chapter 22 nuclear chemistry · nuclear chemistry 701 section 22-1 o bjectives explain what a...

C H A P T E R 2 2

Nuclear Chemistry

Particle detectors are important tools in the study of nuclear chemistry.

Copyright © by Holt, Rinehart and Winston. All rights reserved.


The Nucleus

A tomic nuclei are made of protons and neutrons, which are collec-tively called nucleons. In nuclear chemistry, an atom is referred to as anuclide and is identified by the number of protons and neutrons in its nucleus. Nuclides can be represented in two ways: when a symbolsuch as 228

88Ra is used, the superscript is the mass number and the sub-script is the atomic number; the same nuclide can also be written asradium-228.

Mass Defect and Nuclear Stability

Because an atom is made of protons, neutrons, and electrons, youmight expect the mass of an atom to be the same as the mass of anequal number of isolated protons, neutrons, and electrons. However,this is not the case. Let’s consider a 42He atom as an example. The com-bined mass of two protons, two neutrons, and two electrons is calcu-lated below.

2 protons: (2 × 1.007 276 amu) = 2.014 552 amu2 neutrons: (2 × 1.008 665 amu) = 2.017 330 amu2 electrons: (2 × 0.000 5486 amu) = 0.001 097 amu

total combined mass: 4.032 979 amu

However, the atomic mass of a 42He atom has been measured to be

4.002 60 amu. The measured mass, 4.002 60 amu, is 0.030 38 amu lessthan the calculated mass, 4.032 98 amu. The difference between the massof an atom and the sum of the masses of its protons, neutrons, and elec-trons is called the mass defect.

Nuclear Binding EnergyWhat causes the loss in mass? According to Albert Einstein’s equationE = mc2, mass can be converted to energy, and energy to mass. The massdefect is caused by the conversion of mass to energy upon formation ofthe nucleus. The mass units of the mass defect can be converted toenergy units by using Einstein’s equation. First, convert 0.030 38 amu tokilograms to match the units for energy, kg•m2/s2.

0.030 38 amu × = 5.0446 × 10−29 kg1.6605 × 10−27 kg

1 amu

N U C L E A R C H E M I S T R Y 701

SECTION 22-1

OBJECTIVES

Explain what a nuclide is, anddescribe the different waysnuclides can be represented.

Define and relate the termsmass defect and nuclear binding energy.

Explain the relationshipbetween nucleon number and stability of nuclei.

Explain why nuclear reactionsoccur, and know how to balance a nuclear equation.

Module 2: Models of the Atom

CHEMISTRYCHEMISTRY

INTERACTIV

E•

T U T O R


The energy equivalent can now be calculated.

E = mc2

E = (5.0446 × 10−29 kg)(3.00 × 108 m/s)2

= 4.54 × 10−12 kg•m2/s2 = 4.54 × 10−12 J

This is the nuclear binding energy, the energy released when a nucleusis formed from nucleons. This energy can also be thought of as theamount of energy required to break apart the nucleus. Therefore, thenuclear binding energy is also a measure of the stability of a nucleus.

Binding Energy per NucleonThe binding energy per nucleon is used to compare the stability of dif-ferent nuclides, as shown in Figure 22-1. The binding energy per nucleonis the binding energy of the nucleus divided by the number of nucleons itcontains. The higher the binding energy per nucleon, the more tightlythe nucleons are held together. Elements with intermediate atomicmasses have the greatest binding energies per nucleon and are there-fore the most stable.

Nucleons and Nuclear Stability

Stable nuclides have certain characteristics. When the number of pro-tons in stable nuclei is plotted against the number of neutrons, as shownin Figure 22-2, a beltlike graph is obtained. This stable nuclei cluster overa range of neutron-proton ratios is referred to as the band of stability.Among atoms having low atomic numbers, the most stable nuclei arethose with a neutron-proton ratio of approximately 1:1. For example,42He, a stable isotope of helium with two neutrons and two protons, hasa neutron-proton ratio of 1:1. As the atomic number increases, the sta-ble neutron-proton ratio increases to about 1.5:1. For example, 206

82Pb,with 124 neutrons and 82 protons, has a neutron-proton ratio of 1.51:1.

This trend can be explained by the relationship between the nuclearforce and the electrostatic forces between protons. Protons in a nucleusrepel all other protons through electrostatic repulsion, but the short

C H A P T E R 2 2702

FIGURE 22-1 This graph showsthe relationship between bindingenergy per nucleon and mass num-ber. The binding energy per nucleonis a measure of the stability of anucleus.

Mass number

Bin

din

g e

ner

gy

per

nu

cleo

n

(kJ/

mo

l)

0 20 40 60 80 100 120 140 160 180 200 220 2400

1x108

2x108

3x108

4x108

5x108

6x108

7x108

8x108

9x108

10x108

42He

10 5B

21H

5626Fe

23892 U

63Li

Binding Energy per Nucleon


C

A

B

Number of protons

Nu

mb

er o

f n

eutr

on

s

00

10

10 20 30 40

n/p = 1.5:1

n/p = 1:1

50 60 70 80 90 100

20

30

40

50

60

70

80

90

100

110

120

130

The Band of Stability

range of the nuclear force allows them to attract only protons very closeto them, as shown in Figure 22-3. So as the number of protons in anucleus increases, the electrostatic force between protons increasesfaster than the nuclear force. More neutrons are required to increasethe nuclear force and stabilize the nucleus. Beyond the atomic number83, bismuth, the repulsive force of the protons is so great that no stablenuclides exist.

Stable nuclei tend to have even numbers of nucleons. Out of 265 sta-ble nuclides, 159 have even numbers of both protons and neutrons. Onlyfour nuclides have odd numbers of both. This indicates that stability ofa nucleus is greatest when the nucleons—like electrons—are paired.

The most stable nuclides are those having 2, 8, 20, 28, 50, 82, or 126protons, neutrons, or total nucleons. This extra stability at certain num-bers supports a theory that nucleons—like electrons—exist at certainenergy levels. According to the nuclear shell model, nucleons exist indifferent energy levels, or shells, in the nucleus. The numbers of nucleonsthat represent completed nuclear energy levels—2, 8, 20, 28, 50, 82, and126—are called magic numbers.


FIGURE 22-2 The neutron-proton ratios of stable nuclides clus-ter together in a region known asthe band of stability. As the numberof protons increases, the ratioincreases from 1:1 to about 1.5:1.

FIGURE 22-3 Proton A attractsproton B through the nuclear forcebut repels it through the electrostaticforce. Proton A mainly repels protonC through the electrostatic forcebecause the nuclear force reachesonly a few nucleon diameters.


