3 3-1 © 2003 Thomson Learning, Inc. All rights reserved General, Organic, and Biochemistry, 8e Bettelheim, Brown, Campbell, and Farrell

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3-1© 2003 Thomson Learning, Inc.All rights reserved

General, Organic, and General, Organic, and Biochemistry, 8eBiochemistry, 8e

Bettelheim, Brown,Bettelheim, Brown,

Campbell, and FarrellCampbell, and Farrell

33


Chapter 3Chapter 3Nuclear ChemistryNuclear Chemistry

• Figure 3.1 Electricity and Radioactivity

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IntroductionIntroduction• In this chapter, we study some of the properties

of the nucleus, its particles, and nuclear radiation.

• Nuclear radiation:Nuclear radiation: radiation emitted from a nucleus during nuclear decay• alpha particle (alpha particle ():): a helium nucleus, He2+; contains two

protons and two neutrons, has mass of 4 amu, and atomic number 2

• beta particle (beta particle ():): an electron; has a charge of -1, and a mass of 0.00055 amu

• positron (positron ():): has the mass of an electron (0.00055 amu) but a charge of +1+1

• gamma ray (gamma ray ():): high-energy electromagnetic radiation

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Electromagnetic RadiationElectromagnetic Radiation• All electromagnetic radiation consists of waves

• the only difference between forms of electromagnetic radiation is the wavelength, wavelength, the distance between wave crests.

• frequency, frequency, :: the number of crests that pass a given point in a second.

• the higher the frequency, the shorter the wavelength.• the higher the frequency, the higher the energy.

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Electromagnetic RadiationElectromagnetic Radiation• Figure 3.3 The Electromagnetic Spectrum

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Nuclear RadiationNuclear Radiation• Table 3.1 summarizes the types of nuclear

radiation we deal with in this chapter

Proton

Electron

Neutron

Positron

Helium nucleus

Gamma ray

Particle or ray

Common nameof radiation Symbol Charge

Mass(amu)

Proton beam

Beta particle ()

Neutron beam

Alpha particle ()

Gamma ray

H

e or

n

e or +

He or

+1

-1

0

+1

+2

0

1

0.00055

1

0.00055

4

0

11

-10

01

+10

24

Positron emission

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Nuclear RadiationNuclear Radiation• There are more than 300 naturally occurring

isotopes• Of these 264 are stable, meaning that the nuclei of

these isotopes are not radioactive (they do not give off radiation); the remainder are radioactive isotopes.

• among the lighter elements, stable isotopes have approximately the same number of protons and neutrons; this is the case of 12

6C, 168O, and 20

10Ne

• among the heavier elements,stability requires more neutrons than protons; the most stable isotope of lead, for example, is lead-206, 124

82Pb

• More than 1000 artificial isotopes have been made in the laboratory; all are radioactive

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Beta EmissionBeta Emission• beta emission:beta emission: a type of nuclear decay in which a

neutron is converted to a proton and an electron, and the electron is emitted from the nucleus

• emission of a beta particle transforms the element into a new element with the same mass number but an atomic number one unit greater

• phosphorus-32, for example, is a beta emitter

• note in this nuclear decay equation that the sum of both the mass numbers and atomic numbers are the same on each side of the equation

e-10n0

1 H11 +

e-10

1532 S16

32 +P

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Beta EmissionBeta Emission• Problem:Problem: carbon-14 is a beta emitter. When it

undergoes beta emission, into what element is it converted?

e-10

614 +?C

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Beta EmissionBeta Emission• Problem:Problem: carbon-14 is a beta emitter. When it

undergoes beta emission, into what element is it converted?

• Solution:Solution: it is converted into nitrogen-14

e-10

614 +?C

e-10

614 +N7

14C

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Alpha EmissionAlpha Emission• alpha emission:alpha emission: a type of nuclear decay in which

a helium nucleus is emitted from the nucleus• in alpha emission, the new element formed has an

atomic number two units lower and a mass number four units lower than the original nucleus.

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92238 +Th90

234U He

24

84210 +Pb82

206Po +He

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Positron EmissionPositron Emission• positron emission:positron emission: a type of nuclear decay in

which a positive electron is emitted from the nucleus• in positron emission, the new element formed has an

atomic number one unit lower but the same mass number as the original nucleus.

e+10

611 +B5

11C

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Gamma EmissionGamma Emission• In pure gamma emission, there is no change in

either the atomic number or the mass number of the element• a nucleus in a higher-energy state emits gamma

radiation as it returns to its ground state (its most stable energy state)

• in this example, the notation “11m” indicates that the nucleus of boron-11 is in a higher-energy (excited) state. In this nuclear decay, no transmutation takes place.

