown below. Identify

the mode of radioactive decay at each step.

232

Th n

228

Ra n

228

Ac n

228

Th n

224

Ra n

220

Rn n

216

Po n

212

Pb n

212

Bi n

212

Po n

208

86. In the radioactive decay series in the previous problem, lead-

212 was formed. Why didn’t the decay series stop at lead?

87. In the radioactive decay series given in Problem 85, which el-

ements are represented by the symbols Rn and Ra? Which

one is a gas? Which one is a metal? Which one is chemically

inert?

88. Radon is produced in three of the naturally occurring decay

series. Which three? Which isotopes of radon are formed?

How would you expect these isotopes to differ? How would

you expect them to be the same?

89. Of the three radon isotopes mentioned in Problem 88, only

radon-222 is a health hazard. Propose a reason why, and then

research your answer to see whether you are correct.

90. Many tropical islands are volcanic in origin and contain ura-

nium in the rocks and minerals beneath the soils. However,

radon is less likely to be a problem in homes built in the trop-

ics. Propose two reasons why (and more if you can).

91. How does the nucleus of a carbon atom compare in density

with that of elemental lead (d = 11.3 g/cm

)? To answer this

question, calculate the volume of a

C nucleus, assuming

that the nucleus is spherical and that the radius is 1.2 ×

−13

cm. The mass of the nucleus in

C is 11.96709 u, or

1.98718 × 10

−23

92. The transformation of elements into other elements also

takes place in stars. Write the nuclear equation for the forma-

tion of oxygen-16 when carbon-12 is hit with an alpha parti-

cle. A gamma ray is also released in this reaction.

93. Does food irradiation make the food radioactive? Find an an-

swer to this question using the resources of the World Wide

Web. Cite your sources.

94. Look up on the Internet the current maximum allowed ex-

posure of workers in the nuclear industry. The Department

of Energy (DOE) sets this standard. How does this standard

vary for some individuals?

95. Irene Curie and her mother Marie are not the only scientists

to have won the Nobel Prize for their pioneering work in

nuclear chemistry and physics. Others include Ernest

Rutherford, Ernest O. Lawrence, and Emilio G. Segrè. Use the

Internet to ﬁnd out why these and/or other prizes for nuclear

work were awarded.

96. We pointed out in the text that palladium-103 (half-life =

16.97 days) is used in the treatment of prostate cancer, re-

placing iodine-125 (half-life = 59.4 days), though both are

widely used. Recently, Cs-131 (half-life = 9.7 days) has been

used. Why are these different products being used? That is,

what are the advantages and disadvantages of each?

97. Palladium-103 has a half-life of 16.97 days.

a. What is the half-life in years?

b. How many protons, neutrons, and electrons are in an atom

of palladium-103 in PdCl

c. A researcher develops cisplatin (Pd(NH

)

) using the

palladium-103 isotope. Assume that the process involves

the direct conversion of palladium(II) chloride to cisplatin

in one step, but the process itself requires 24 hours to

complete. How many grams of radioactive cisplatin would

remain if the researcher started with 100.0 g of pure

palladium-130(II) chloride?

Thinking Beyond the Calculation

98. Americium oxide is typically used in household smoke detec-

tors. A document reports that a gram of americium oxide

provides enough active material for “more than 5000 house-

hold smoke detectors.” The particular isotope used in this ap-

plication is americium-241.

a. How much americium is present in a typical smoke

detector?

b. Give two reasons why Am-240 and Am-242 would not be

appropriate isotopes to use.

c. Americium-241 decays by alpha emission. Write the

nuclear reaction for this process.

d. The half-life of americium-241 is 432.2 years. How long

will it take the reactivity of a sample of this nuclide to drop

to 1% of its original activity?

e. Beta-particle emission by americium-242 is found in 83%

of the sample. The rest of an americium-242 sample

decays by electron capture. Write nuclear reactions for

these processes.

Focus Your Learning

933

Smoke detectors contain about 150 mil-

lionths of a gram of americium-241,

which is extracted from spent fuel rods.

The

Chemistry

of Life

The cell is an intricate ballet of huge poly-

mers, proteins, hormones, and lipids.

Their dance keeps the cell alive. Shown

here is a colored transmission electron mi-

crograph of the protozoa responsible for

causing meningoencephalitis (inﬂamma-

tion of the brain and its membranes). The

nucleus (red) contains a large body known

as a nucleolus (yellow), where RNA is

made.

934

Contents and Selected Applications

22.1 DNA—The Basic Structure

Chemical Encounters: The Human Genome Project

22.2 Proteins

22.3 How Genes Code for Proteins

22.4 Enzymes

22.5 The Diversity of Protein Functions

22.6 Carbohydrates

22.7 Lipids

22.8 The Maelstrom of Metabolism

22.9 Biochemistry and Chirality

22.10 A Look to the Future

Go to college.hmco.com/pic/kelterMEE for online learning resources.

Guanine

Adenine

Cytosine

Thymine

FIGURE 22.2

The four nitrogenous

organic bases found

in DNA.