C H A P T E R 2 2704

Identify the product that balances the following nuclear reaction: 21284Po → 4

2He +

1. The total mass number and atomic number must be equal on both sides of the equation.

21284Po → 4

2He +mass number: 212 − 4 = 208atomic number: 84 − 2 = 82

2. The nuclide has a mass number of 208 and an atomic number of 82, 20882Pb.

3. The balanced nuclear equation is 21284Po → 4

2He + 20882Pb

?

?

SOLUTION

SAMPLE PROBLEM 22- 1

Answer 21884Po → 4

2He + 21482Pb

Answer 25399Es + 4

2He → 10n + 256

101Md

Answer 14261Pm + −1

0e → 14260Nd

Complete the following nuclear equations:

1. 21884Po → 4

2He +

2. 25399Es + 4

2He → 10n +

3. 14261Pm + → 142

60Nd?

?

?

PRACTICE

SECTION REVIEW

1. Define the following terms:a. nuclide c. mass defectb. nucleon d. nuclear binding energy

2. How is nuclear stability related to the neutron-proton ratio?

3. Why do unstable nuclides undergo nuclear reactions?

4. Complete and balance the following nuclearequations:

a. 18775Re + → 188

75Re + 11H

b. 94Be + 4

2He → + 10n

c. 2211Na + → 22

10Ne?

?

?

Nuclear Reactions

Unstable nuclei undergo spontaneous changes that change their num-ber of protons and neutrons. In this process, they give off large amountsof energy and increase their stability. These changes are a type ofnuclear reaction. A nuclear reaction is a reaction that affects the nucleusof an atom. In equations representing nuclear reactions, the total of theatomic numbers and the total of the mass numbers must be equal onboth sides of the equation. An example is shown below.

94Be + 4

2He → 126C + 1

0n

Notice that when the atomic number changes, the identity of the ele-ment changes. A transmutation is a change in the identity of a nucleus asa result of a change in the number of its protons.



SECTION 22-2

OBJECTIVES

Define and relate the termsradioactive decay and nuclearradiation.

Describe the different types of radioactive decay and theireffects on the nucleus.

Define the term half-life, andexplain how it relates to thestability of a nucleus.

Define and relate the termsdecay series, parent nuclide,and daughter nuclide.

Explain how artificial radioactive nuclides are made,and discuss their significance.

Radioactive Decay

I n 1896, Henri Becquerel was studying the possible connectionbetween light emission of some uranium compounds after exposure tosunlight and X-ray emission. He wrapped a photographic plate in alightproof covering and placed a uranium compound on top of it. Hethen placed them in sunlight. The photographic plate was exposed eventhough it was protected from visible light, suggesting exposure by X rays. When he tried to repeat his experiment, cloudy weather pre-vented him from placing the experiment in sunlight. To his surprise, theplate was still exposed. This meant that sunlight was not needed toproduce the rays that exposed the plate. The rays were produced byradioactive decay. Radioactive decay is the spontaneous disintegrationof a nucleus into a slightly lighter nucleus, accompanied by emission ofparticles, electromagnetic radiation, or both. The radiation that exposedthe plate was nuclear radiation, particles or electromagnetic radiationemitted from the nucleus during radioactive decay.

Uranium is a radioactive nuclide, an unstable nucleus that undergoesradioactive decay. Studies by Marie Curie and Pierre Curie found thatof the elements known in 1896, only uranium and thorium wereradioactive. In 1898, the Curies discovered two new radioactive metal-lic elements, polonium and radium. Since that time, many other radioac-tive nuclides have been identified. In fact, all of the nuclides beyondatomic number 83 are unstable and thus radioactive.

Types of Radioactive Decay

A nuclide’s type and rate of decay depend on the nucleon content andenergy level of the nucleus. Some common types of radioactive nuclideemissions are summarized in Table 22-1.

Type Symbol Charge Mass (amu)

Alpha particle 42He 2+ 4.002 60

Beta particle −10β 1− 0.000 5486

Positron +10β 1+ 0.000 5486

Gamma ray γ 0 0

TABLE 22-1 Radioactive Nuclide Emissions

NSTA

TOPIC: Radioactive decayGO TO: www.scilinks.orgsci LINKS CODE: HC2221


C H A P T E R 2 2706

Alpha EmissionAn alpha particle (�) is two protons and two neutrons bound togetherand is emitted from the nucleus during some kinds of radioactive decay.Alpha particles are helium nuclei and have a charge of 2+. They areoften represented with the symbol 4

2He. Alpha emission is restrictedalmost entirely to very heavy nuclei. In these nuclei, both the number ofneutrons and the number of protons need to be reduced in order toincrease the stability of the nucleus.An example of alpha emission is thedecay of 210

84Po into 20682Pb, shown in Figure 22-4. The atomic number

decreases by two, and the mass number decreases by four.

21084Po → 206

82Pb + 42He

Beta EmissionElements above the band of stability are unstable because they havetoo many neutrons. To decrease the number of neutrons, a neutron canbe converted into a proton and an electron. The electron is emittedfrom the nucleus as a beta particle. A beta particle (�) is an electronemitted from the nucleus during some kinds of radioactive decay.

10n → 1

1p + −10β

An example of beta emission, shown in Figure 22-5, is the decay of 146C

into 147N. Notice that the atomic number increases by one and the mass

number stays the same.

146C → 14

7N + −10β

Positron EmissionElements below the band of stability have too many protons to be sta-ble. To decrease the number of protons, a proton can be converted intoa neutron by emitting a positron. A positron is a particle that has thesame mass as an electron, but has a positive charge, and is emitted fromthe nucleus during some kinds of radioactive decay.

FIGURE 22-5 Beta emissioncauses the transmutation of 14

6C into 14

7N. Beta emission is a type ofradioactive decay in which a protonis converted to a neutron with theemission of a beta particle.

146C

147N

0–1β

Po21084 Pb206

82

He42

FIGURE 22-4 An alpha particle, identical to a helium nucleus, is emittedduring the radioactivedecay of some very heavy nuclei.


Wav

elen

gth

(m

)

X rays Microwaves

Ultraviolet rays

AMradio waves

Infrared heat waves

TV and FMradio waves

Longradio waves

Gamma rays Visible light waves

10–13 10–12 10–11 10–10 10–9 10–8 10–7 10–6 10–5 10–4 10–3 10–2 10–1 1 101 102 103 104 105

11p → 1

0n + +10β

An example of positron emission is the decay of 3819K into 38

18Ar. Noticethat the atomic number decreases by one but the mass number staysthe same.

3819K → 38

18Ar + +10β

Electron CaptureAnother type of decay for nuclides with too many protons is electroncapture. In electron capture, an inner orbital electron is captured by thenucleus of its own atom. The inner orbital electron combines with a pro-ton, and a neutron is formed.