6

11m +B 611B

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Half-LifeHalf-Life• half-life of a radioisotope, thalf-life of a radioisotope, t1/21/2:: the time it takes

one half of a sample of a radioisotope to decay• iodine-131 decays by beta, gamma emission

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Hydrogen-3 (tritium)Carbon-14Phosphorus-28Phosphorus-32Potassium-40Scandium-42Cobalt-60Strontium-90Technetium-99m

Indium-116

Iodine-131Mercury-197

Radon-205Radon-222Uranium-235

Name Half-life Radiation

12.26 y5730 y0.28 s14.3 d

1.28 x 109 y0.68 s5.2 y28.1 y6.0 h

14 s

8 d65 h

Polonium-210 138 d2.8 m3.8 d

4 x 109 y

BetaBetaPositronBetaBeta + gammaPositronGammaBetaGamma

Beta

Beta + gammaGammaAlphaAlphaAlphaAlpha

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Characteristics of RadiationCharacteristics of Radiation• Intensity

• to measure intensity, we take advantage of the ionizing property of radiation

• instruments such as a Geiger-MüllerGeiger-Müller or proportional proportional countercounter contain a gas such as helium or argon

• when a radioactive nucleus emits beta particles, these particles ionize the gas in the instrument; it registers the ionization by indicating that an electric current has passed between two electrodes

• another measuring device, called a scintillation scintillation countercounter, has a phosphor that emits a unit of light when a beta particle or gamma ray strikes it

• intensity is recorded in counts/min or counts/s

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Characteristics of RadiationCharacteristics of Radiation• Energy and penetrating power

X-ray

Type ofRadiation Charge

Mass(amu)

Proton beam

Beta partricle ()

Neutron beam

Alpha particle ()

+1

-1

0

+1+2

0

1

0.00055

1

0.00055

40

Positron emission (+)

Energy

Rangeb

1-3 MeV--_

3-9 MeV

0.01-10 MeVGamma ray (0 0 0.1-10 keV

Penetrating

Powera

1-3 cm

0-4 mm--_

0.02-0.04 mm

1-20 cm

0.01-1 cm

60 MeV

a Distance at which half the radiation is stopped in water.b MeV = 1.62 x10-13 J = 3.829 x 10-14 cal

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Characteristics of RadiationCharacteristics of Radiation• Figure 3.6 Penetration of radioactive emissions

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Radiation DosimetryRadiation Dosimetry• Although alpha particles cause more damage than X-

rays or gamma radiation, they have very low penetrating power and cannot pass through skin.

• Consequently alpha particles are not harmful to humans or animals as long as they do not get into the body; if they do get into the body, they can be quite harmful.

• Beta particles are less damaging to tissue than alpha particles but penetrate farther and so are generally more harmful.

• Gamma rays, which can easily penetrate skin, are by far the most dangerous and harmful form of radiation.

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Radiation DosimetryRadiation Dosimetry• Terms and units

• Becquerel (Bq): Becquerel (Bq): one Bq is 1 dpsone Bq is 1 dps• Curie (Ci):Curie (Ci): one Ci = 3.7 x 1010 dps• Roentgen (R):Roentgen (R): the amount of radiation that produces

ions having 2.58 x 10-4 coulomb/kg; a measure of the energy delivered by a radiation source.

• Radiation absorbed dose (Rad):Radiation absorbed dose (Rad): a measure of the ionizing radiation absorbed; the SI unit is the gray (Gy)gray (Gy)

• Gray (Gy):Gray (Gy): one Gy = 1 joule/kilogram (1 J/kg) • Roentgen-equivalent-man (Rem):Roentgen-equivalent-man (Rem): a measure of the

effect of the radiation when one roentgen is absorbed by a person; the SI unit is the sievert (Sv)sievert (Sv) where one Sv = 1 J/kg

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Radiation DosimetryRadiation Dosimetry• the relationship between delivered dose in roentgens

(R) and the absorbed dose in rads; exposure to 1 R of high energy photons yields 0.97 rad in water, 0.96 rad in muscle, and 0.93 rad in bone

