Daniel L. RegerScott R. GoodeDavid W. Ball

http://academic.cengage.com/chemistry/reger

Chapter 21Nuclear Chemistry

• Most naturally occurring elements are mixtures of isotopes, which are represented by symbols of the form

where X is the symbol of the element, A = mass number, and Z = atomic number.

• Nuclide is the nucleus of a specific isotope.• A nucleon is a proton or neutron.

Review

XAZ

Definitions

• A stable isotope is one that does not spontaneously decompose into another nuclide.• An radioactive nuclide is one that

spontaneously decomposes into another nuclide.

Stable Nuclide Characteristics

1 The number of neutrons is equal to or greater than the number of protons (except for 1H and 3He).

2 Up to Z = 20, the number of neutrons and protons are nearly equal; above 20 the ratio of n/p increases slowly to about 1.6:1.

3 Nuclear stability is greater for nuclides containing even numbers of protons, neutrons, or both.

Stable Nuclide Characteristics

4 Certain numbers of protons and neutrons (called magic numbers) confer unusual stability: 2, 8, 20, 26, 28, 50, 82, and 126.

5 The zone of stability contains all of the stable nuclides, but some nuclides in this band are unstable.

6 Tc (Z=43), Pm (Z=61), and all elements beyond Bi (Z=83) have no stable isotopes.

Radioactivity

• There are three major kinds of emissions from radioactive.α particles are high-energy 4He nuclei.

β particles are high-energy electrons that originate from the nucleus.

γ rays are very short wavelength (high energy) electromagnetic radiation.

• A nuclear equation describes any process in which a nuclide undergoes change. In a balanced nuclear equation the

sum of the mass numbers and atomic numbers on the reactant and product sides of the equation must be equal.

Z: 92 = 90 + 2 A: 238 = 234 + 4

Nuclear Equations

α 42

23490

23892 Th U

• An alpha decay decreases the atomic number by 2 and the mass number by 4.

• Alpha decay is seen in all elements heavier than bismuth (Z > 83).

α 42

22899

23290 RaTh

Alpha Decay

• Beta decay increases the atomic number by one, without changing the mass number.

• The β particle does not exist in the nucleus, but is created at the instant of its emission.• Beta emission is observed in nuclides that

have too many neutrons to be stable.

01-

147

146

+ N C

Beta Decay

• A positron is identical to an electron, except its charge is positive.

• Positron emission decreases the atomic number by one, without changing the mass number.

• The symbol for the positron and beta particle is the same, except for the sign of Z.

• Positron emission is seen in nuclides that have too many protons to be stable.

Positron Emission

β Ar K 01

4018

4019

• In electron capture an electron in a low energy orbital of the atom is capture by the nucleus and converts a proton to a neutron.

• X rays (not rays) accompany electron capture, because the atom produced is in an excited electronic state.

• Electron capture and positron emission both decrease the atomic number by 1.

• Both processes occur when the nuclide contains too many protons to be stable.

Electron Capture

Sc e Ti 4421

01

4422

• When Z > 83, emission is often observed.

• If A > atomic mass of element decay occurs.

• If A < atomic mass of element decay or electron capture occurs.

For radioactive elements

01

01

Predicting Modes of Decay

• Predict the mode of decay, and write the nuclear equation for each radioactive nuclide:

(a) (b) (c)

Example Problem

Ra22688 Na22

11 I13053

• Among the heavier elements radioactive decay series are common.

Radioactive Series

• Radiation detection is based on the ionization caused by high energy particles and light and includes:• Exposure of photographic film.

• Geiger counters.

• Scintillation counters.

Detecting Radiation

• Radioactive decays obey a first order rate law:

where N is the number of radioactive nuclei.

• Usually the half-life, t1/2, is given rather than k.

kNt

N

-

= rate

kkt

693.02 ln1/2

Decay Rates

• A nuclear transmutation is a reaction in which two particles or nuclides produce nuclides that are different than the reactant species.• Two of the first nuclear transmutations

observed were:

Nuclear Transmutation

p O N 11

178

42

147 α

n C Be 10

126

42

94 α

• When both reactants in a nuclear transmutation are positively charged, very high energies are needed because of electrostatic repulsion.• Devices used to produce these high

energy particles include cyclotrons and linear accelerators.

Nuclear Reactions of Charged Particles

• The energy equivalent of mass is calculated from

E = mc2

where E is energy, m is mass, and c is the speed of light.• When a nuclear change occurs a

measurable difference in the mass of the products and reactants is observed.