−10e + 1

1p → 10n

An example of electron capture is the radioactive decay of 10647Ag into

10646Pd. Just as in positron emission, the atomic number decreases by one

but the mass number stays the same.

10647Ag + −1

0e → 10646Pd

Gamma EmissionGamma rays (�) are high-energy electromagnetic waves emitted from anucleus as it changes from an excited state to a ground energy state. Theposition of gamma rays in the electromagnetic spectrum is shown inFigure 22-6. The emission of gamma rays is another piece of evidencesupporting the nuclear shell model. According to the nuclear shellmodel, gamma rays are produced when nuclear particles undergotransitions in nuclear-energy levels. This is similar to the emission oflight when an electron drops to a lower-energy level, which was coveredin Chapter 4. Gamma emission usually occurs immediately followingother types of decay, when other types of decay leave the nucleus inan excited state.


FIGURE 22-6 Gamma rays, likevisible light, are a form of electro-magnetic radiation, but they have a much shorter wavelength and are much higher in energy thanvisible light.


Half-Life

No two radioactive isotopes decay at the same rate. Half-life, t1/2, is thetime required for half the atoms of a radioactive nuclide to decay. Lookat the graph of the decay of radium-226 in Figure 22-7. Radium-226 hasa half-life of 1599 years. Half of a given amount of radium-226 decays in1599 years. In another 1599 years, half of the remaining radium-226decays. This process continues until there is a negligible amount ofradium-226. Each radioactive nuclide has its own half-life. More-stablenuclides decay slowly and have longer half-lives. Less-stable nuclidesdecay very quickly and have shorter half-lives, sometimes just a fractionof a second. Some representative radioactive nuclides, along with theirhalf-lives and types of decay, are given in Table 22-2.

C H A P T E R 2 2708

FIGURE 22-7 The half-life ofradium-226 is 1599 years. Half of theremaining radium-226 decays by theend of each additional half-life.

Nuclide Half-life Nuclide Half-life31H 12.32 years 214

84Po 163.7 µs146C 5715 years 218

84Po 3.0 min3215P 14.28 days 218

85At 1.6 s4019K 1.3 × 109 years 238

92U 4.46 × 109 years6027Co 10.47 min 239

94Pu 2.41 × 104 years

TABLE 22-2 Representative Radioactive Nuclides and Their Half-Lives

Am

ou

nt

of

rad

ium

-226

(m

g)

1599 3198 4797 6396

2

4

6

8

10

12

14

16

18

20

0

Time (years)

Rate of Decay

NSTA

TOPIC: Half-lifeGO TO: www.scilinks.orgsci LINKS CODE: HC2222



Phosphorus-32 has a half-life of 14.3 days. How many milligrams of phosphorus-32 remain after 57.2 days if you start with 4.0 mg of the isotope?

Given: original mass of phosphorus-32 = 4.0 mghalf-life of phosphorus-32 = 14.3 daystime elapsed = 57.2 days

Unknown: mass of phosphorus-32 remaining after 57.2 days

To determine the number of milligrams of phosphorus-32 remaining, we must first find thenumber of half-lives that have passed in the time elapsed. Then the amount of phosphorus-32 is determined by reducing the original amount by half for every half-life that has passed.

number of half-lives = time elapsed (days) ×

amount of phosphorus-32 remaining =original amount of phosphorus-32 × for each half-life

number of half-lives = 57.2 days × = 4 half-lives

amount of phosphorus-32 remaining = 4.0 mg × × × × =0.25 mg

A period of 57.2 years is four half-lives for phosphorus-32. At the end of one half-life,2.0 mg of phosphorus-32 remains; 1.0 mg remains at the end of two half-lives; 0.50 mgremains at the end of three half-lives; and 0.25 mg remains at the end of four half-lives.

12

12

12

12

1 half-life

14.3 days

12

1 half-life

14.3 days

SOLUTION

1 ANALYZE

2 PLAN

3 COMPUTE

4 EVALUATE

SAMPLE PROBLEM 22- 2

Answer0.25 mg

Answer6396 years

Answer7.648 days

Answer0.00977 mg

Answer4.46 × 109

years

Answer3.0 min

1. The half-life of polonium-210 is 138.4 days. How many milligrams of polonium-210 remain after 415.2 days if you start with 2.0 mg of the isotope?

2. Assuming a half-life of 1599 years, how many years will be needed

for the decay of of a given amount of radium-226?

3. The half-life of radon-222 is 3.824 days. After what time will one-fourth of a given amount of radon remain?

4. The half-life of cobalt-60 is 10.47 min. How many milligrams of cobalt-60 remain after 104.7 min if you start with 10.0 mg?

5. A sample contains 4.0 mg of uranium-238. After 4.46 × 109 years, thesample will contain 2.0 mg of uranium-238. What is the half-life ofuranium-238?

6. A sample contains 16 mg of polonium-218. After 12 min, the samplewill contain 1.0 mg of polonium-218. What is the half-life of polonium-218?

1516

PRACTICE


Atomic number

Mas

s n

um

ber

242

238

234

230

226

222

218

214

210

206

204

Uranium-238 Decay Series

0 80 81 82 83 84 85 86 87 88 89 90 91 92 93

s = secondsmin = minutes

d = daysy = years

= alpha emission= beta emission

20681Tl

4.2 min

21081Tl

1.3 min

23490Th

24.1 d

23491Pa

1.2 min

22688Ra

1599 y

23090Th

7.5X104y

23492U

2.5X105y

22286Rn

3.8 d

21084 Po

138.4 d

20682Pb

stable

21083 Bi

5.01 d

21482 Pb

27. min

21483Bi

19.9 min

21885At

1.6 s

21884Po

3.0 min

21484 Po

163.7 µs

21082 Pb

22.6 y

23892U

4.5X109y

Decay Series

One nuclear reaction is not always enough to produce a stable nuclide.A decay series is a series of radioactive nuclides produced by succes-sive radioactive decay until a stable nuclide is reached. The heaviestnuclide of each decay series is called the parent nuclide. The nuclidesproduced by the decay of the parent nuclides are called daughternuclides. All naturally occurring nuclides with atomic numbersgreater than 83 are radioactive and belong to one of three naturaldecay series. The parent nuclides are uranium-238, uranium-235, andthorium-232. The transmutations of the uranium-238 decay series arecharted in Figure 22-8.

Locate the parent nuclide, uranium-238, on the chart. As the nucleusof uranium-238 decays, it emits an alpha particle. The mass number ofthe nuclide, and thus the vertical position on the graph, decreases byfour. The atomic number, and thus the horizontal position, decreases bytwo. The daughter nuclide is an isotope of thorium.