Life is an adventure of changes. From the mo-

ment of conception, through birth, growth, adoles-

cence, maturity, aging, and death, we are sustained by an

endless process of change. The chemistry of life is the chem-

istry of these changes, and as you might expect, it is exceptionally

complex. The human body is one of the most intricate chemical sys-

tems we know, and yet there is a wonderful simplicity at the root of it all.

Our introduction to chemistry has given us the basic tools with which to

examine the chemistry of life in some detail. At the root of this detail is a chemical

we call DNA. This substance serves as the set of instructions that operate the chemi-

cal processes in living things. To understand the chemical reactions that take place in

the body, we must understand the nature of DNA because the chemistry of life is

based on DNA and the chemical species it produces.

935

22.1 DNA—The Basic Structure

In the summer of the year 2000, scientists participating in the International

Human Genome Project announced that they had reached the ﬁrst major mile-

stone in working out the structure of all the genetic material that is needed to

make a human. They had deciphered a rough “ﬁrst draft” of a com-

plete set of human genes. Work will continue for many years to

complete the draft and then to look at the different versions of genes

that underlie many of the differences among us. The information

gained about the structure of our genes will be used to develop new

medicines and biotechnologies and to understand much more

about how life works. What exactly are genes?

Why are they the focus

of such intense scrutiny?

To best uncover the meaning and signiﬁcance of the gene, we

will begin with DNA.

Deoxyribonucleic acid (DNA) is the name for a

series of polymers composed of chemicals called

nucleotides.In

DNA, each nucleotide is itself composed of a phosphate group

bonded to a

deoxyribose sugar group, bonded to one of four ni-

trogenous organic bases

(schematically shown in Figure 22.1). The

four nitrogenous organic bases, shown in Figure 22.2, are adenine,

guanine, thymine, and cytosine.

DNA is a polymer of the four nucleotides bonded together in the

manner shown in Figure 22.3. The phosphate groups on the nu-

cleotides participate in condensation reactions to form phosphodi-

ester bonds that bind the individual nucleotides into the DNA chain.

Recall that condensation reactions result in the coupling of two

molecules with the elimination of a molecule of water (Chapter 12).

N O

Phosphate

Deoxyribose sugar

Nitrogenous organic base

Deoxynucleoside

Deoxynucleotide

–

HC C

FIGURE 22.1

A nucleotide contains a nitrogenous organic base, a

deoxyribose sugar, and a phosphate.

Application

HEMICAL ENCOUNTERS:

The Human Genome

Project

Video Lesson:

Nucleic Acids

936 Chapter 22 The Chemistry of Life

FIGURE 22.3

The structure of DNA. Note how a strand

of DNA naturally twists when the nitro-

genous bases are on the same side of

the molecule.

–

H H

–

H H

–

FIGURE 22.4

The condensation of two

nucleotides and the elim-

ination of water form the

backbone of the DNA

molecule.

–

H H

–

H H

+ H

In DNA, the phosphate’s —OH group is lost as the sugar’s oxygen adds to the

phosphorus atom (Figure 22.4).

The resulting condensation product is known as single-stranded DNA. How-

ever, the DNA that makes up genes is more complex in a very beautiful and sig-

niﬁcant way. This occurs when a second strand of DNA (upside down in relation

to the ﬁrst strand) interacts with the ﬁrst strand.

Why does DNA occur as pairs of

strands?

Double-stranded DNA is much more stable than a single strand, because

the nitrogenous bases are able to participate in hydrogen bonds with each other.

The hydrogen bonding results in a

base pair. As a consequence of their relative

size, the amount of space available between the two DNA strands, and the types

of hydrogen bonds in the bases, adenine always base-pairs with thymine, and cy-

tosine always base-pairs with guanine (Figure 22.5). We say that these pairs of

bases are complementary; that is, they pair in a reciprocal fashion with one

another.

To improve the hydrogen bonding, the two strands of DNA wind around each

other to form a base-paired double helix, as shown in Figure 22.6. This structure

had eluded scientists for quite some time until James Watson and Francis Crick

22.1 DNA—The Basic Structure 937

FIGURE 22.5

Base pairs in DNA, looking down the axis of the DNA molecule. Note the proximity of the oxygen

atom to the amine group on the opposite base, and that of the nitrogen to the opposite amine.

Adenine

Thymine

Guanine

Cytosine

Deoxyribose

T–A

C–G

obtained the 1953 X-ray diffraction photograph shown in Figure 22.7. The image

of this diffraction pattern was made by the chemist Rosalind Franklin and given

to Watson and Crick without her knowledge. It turned out to be the key piece of

data used to determine the structure of the double helix. In fact, the structure of

the DNA double helix has become an icon of science. It was even the centerpiece

of the 2004 Olympic Summer Games (Figure 22.8). Why is this structure so

important? Within the double helix, we ﬁnd the secret of how life is able to

reproduce—and also the secret of how mere chemicals can contain the coded

information that controls the structure and activities of all living things.

FIGURE 22.6

The double helix of DNA.

Side view View down the axis

FIGURE 22.7

Rosalind Franklin (1920–1958) and the diffrac-

tion pattern she made that solved the double-

helical structure of DNA. Her contributions to

this discovery were never acknowledged during

her life. She died of cancer at a very early age.

FIGURE 22.8

The opening ceremony of the 2004 Sum-

mer Olympic Games in Athens, Greece.