Energy and Mass

• Nuclear binding energy is the amount of energy required to keep the protons and neutrons together in the nucleus.• Some of the mass of the nucleons is

converted into binding energy.

• The mass of a nuclide is always less than the masses of the neutrons and protons present.

Nuclear Binding Energy

• The mass defect is the difference between the mass of the nucleons and the mass of the nuclide.• The larger the mass defect, the greater is

the nuclear binding energy.

Mass Defect

• The mass of a 7Li atom is 7.016003 u. Calculate the mass defect (in u) given the following masses:

1H 1.007825 u1

on 1.008665 u

Calculating Mass Defect

• The 7Li nuclide has a mass defect of 0.042132 u. Calculate the binding energy of this nuclide, in kJ/mol, using the equation

E = mc2

Nuclear Binding Energy from Mass Defect

• Nuclear fission forms two nuclei of roughly similar size from a single heavy nucleus.• Fission reactions release very large quantities

of energy (“exergonic”) and produce several neutrons in addition to two nuclides.

n3 + Kr+ Ba U 10

9236

14156

23692

Nuclear Fission

• More than 370 nuclides, with A = 72 to 161, are found from the fission of 235U.

Fission Yield Curve

• 235U absorbs a neutron which induces fission, and a chain reaction.

Chain Reaction of 235U

• The critical mass is the minimum mass needed for a nuclear chain reaction to maintain a self-sustaining reaction.

Critical Mass

• In a nuclear power reactor, conditions maintain criticality at a constant power level.

Nuclear Power Reactor

• Long term storage of radioactive fission products and fear of disastrous accidents are the major deterrents to increased use of nuclear power.• Current technology incorporates the

radioactive waste in glass loaded into stainless steel containers, which are buried deep underground.

• Strict government regulations are enforced to assure the safe operation of reactors.

Nuclear Power and Safety

• Nuclear fusion is the combination of two light nuclides to form a larger one.• The energy produced by the sun

comes from fusion reactions, such as

kJ/mol10-1.9= β+ He H+ He

kJ/mol 10-5.2= He H H

kJ/mol 10-9.9= n H H H

901

42

11

32

832

21

11

701

21

11

11

E

E

E

Reactor Designs

• The possibility of fusion reactors is at least several decades away from reality.• An international consortium of the U.S.,

Japan, Russia, and the European Community are jointly designing a experimental thermonuclear power reactor.

Reactor Designs

Radioactivity Name Abbrev Definition or conversionSI unit Becquerel Bq 1 disintegration /second

Common unit Curie Ci3.7 1010 disintegrations /second

Radiation Dose

SI unit rad rad

Quantity of radiation that transfers 1  10-2 J of energy per kilogram of matter

Effective Radiation DoseSI unit Sievert Sv 1 Sv = 100 rem

Common unit rem rem 1 rem = 0.01 Sv

Units of Radioactivity and Radiation Dose

Most Radiation has a Natural Source

• 222Ra is produced by the decay of natural 238U found in rocks such as granite. • Home radon test kits are sold because of

the great public awareness and the potential dangers of radon accumulation. • Since radon generally enters homes

through the basement, one of the most effective means of eliminating the radiation danger is to add ventilation fans in the basement.

Radon

• When a patient has an overactive thyroid gland, he or she is often given a dose of 131I, which is a beta-emitter. • Iodine concentrates in the thyroid gland,

and the beta radiation from the 131I isotope reduces the amount of hormone produced by the thyroid gland.

Nuclear Medicine

• Radiopharmaceuticals are synthesized with isotopes that emit gamma radiation coupled to organic molecules that are taken up specifically in target organs.• Modern scanners use gamma ray

cameras that take measurements at thousands of different locations.• Computerized tomography is often

abbreviated CT and referred to as a CT scan.

CT Scans

• Technicium is a transition metal with a variety of stable oxidation states that makes it an excellent species to use to react with organic molecules. • Heart imaging is done with a compound

formed by technicium(I) and six isonitrile ligands.• Other compounds have been developed

for mapping the brain, lungs, etc. • Over 100 different nuclear medicine

diagnoses are available to clinicians.

Gamma Radiation Scans - 99mTc

• A fluorinated sugar (fluorodeoxyglucose) that contains 18F is often used.• This sugar is taken up most by the organs

that are subdividing most quickly. And cancer cells are among the fastest.• PET scans are particularly effective in

diagnosing lung, head and neck, colorectal, esophageal, lymphoma, melanoma, breast, thyroid, cervical, pancreatic, and brain cancers.

Positron Emission Tomography (PET) Scans