23892U → 234

90Th + 42He

C H A P T E R 2 2710

FIGURE 22-8 This chart showsthe transmutations that occur as 238

92Udecays to the final, stable nuclide,206

82Pb. Decay usually follows thesolid arrows. The dotted arrows represent alternative routes of decay.


The half-life of 23490Th, about 24 days, is indicated on the chart. It

decays by giving off beta particles. This increases its atomic number,and thus its horizontal position, by one. The mass number, and thus itsvertical position, remains the same.

23490Th → 234

91Pa + −10β

The remaining atomic number and mass number changes shown onthe decay chart are also explained in terms of the particles given off. Inthe final step, 210

84Po loses an alpha particle to form 20682Pb.This is a stable,

nonradioactive isotope of lead. Notice that 20682Pb contains 82 protons, a

magic number. It contains the extra-stable nuclear configuration of acompleted nuclear shell.

Artificial Transmutations

Artificial radioactive nuclides are radioactive nuclides not found natu-rally on Earth. They are made by artificial transmutations, bombard-ment of stable nuclei with charged and uncharged particles. Becauseneutrons have no charge, they can easily penetrate the nucleus of anatom. However, positively charged alpha particles, protons, and otherions are repelled by the nucleus. Because of this repulsion, great quan-tities of energy are required to bombard nuclei with these particles. Thenecessary energy may be supplied by accelerating these particles in themagnetic or electrical field of a particle accelerator. An example of anaccelerator is shown in Figure 22-9.


FIGURE 22-9 This is an aerialview of the Fermi InternationalAccelerator Laboratory (Fermilab),in Illinois. The particle acceleratorsare underground. The Tevatron ring,the larger particle accelerator, has acircumference of 4 mi. The smallerring in the background is a newaccelerator, the Main Injector.

Artificial Radioactive NuclidesRadioactive isotopes of all the natural elements havebeen produced by artificial transmutation. In addition,production of technetium, astatine, francium, and prome-thium by artificial transmutation has filled gaps in theperiodic table. Their positions are shown in Figure 22-10.

Artificial transmutations are also used to produce thetransuranium elements. Transuranium elements are ele-ments with more than 92 protons in their nuclei. All ofthese elements are radioactive. The nuclear reactions forthe synthesis of several transuranium elements are shownin Table 22-3. Currently, 17 artificially prepared transura-nium elements have been reported. The positions of thetransuranium elements in the periodic table are alsoshown in Figure 22-10.

C H A P T E R 2 2712Copyright © by Holt, Rinehart and Winston. All rights reserved.

Atomic number Name Symbol Nuclear reaction

93 neptunium Np 23892U + 1

0n → 23992U

23992U → 239

93Np + −10β

94 plutonium Pu 23893Np → 238

94Pu + −10β

95 americium Am 23994Pu + 21

0n → 24195Am + −1

0β

96 curium Cm 23994Pu + 4

2He → 24296Cm + 1

0n

97 berkelium Bk 24195Am + 4

2He → 24397Bk + 21

0n

98 californium Cf 24296Cm + 4

2He → 24598Cf + 1

0n

99 einsteinium Es 23892U + 151

0n → 25399Es + 7−1

0β

100 fermium Fm 23892U + 171

0n → 255100Fm + 8−1

0β

101 mendelevium Md 25399Es + 4

2He → 256101Md + 1

0n

102 nobelium No 24696Cm + 12

6C → 254102No + 41

0n

103 lawrencium Lr 25298Cf + 10

5B → 258103Lr + 41

0n

TABLE 22-3 Reactions for the First Preparation of Several Transuranium Elements

SECTION REVIEW

1. Define radioactive decay.

2. a. What are the different types of radioactivedecay?

b. List the types of radioactive decay that involveconversion of particles.

3. What fraction of a given sample of a radioactivenuclide remains after four half-lives?

4. When does a decay series end?

5. Distinguish between natural and artificialradioactive nuclides.

FIGURE 22-10 Artificial transmutations filled thegaps in the periodic table, shown in red, and extendedthe periodic table with the transuranium elements,shown in blue.



Nuclear Radiation

I n Becquerel’s experiment, nuclear radiation from the uranium com-pound penetrated the lightproof covering and exposed the film. Differ-ent types of nuclear radiation have different penetrating abilities. Nuclearradiation includes alpha particles, beta particles, and gamma rays.

Alpha particles have a range of only a few centimeters in air andhave a low penetrating ability due to their large mass and charge. Theycannot penetrate skin. However, they can cause damage if ingested orinhaled. Beta particles travel at speeds close to the speed of light andhave a penetrating ability about 100 times greater than that of alphaparticles. They have a range of a few meters in air. Gamma rays havethe greatest penetrating ability. The penetrating abilities and shieldingrequirements of different types of nuclear radiation are shown inFigure 22-11.

Radiation Exposure

Nuclear radiation can transfer its energy to the electrons of atoms ormolecules and cause ionization. The roentgen is a unit used to measurenuclear radiation; it is equal to the amount of radiation that produces2 × 10 9 ion pairs when it passes through 1 cm3 of dry air. Ionization candamage living tissue. Radiation damage to human tissue is measured inrems (roentgen equivalent, man). One rem is the quantity of ionizingradiation that does as much damage to human tissue as is done by1 roentgen of high-voltage X rays. Cancer and genetic effects caused byDNA mutations are long-term radiation damage to living tissue. DNA

FIGURE 22-11 The differentpenetrating abilities of alpha parti-cles, beta particles, and gamma raysrequire different levels of shielding.Alpha particles can be shielded withjust a sheet of paper. Lead or glass isoften used to shield beta particles.Gamma rays are the most penetrat-ing and require shielding with thicklayers of lead or concrete, or both.

SECTION 22-3

OBJECTIVES

Compare the penetrating ability and shielding requirements of alpha particles, beta particles,and gamma rays.

Define the terms roentgenand rem, and distinguishbetween them.

Describe three devices used in radiation detection.

Discuss applications ofradioactive nuclides.

Leadand/orconcrete

Leador glass

Paper

Alpha

Beta

Gamma


C H A P T E R 2 2714

can be mutated directly by interaction with radiation or indirectly byinteraction with previously ionized molecules.

Everyone is exposed to environmental background radiation.Averageexposure for people living in the United States is estimated to be about0.1 rem per year. However, actual exposure varies. The maximum per-missible dose of radiation exposure for a person in the general populationis 0.5 rem per year. Airline crews and people who live at high altitudeshave increased exposure levels because of increased cosmic-ray levelsat high altitudes. Radon-222 trapped inside homes may also causeincreased exposure. Because it is a gas, radon can move up from the soilinto homes through cracks and holes in the foundation. Radon trappedin homes increases the risk of lung cancer among smokers.