• for lower-energy photons such as soft x-rays, 1 R yields 3 rads of absorbed radiation in bone; soft tissue lets radiation pass, but bone absorbs it, giving an X-ray

Measurement International Unit Conventional Unit

Intensity 1 Bq = 1 disintegrations/s 1 Ci = 3.7 x 1010 Bq

Production of ions 1 R = 2.58 x 10-4 coulomb/kg 1 R = 2.58 x 10-4 coulomb/kg

Absorbed dose

Absorbed doseby humans

1 Gy = 1 J/kg 1 rad = 0.01 Gy

1 Sv = 1 J/kg 1 rem = 0.01 Sv

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Radiation DosimetryRadiation Dosimetry• Average exposure to radiation from common sources

Naturally Occurring RadiationDose (mrem/y)

Cosmic raysTerrestrial radiation (rocks, buildings)Inside human body (K-40 and Ra-226 in bones)Radon in the airTotal

Artificial RadiationMedical x-raysNuclear medicineConsumer productsNuclear power plantsAll othersTotal

272839

200294

3914100.51.5

65.0

Source

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Radiation DosimetryRadiation Dosimetry• A single whole-body irradiation of 25 rem is noticeable

in white blood cell count.• A single dose of 100 rem causes typical symptoms of

radiation sickness, which include nausea, vomiting, a decrease in white blood cell count, and loss of hair.

• A single dose of 400 rem causes death within one month in 50% of the exposed persons.

• A single dose of 600 rem is almost invariably lethal within a month.

• It is estimated that a single dose of 50,000 rem is needed to kill bacteria, and up to 106 rem is needed to inactivate viruses.

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Nuclear MedicineNuclear Medicine• Radioisotopes have two main uses in medicine;

diagnosis and therapy

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Nuclear FusionNuclear Fusion• The transmutation of two hydrogen nuclei into a

helium nucleus liberates energy in the form of photons• this process is called nuclear fusionnuclear fusion

• all transuranium elements (elements with atomic number greater than 92) are artificial and have been prepared by nuclear fusion

• to prepare them, heavy nuclei are bombarded with lighter ones

+ H31H2

1 He42 + n1

0 + 108 cal/mole He

Deuterium Tritium

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NUcldear fusionNUcldear fusion• The fusion of deuterium and tritium gives off a

very large amount of energy. What is the source of the energy? The answer is that there is a decrease in mass in the process, and the missing mass is converted to energy.

21H + 3

1H 4

2He + n1

0

2.0140 g3.0161 g 4.0026 g1.0087 g

5.0301 g 5.0113 g

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Nuclear FusionNuclear Fusion• All transuranium elements (elements with atomic

number greater than 92) are artificial and have been prepared by nuclear fusion

• To prepare them, heavy nuclei are bombarded with lighter ones. Examples are:

• These transuranium elements are unstable and have very short half-lives; that of lawrencium-257, for example, is only 0.65 second

+ He42Cm244

96 Bk24597 + H1

1 + n102

+ B105Cf252

98 Lr257103 + n1

05

+ C126U238

92 Cf24698 + n1

04

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Nuclear FissionNuclear Fission• Nuclear fission:Nuclear fission: the fragmentation of larger nuclei

into smaller ones• When uranium-235 is bombarded with neutrons, it is

broken into two smaller elements.• More importantly, energy is released because the

products have less mass than the starting materials.• The mass decrease in fission is converted into energy.• This form of energy is called atomic energy.atomic energy.

n10

+U23592 Ba139

56 + Kr9436 + n1

03 +

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Nuclear FissionNuclear Fission• Nuclear fission is a chain reaction

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Nuclear FissionNuclear Fission• today more than 15% of the electrical energy in the

United States is supplied by nuclear power plants• disposal of spent but still radioactive fuel materials is a

major long-term problem• spent fuel contains high-level fission products together

with recoverable uranium and plutonium• in addition, there are radioactive wastes from nuclear

weapons programs, research reactors, and so forth• recently the government gave its final approval to store

nuclear wastes at a site deep under Yucca Mountain in Nevada

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End End Chapter 3Chapter 3

Nuclear ChemistryNuclear Chemistry

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3 3-1 © 2003 Thomson Learning, Inc. All rights reserved General, Organic, and Biochemistry, 8e Bettelheim, Brown, Campbell, and Farrell