Radiation Detection

Film badges, Geiger-Müller counters, and scintillation counters are threedevices commonly used to detect and measure nuclear radiation. A filmbadge and a Geiger-Müller counter are shown in Figure 22-12. As pre-viously mentioned, nuclear radiation exposes film just as visible lightdoes. This property is used in film badges. Film badges use exposure offilm to measure the approximate radiation exposure of people workingwith radiation. Geiger-Müller counters are instruments that detect radia-tion by counting electric pulses carried by gas ionized by radiation.Geiger-Müller counters are typically used to detect beta-particle radia-tion. Radiation can also be detected when it transfers its energy to sub-stances that scintillate, or absorb ionizing radiation and emit visiblelight. Scintillation counters are instruments that convert scintillating lightto an electric signal for detecting radiation.

FIGURE 22-12 Film badges (a)and Geiger-Müller counters (b) areboth used to detect nuclear radiation.

(a) (b)



FIGURE 22-13 Radioactivenuclides, such as technetium-99,can be used to detect bone cancer.In this procedure, technetium-99accumulates in areas of abnormalbone metabolism. Detection of thenuclear radiation then shows thelocation of bone cancer.

Applications of Nuclear Radiation

Many applications are based on the fact that the physical and chemicalproperties of stable isotopes are essentially the same as those of radio-active isotopes of the same element.A few uses of radioactive nuclides arediscussed below.

Radioactive DatingRadioactive dating is the process by which the approximate age of anobject is determined based on the amount of certain radioactive nuclidespresent. Such an estimate is based on the fact that radioactive sub-stances decay with known half-lives. Age is estimated by measuringeither the accumulation of a daughter nuclide or the disappearance ofthe parent nuclide.

Carbon-14 is radioactive and has a half-life of approximately 5715years. It can be used to estimate the age of organic material up to about50 000 years old. Nuclides with longer half-lives are used to estimate theage of older objects; methods using nuclides with long half-lives havebeen used to date minerals and lunar rocks more than 4 billion years old.

Radioactive Nuclides in MedicineIn medicine, radioactive nuclides, such as the artificial radioactivenuclide cobalt-60, are used to destroy certain types of cancer cells. Manyradioactive nuclides are also used as radioactive tracers, which areradioactive atoms that are incorporated into substances so that movementof the substances can be followed by radiation detectors. Detection ofradiation from radioactive tracers can be used to diagnose cancer andother diseases. See Figure 22-13.

Radioactive Nuclides in AgricultureIn agriculture, radioactive tracers in fertilizers are used to determine theeffectiveness of the fertilizer. The amount of radioactive tracerabsorbed by a plant indicates the amount of fertilizer absorbed. Nuclearradiation is also used to prolong the shelf life of food. For example,gamma rays from cobalt-60 can be used to kill bacteria and insects thatspoil and infest food.

Nuclear Waste

Nuclear Fission and Nuclear FusionIn nuclear fission, the nucleus of a very heavy atom, such as uranium, issplit into two or more nuclei. The products of the fission include thenuclei as well as the nuclides formed from the fragments’ radioactivedecay. Fission is the primary system powering nuclear reactors, nuclearmissiles, and nuclear-powered submarines and aircraft carriers. Fusionis the opposite process of fission. In fusion, very high temperatures andpressures are used to combine light atoms, such as hydrogen, to makeheavier atoms, such as helium. Fusion is the primary process that fuels

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C H A P T E R 2 2716

SECTION REVIEW

1. What is required to shield alpha particles? Why arethese materials effective?

2. a. What is the average exposure of people living inthe United States to environmental backgroundradiation?

b. How does this relate to the maximum permis-sible dose?

3. What device is used to measure the radiationexposure of people working with radiation?

4. Explain why nuclear radiation can be used to preserve food.

5. Explain how nuclear waste is contained, stored,and disposed of, and how each method affects theenvironment.

our sun and the stars. Creating and maintaining a fusion reaction ismore complex and expensive than performing fission. Both fission andfusion release enormous amounts of energy that can be converted intoheat and electricity, and both produce nuclear waste. Fission producesmore waste than fusion. As new processes are developed to use energyfrom fission and fusion, a more vexing question arises: how to contain,store, and dispose of nuclear waste.

Containment of Nuclear WasteEvery radioactive substance has a half-life, which is the amount of timeneeded for half of a given material to decay into a stable nuclear form andlose its radioactivity. Radioactive waste from medical research, for exam-ple, has a half-life of a few months.Waste that is produced in a nuclear reac-tor will take hundreds of thousands of years to decay, and it needs to becontained so that living organisms can be shielded from radioactivity.Thereare two main types of containment: on-site storage and off-site disposal.

Storage of Nuclear WasteThe most common form of nuclear waste is spent fuel rods from nuclearpower plants. These fuel rods can be contained above the ground by plac-ing them in water pools or in dry casks. Each nuclear reactor in the UnitedStates has large pools of water where spent rods can be stored, and someof the radioactive materials will decay. When these pools are full, the rodsare moved to dry casks, which are usually made of concrete and steel. Bothstorage pools and casks are meant for only temporary storage before thewaste is moved to permanent underground storage facilities.

Disposal of Nuclear WasteDisposal of nuclear waste is done with the intention of never retrievingthe materials. Because of this, building disposal sites takes careful plan-ning. Currently, there are 77 disposal sites around the United States.TheU. S. Department of Energy is considering a new site near Las Vegas,Nevada, called Yucca Mountain, for the permanent disposal of much ofthis waste. This plan, however, is controversial—some organizationsoppose the idea of the disposal site, and others have alternate plans. Ifthe Yucca Mountain site is approved, nuclear waste will be transportedthere by truck and train beginning in 2010.



SECTION 22-4

OBJECTIVES

Define nuclear fission, chainreaction, and nuclear fusion,and distinguish between them.

Explain how a fission reactionis used to generate power.

Discuss the possible benefitsand the current difficulty ofcontrolling fusion reactions.

Nuclear Fission and Nuclear Fusion

Nuclear Fission

Review Figure 22-1 on page 702, which shows that nuclei of intermedi-ate mass are the most stable. In nuclear fission, a very heavy nucleussplits into more-stable nuclei of intermediate mass. This process releasesenormous amounts of energy. Nuclear fission can occur spontaneouslyor when nuclei are bombarded by particles. When uranium-235 is bom-barded with slow neutrons, a uranium nucleus may capture one of theneutrons, making it very unstable. The nucleus splits into medium-massparts with the emission of more neutrons.The mass of the products is lessthan the mass of the reactants. The missing mass is converted to energy.

Nuclear Chain ReactionWhen fission of an atom bombarded by neutrons produces moreneutrons, a chain reaction can occur. A chain reaction is a reac-tion in which the material that starts the reaction is also oneof the products and can start another reaction. As shownin Figure 22-14, two or three neutrons can be given offwhen uranium-235 fission occurs. These neutronscan cause the fission of other uranium-235nuclei. Again neutrons are emitted, which

10n

10n

10n

10n

10n

10n

10n

10n

10n

10n

10n

10n

23592 U

23592 U

23592 U

23592 U

23592 U

23592 U

23592 U

23592 U

23592 U

23592 U

14056 Ba

9336 Kr

14056 Ba

9336 Kr

14455 Cs

9037 Rb

14657 La

8735 Br

FIGURE 22-14 Fission induction of uranium-235 by bombardment with neutrons can lead to a chainreaction when a critical mass of uranium-235 is present.


C H A P T E R 2 2718

can cause the fission of still other uranium-235 nuclei. This chain reactioncontinues until all of the uranium-235 atoms have split or until the neu-trons fail to strike uranium-235 nuclei. If the mass of uranium-235 isbelow a certain minimum, too many neutrons will escape without strikingother nuclei, and the chain reaction will stop. The minimum amount ofnuclide that provides the number of neutrons needed to sustain a chainreaction is called the critical mass. Uncontrolled chain reactions providethe explosive energy of atomic bombs. Nuclear reactors use controlled-fis-sion chain reactions to produce energy or radioactive nuclides.

Nuclear Power PlantsNuclear power plants use heat from nuclear reactors to produce electricalenergy. They have five main components: shielding, fuel, control rods,moderator, and coolant. The components, shown in Figure 22-15, aresurrounded by shielding. Shielding is radiation-absorbing material thatis used to decrease exposure to radiation, especially gamma rays, fromnuclear reactors. Uranium-235 is typically used as the fissionable fuel toproduce heat, which is absorbed by the coolant. Control rods are neu-tron-absorbing rods that help control the reaction by limiting the numberof free neutrons. Because fission of uranium-235 is more efficientlyinduced by slow neutrons, a moderator is used to slow down the fast neu-trons produced by fission. Current problems with nuclear power plantdevelopment include environmental requirements, safety of operation,plant construction costs, and storage and disposal of spent fuel andradioactive wastes.

FIGURE 22-15 In this model of a nuclear power plant, pressurizedwater is heated by fission of uranium-235. This water is circulated to asteam generator. The steam drives a turbine to produce electricity.Cool water from a lake or river isthen used to condense the steam into water. The warm water from the condenser may be cooled in coolingtowers before being reused orreturned to the lake or river. Water heated by nuclear reactor

Water converted to steam

Water used to condense steam

Steam turbine

Electric current

Condenser

Warm water

Cool water

Pump

Steam generator

Pump

Nuclear reactor

Containmentstructure

Control rod

Fuel(uraniumfuel rod)

Moderator and coolant(liquid waterunder highpressure)

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Nuclear Fusion

The high stability of nuclei with intermediate masses can also be usedto explain nuclear fusion. In nuclear fusion, light-mass nuclei combine toform a heavier, more stable nucleus. Nuclear fusion releases even moreenergy per gram of fuel than nuclear fission. In our sun and other stars,four hydrogen nuclei combine at extremely high temperature and pres-sure to form a helium nucleus with a loss of mass and release of energy.This reaction is illustrated in Figure 22-16.

Uncontrolled fusion reactions of hydrogen are the source of energyfor the hydrogen bomb, as shown in Figure 22-17. A fission reaction isused to provide the heat and pressure necessary to trigger the fusion ofnuclei. Current research indicates that fusion reactions may be control-lable if some major problems can be overcome. One of the problems isthat no known material can withstand the initial temperatures, about108 K, required to induce fusion at high temperatures. Methods thatcontain the fusion reactions within a magnetic field or induce fusion atlower temperatures are being investigated.

N U C L E A R C H E M I S T R Y 719Copyright © by Holt, Rinehart and Winston. All rights reserved.

SECTION REVIEW

1. Distinguish between nuclear fission and nuclearfusion.

2. Define chain reaction.

3. List the five main components of a nuclear powerplant.

4. Explain how fusion is one of our sources of energy.

FIGURE 22-16 Fusion of hydrogen nuclei intomore-stable helium nuclei provides the energy ofour sun and other stars.

+

+

Nuclear fusion

4 H nuclei He nucleus 2 β particles11

42

0+1

FIGURE 22-17 The enormousamount of energy released by fusionreactions is illustrated by this explo-sion of a hydrogen bomb.


C H A P T E R 2 2720

G R E A T D I S C O V E R I E S

Neutrons in ItalyIn 1934, uranium had the mostprotons of any known element,92. But that year, Italian physicistEnrico Fermi believed he had syn-thesized elements with even higheratomic numbers. After bombardinga sample of uranium with neu-trons, Fermi and co-workers re-corded measurements that seemedto indicate that some uraniumnuclei had absorbed neutrons andthen undergone beta decay:

23892U + 1

0n → 23992U →

23993? + −1

0β

His report noted further, subsequentbeta decays, by which he hypothe-sized the existence of a whole newseries of “transuranic” elements:

23892U → 238

93? + −10β →

23894?? + −1

0β → 23895??? + −1

0β

Unfortunately, the Italian groupcould not verify the existence ofthe transuranes because, as Fermiput it, “We did not know enoughchemistry to separate the productsof uranium disintegration fromone another.”

Curiosity in BerlinFermi’s experiments caught theattention of a physicist in Berlin,Lise Meitner. Knowing that shecould not perform the difficult taskof chemically separating radionu-clides either, Meitner persuaded acolleague, radiochemist Otto Hahn,to help her explain Fermi’s results.Joined by expert chemical analystFritz Strassman, the team beganinvestigating neutron-induced uranium decay at the end of 1934.

From the onset, the Berlin team,along with all other scientists at the

time, operated under two falseassumptions.The first involved themakeup of the bombarded nuclei. Inevery nuclear reaction observed pre-viously, the resulting nucleus hadnever differed from the original bymore than a few protons or neutrons.Thus, it was assumed that theproducts of neutron bombardmentwere radioisotopes of elements atmost a few places in the periodictable before or beyond the atomsbeing bombarded (as Fermi hadpresumed in hypothesizing thetransuranes).

An Unexpected FindingH I S T O R I C A L P E R S P E C T I V E

The discovery of the artificial transmutation of uranium in 1934 triggered great excitement in science.Chemists preoccupied with identifying what they thought were the final missing elements of the periodic

table suddenly had to consider the existence of elements beyond atomic number 92, while physicists began toprobe the stability of the nucleus more deeply. By 1939, nuclear investigators in both fields had collaborated

to provide a stunning explanation for the mysterious results of uranium’s forced transformation.

This apparatus from Otto Hahn’s lab was used to produce fission reactions.

N U C L E A R C H E M I S T R Y 721Copyright © by Holt, Rinehart and Winston. All rights reserved.

The second assumption con-cerned the periodicity of thetransuranes. Because the ele-ments Ac, Th, Pa, and U chemi-cally resembled the third-rowtransition elements, La, Hf, Ta,and W, it was believed that ele-ments beyond U would corre-spondingly resemble thosefollowing W. Thus, thetransuranes were thought to behomologues of Re, Os, Ir, Pt, andso on. This belief was generallyunquestioned and seemed to beconfirmed. In fact, by 1937 Hahnwas sure of the chemical evidenceof transuranes:

. . . the chemical behavior of thetransuranes . . . is such that theirposition in the periodic systemis no longer in doubt. . . . theirchemical distinction from allpreviously known elementsneeds no further discussion.

Meitner’s Exile By 1938, the political situation inGermany had become dangerousfor Meitner. Because she was ofJewish descent, she was being targeted by the Nazis and fled toSweden to escape persecution.Meanwhile in Berlin, the anti-NaziHahn and Strassman had to becareful of every move they madeunder the watchful eyes of the fas-cists around them.

Despite censorship, the Berlinteam continued to communicatethrough letters. Meitner could notcome up with a satisfying physicalexplanation for the chemicalresults of Hahn and Strassman,

and she insisted that her partnersre-examine their findings. AsStrassman later recalled, Meitner

. . . requested that [the] experi-ments be scrutinized . . . onemore time. . . . FortunatelyL. Meitner’s opinion and judg-ment carried so much weight . . . the necessary control exper-iments were immediatelyundertaken.

A Shocking DiscoveryPrompted by Meitner, Hahn andStrassman realized they had beenlooking in the wrong place to findthe cause of their results. In ana-lyzing a fraction of a solutionassay that they had previouslyignored, they found the criticalevidence they had been seeking.

The analysis indicated that barium appeared to be a result of

neutron bombardment of ura-nium. Suspecting the spectaculartruth but lacking confidence,Hahn wrote to Meitner for anexplanation. After consultationwith her nephew, Otto Frisch,Meitner proposed that the ura-nium nuclei had been broken apartinto elemental fragments, one ofwhich was Ba. On January 3, 1939,she wrote to Hahn:

. . . the two of you really dohave a splitting to Ba . . . a trulybeautiful result, for which Imost heartily congratulate youand Strassman.

Thus, the transuranes turnedout to be merely radioisotopes ofknown elements—atomic frag-ments of uranium atoms that hadburst apart on being struck byneutrons.

For the discovery of this unexpected phenomenon, whichMeitner named nuclear fission,the talented Hahn was awardedthe 1944 Nobel Prize in chemistry.Due to wartime politics, however,Lise Meitner did not receive thecorresponding award in physics,and she was not popularly recog-nized until well after her death in1968 for her role in clarifying theprocess that she first explainedand named.

The politics of World War II prevented LiseMeitner from receiving the Nobel Prize in physics for explaining nuclear fission.

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C H A P T E R 22 R E V I E W

CHAPTER SUMMARY

C H A P T E R 2 2722

• The difference between the sum of the masses ofthe nucleons and electrons in an atom and theactual mass of an atom is the mass defect, ornuclear binding energy.

• Nuclear stability tends to be greatest when

nucleons are paired, when there are magic num-bers of nucleons, and when there are certainneutron-proton ratios.

• Nuclear reactions, represented by nuclear equa-tions, can involve the transmutation of nuclides.

22-1

band of stability (702) mass defect (701) nuclear shell model (703) nuclide (701)binding energy per nuclear binding energy nucleons (701) transmutation (704)

nucleon (702) (702)magic numbers (703) nuclear reaction (704)

film badges (714) nuclear waste (716) radioactive tracers (715) roentgen (713)Geiger-Müller counters (714) radioactive dating (715) rem (713) scintillation counters (714)

alpha particle (706) decay series (710) nuclear radiation (705) radioactive nuclide (705)artificial transmutations (711) electron capture (707) parent nuclide (710) transuranium elements (712)beta particle (706) gamma ray (707) positron (706)daughter nuclides (710) half-life (708) radioactive decay (705)

• Radioactive nuclides become more stable byradioactive decay, a type of nuclear reaction.

• Alpha, beta, positron, and gamma emission, andelectron capture are all types of radioactive decay.A nuclide’s type of decay is related to its nucleoncontent and the energy level of the nucleus.

• The half-life of a radioactive nuclide is the

length of time that it takes for half of a givennumber of atoms of the nuclide to decay.

• A decay series starts with a parent nuclide andends with a stable daughter nuclide.

• Artificial transmutations are used to produceartificial radioactive nuclides, including thetransuranium elements.

• Nuclear fission and nuclear fusion are nuclearreactions in which the splitting and fusing ofnuclei produce more stable nuclei and releaseenormous amounts of energy.

• Controlled fission reactions are used to produce

energy and radioactive nuclides.• Fusion reactions produce the sun’s heat and

light. If fusion reactions could be controlled,they would produce more usable energy pergram of fuel than fission reactions.

• Alpha particles, beta particles, and gamma rayshave different penetrating abilities and thereforedifferent shielding requirements.

• Film badges, Geiger-Müller counters, and scintil-lation detectors are used to detect radiation.

• Everyone is exposed to environmental back-

ground radiation. Exposure levels vary.• Radioactive nuclides have many uses, including

radioactive dating and cancer detection.• Nuclear waste must be contained, stored, and

disposed of in a way that does not harm peopleor the environment.

chain reaction (717) moderator (718) nuclear fusion (719) nuclear reactors (718)control rods (718) nuclear fission (717) nuclear power plant (718) shielding (718)critical mass (718)

Vocabulary

Vocabulary

Vocabulary

Vocabulary

22-2

22-3

22-4


CHAPTER 22 REVIEW


1. a. How does mass defect relate to nuclear binding energy?

b. How does binding energy per nucleon varywith mass number?

c. How does binding energy per nucleon affectthe stability of a nucleus? (22-1)

2. Describe three ways in which the number of protons and the number of neutrons in anucleus affect its stability. (22-1)

3. Where on the periodic table are most of thenatural radioactive nuclides located? (22-2)

4. What changes in atomic number and mass number occur in each of the following types of radioactive decay?a. alpha emissionb. beta emissionc. positron emissiond. electron capture (22-2)

5. Which types of radioactive decay cause thetransmutation of a nuclide? (Hint: Review thedefinition of transmutation.) (22-2)

6. Explain how beta emission, positron emission,and electron capture affect the neutron-protonratio. (22-2)

7. Write out the nuclear reactions that show particle conversion for the following types ofradioactive decay:a. beta emissionb. positron emissionc. electron capture (22-2)

8. Compare and contrast electrons, beta particles,and positrons. (22-2)

9. a. What are gamma rays?b. How do scientists think they are

produced? (22-2)

10. How does the half-life of a nuclide relate to itsstability? (22-2)

11. List the three parent nuclides of the naturaldecay series. (22-2)

12. How are artificial radioactive isotopesproduced? (22-2)

REVIEWING CONCEPTS 13. Neutrons are more effective than protons oralpha particles for bombarding atomic nuclei.Why? (22-2)

14. Why are all of the transuranium elementsradioactive? (Hint: See Section 22-1.) (22-2)

15. Why can a radioactive material affect photo-graphic film even though the film is wellwrapped in black paper? (22-3)

16. How does the penetrating ability of gamma rayscompare with that of alpha particles and betaparticles? (22-3)

17. How does nuclear radiation damage biologicaltissue? (22-3)

18. Explain how film badges, Geiger-Müller counters,and scintillation detectors are used to detectradiation and measure radiation exposure.(22-3)

19. How is the age of an object containing aradioactive nuclide estimated? (22-3)

20. How is the fission of a uranium-235 nucleusinduced? (22-4)

21. How does the fission of uranium-235 produce achain reaction? (22-4)

22. Describe the purposes of the five major compo-nents of a nuclear power plant. (22-4)

23. Describe the reaction that produces the sun’senergy. (22-4)

24. What is one problem that must be overcomebefore energy-producing controlled fusion reac-tions are a reality? (22-4)

Mass Defect25. The mass of a 20

10Ne atom is 19.992 44 amu.Calculate its mass defect.

26. The mass of a 73Li atom is 7.016 00 amu.Calculate its mass defect.

Nuclear Binding Energy27. Calculate the nuclear binding energy of one

lithium-6 atom. The measured atomic mass oflithium-6 is 6.015 amu.

28. Calculate the binding energies of the followingtwo nuclei, and indicate which releases more

PROBLEMS


CHAPTER 22 REVIEW

C H A P T E R 2 2724

energy when formed. You will need informationfrom the periodic table and the text.a. atomic mass 34.988011 amu, 35

19Kb. atomic mass 22.989767 amu, 23

11Na

29. a. What is the binding energy per nucleon foreach nucleus in the previous problem?

b. Which nucleus is more stable?

30. The mass of 73Li is 7.016 00 amu. Calculate thebinding energy per nucleon for 73Li. Convert themass in amu to binding energy in joules.

Neutron-Proton Ratio31. Calculate the neutron-proton ratios for the

following nuclides:a. 12

6C c. 20682Pb

b. 31H d. 134

50Sn

32. a. Locate the nuclides in problem 31 on thegraph in Figure 22-2. Which ones lie on theband of stability?

b. For the stable nuclides, determine whether theirneutron-proton ratio tends toward 1:1 or 1.5:1.

Nuclear Equations33. Balance the following nuclear equations.

(Hint: See Sample Problem 22-1.)a. 43

19K → 4320Ca +

b. 23392U → 229

90Th +

c. 116C + → 11

5B

d. 137N → +1

0β +

34. Write the nuclear equation for the release of an alpha particle by 210

84Po.

35. Write the nuclear equation for the release of a beta particle by 210

82Pb.

Half-Life36. The half-life of plutonium-239 is 24 110 years. Of

an original mass of 100.g, how much remains after96 440 years? (Hint: See Sample Problem 22-2.)

37. The half-life of thorium-227 is 18.72 days. Howmany days are required for three-fourths of agiven amount to decay?

38. Exactly �116� of a given amount of protactinium-

234 remains after 26.76 hours. What is the half-life of protactinium-234?

?

?

?

?

39. How many milligrams remain of a 15.0 mg sam-ple of radium-226 after 6396 years? The half-lifeof radium-226 is 1599 years.

40. Balance the following nuclear reactions;

a. 23993Np → −1

0β +

b. 94Be + 4

2He →

c. 3215P + → 33

15P

d. 23692U → 94

36Kr + + 310n

41. After 4797 years, how much of an original 0.250 g of radium-226 remains? Its half-life is1599 years.

42. The parent nuclide of the thorium decay seriesis 232

90Th. The first four decays are as follows:alpha emission, beta emission, beta emission,and alpha emission. Write the nuclear equationsfor this series of emissions.

43. The half-life of radium-224 is 3.66 days. Whatwas the original mass of radium-224 if 0.0500 gremains after 7.32 days?

44. Calculate the neutron-proton ratios for the following nuclides, and determine where theylie in relation to the band of stability.a. 235

92U c. 5626Fe

b. 168O d. 156

60Nd

45. Calculate the binding energy per nucleon of23892U in joules. The atomic mass of a 238

92Unucleus is 238.050 784 amu.

46. The energy released by the formation of anucleus of 56

26Fe is 7.89 × 10−11 J. Use Einstein’sequation, E = mc2, to determine how muchmass is lost (in kilograms) in this process.

47. Calculate the binding energy for one mole of deuterium atoms. The measured mass of deuterium is 2.0140 amu.

48. Why do we compare binding energy per nuclearparticle of different nuclides instead of the totalbinding energy per nucleus?

CRITICAL THINKING

?

?

?

?

MIXED REVIEW


CHAPTER 22 REVIEW


49. Why is the constant rate of decay of radioactivenuclei so important in radioactive dating?

50. Which of the following nuclides of carbon ismost likely to be stable? State reasons for youranswer.a. 11

6C b. 126C

51. Which of the following nuclides of iron aremost likely to be stable? State reasons for youranswer.a. 56

26 Fe b. 5926 Fe

52. Use the data in the table shown to determinea. which isotopes would be best for dating

ancient rocks.b. which isotopes could be used as tracers.State reasons for your answers.

Element Half-LifePotassium-40 1.28 × 109 yrPotassium-42 12.36 hUranium-238 4.468 × 109 yrUranium-239 23.47 min

53. Investigate the history of the Manhattan Project.

54. Research the 1986 nuclear reactor accident atChernobyl, Ukraine. What factors combined tocause the accident?

55. Find out about the various fusion-energyresearch projects that are being conducted inthe United States and other parts of the world.How close are the researchers to finding an eco-nomical method of producing energy? Whatobstacles must still be overcome?

56. Using the library, research the medical uses ofradioactive isotopes such as cobalt-60 andtechnetium-99. Evaluate the benefits and risksof using radioisotopes in the diagnosis andtreatment of medical conditions. Report yourfindings to the class.

ALTERNATIVE ASSESSMENT

RESEARCH & WRITING


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