Nuclear ChemistryChapter 28

RadioactivityRadioactivity• One of the pieces of evidence for One of the pieces of evidence for

the fact that atoms are made of the fact that atoms are made of smaller particles came from the smaller particles came from the work ofwork of Marie CurieMarie Curie (1876-(1876-1934).1934).

• She discoveredShe discovered radioactivityradioactivity, , the spontaneous disintegration the spontaneous disintegration (decay) of some elements into (decay) of some elements into smaller pieces.smaller pieces.

Nuclear Reactions vs. Nuclear Reactions vs. Normal Chemical Normal Chemical

ChangesChanges• Nuclear reactions involve the nucleus.Nuclear reactions involve the nucleus.• The nucleus opens, and protons and neutrons The nucleus opens, and protons and neutrons

are rearranged.are rearranged.• The opening of the nucleus releases a The opening of the nucleus releases a

tremendous amount of energy that holds the tremendous amount of energy that holds the nucleus together – called nucleus together – called binding energy.binding energy.

• ““Normal” Chemical Reactions involve Normal” Chemical Reactions involve electronselectrons, not protons and neutrons., not protons and neutrons.

Types of RadiationTypes of Radiation

e01

He42

• Alpha (Alpha () – a positively ) – a positively charged (+2) helium isotopecharged (+2) helium isotope - - we usually ignore the charge because it we usually ignore the charge because it involves electrons, not protons and neutronsinvolves electrons, not protons and neutrons

•Beta (Beta () – an electron) – an electron

•Gamma (Gamma () – pure energy; ) – pure energy; called a ray rather than a called a ray rather than a particleparticle

00

Other Nuclear ParticlesOther Nuclear Particles

e01

n10• NeutronNeutron

• Positron – a positive Positron – a positive electronelectron

•Proton – usually referred to Proton – usually referred to as hydrogen-1as hydrogen-1

•Any other elemental isotopeAny other elemental isotope

H11

Penetrating AbilityPenetrating Ability

XAZ

Mass Number Atomic Number

Element Symbol

Atomic number (Z) = number of protons in nucleus

Mass number (A) = number of protons + number of neutrons

= atomic number (Z) + number of neutrons

A

Z

1p11H1or

proton1n0

neutron0e-1

0-1orelectron

0e+10+1or

positron4He2

42or particle

1

1

1

0

0

-1

0

+1

4

2

Balancing Nuclear Equations

1. Conserve mass number (A).

The sum of protons plus neutrons in the products must equal the sum of protons plus neutrons in the reactants.

1n0U23592 + Cs138

55 Rb9637

1n0+ + 2

235 + 1 = 138 + 96 + 2x1

2. Conserve atomic number (Z) or nuclear charge. The sum of nuclear charges in the products must equal the sum of nuclear charges in the reactants.

1n0U23592 + Cs138

55 Rb9637

1n0+ + 2

92 + 0 = 55 + 37 + 2x0

212Po decays by alpha emission. Write the balanced nuclear equation for the decay of 212Po.

4He242oralpha particle -

212Po 4He + AX84 2 Z

212 = 4 + A A = 208

84 = 2 + Z Z = 82

212Po 4He + 208Pb84 2 82

Nuclear Stability and Radioactive Decay

Beta decay

14C 14N + 06 7 -1

40K 40Ca + 019 20 -1

1n 1p + 00 1 -1

Decrease # of neutrons by 1

Increase # of protons by 1

Positron decay

11C 11B + 06 5 +1

38K 38Ar + 019 18 +1

1p 1n + 01 0 +1

Increase # of neutrons by 1

Decrease # of protons by 1

Electron capture decay

Increase # of neutrons by 1

Decrease # of protons by 1

Nuclear Stability and Radioactive Decay

37Ar + 0e 37Cl 18 17-1

55Fe + 0e 55Mn 26 25-1

1p + 0e 1n 1 0-1

Alpha decay

Decrease # of neutrons by 2

Decrease # of protons by 2212Po 4He + 208Pb84 2 82

Spontaneous fission

252Cf 2125In + 21n98 49 0

Learning Check

What radioactive isotope is produced in the following bombardment of boron?

10B + 4He ? + 1n 5 2 0

Learning Check

What radioactive isotope is produced in the following bombardment of boron?

10B + 4He 13N + 1n 5 2 7 0

Write Nuclear Equations!

Write the nuclear equation for the beta emitter Co-60.

6060CoCo 00ee ++ 6060NiNi2727 -1 -1 2828

Artificial Nuclear Artificial Nuclear ReactionsReactions

New elements or new isotopes of known elements are New elements or new isotopes of known elements are produced by bombarding an atom with a produced by bombarding an atom with a subatomic particle such as a proton or neutron -- subatomic particle such as a proton or neutron -- or even a much heavier particle such as or even a much heavier particle such as 44He and He and 1111B.B.

Reactions using neutrons are called Reactions using neutrons are called

reactions reactions because a because a ray is usually ray is usually emitted.emitted.

Radioisotopes used in medicine are often made by Radioisotopes used in medicine are often made by reactions.reactions.

Artificial Nuclear Artificial Nuclear ReactionsReactions

Example of a Example of a reaction reaction is production is production

of radioactive of radioactive 3131P for use in studies of P P for use in studies of P

uptake in the body.uptake in the body.

31311515P + P + 11

00n ---> n ---> 32321515P + P +

Transuranium ElementsTransuranium ElementsElements beyond 92 Elements beyond 92 (transuranium)(transuranium) made made

starting with an starting with an reaction reaction

2382389292U + U + 11

00n ---> n ---> 2392399292U + U +

2392399292U U ---> ---> 239239

9393Np + Np + 00-1-1

2392399393Np Np ---> ---> 239239

9494Pu + Pu + 00-1-1

Nuclear Stability• Certain numbers of neutrons and protons are extra stable

• n or p = 2, 8, 20, 50, 82 and 126• Like extra stable numbers of electrons in noble gases

(e- = 2, 10, 18, 36, 54 and 86)• Nuclei with even numbers of both protons and neutrons

are more stable than those with odd numbers of neutron and protons

• All isotopes of the elements with atomic numbers higher than 83 are radioactive

• All isotopes of Tc and Pm are radioactive

Band of Stability Band of Stability and Radioactive and Radioactive DecayDecay

Stability Stability of of NucleiNuclei

• Out of > 300 stable isotopes:

EvenEven OddOdd

OddOdd

EvenEven

ZZNN

157157 5252

5050 55

31311515PP

191999FF

2211H, H, 66

33Li, Li, 101055B, B, 1414

77N, N, 1801807373TaTa

Half-LifeHalf-Life

•HALF-LIFEHALF-LIFE is the time that it takes for is the time that it takes for 1/2 a sample to decompose.1/2 a sample to decompose.

• The rate of a nuclear transformation The rate of a nuclear transformation depends only on the “reactant” depends only on the “reactant” concentration.concentration.

Half-LifeHalf-Life

Decay of 20.0 mg of Decay of 20.0 mg of 1515O. What remains after 3 half-lives? O. What remains after 3 half-lives? After 5 half-lives?After 5 half-lives?

Kinetics of Radioactive Kinetics of Radioactive DecayDecay

For each duration (half-life), one half of the substance decomposes.

For example: Ra-234 has a half-life of 3.6 days

If you start with 50 grams of Ra-234

After 3.6 days > 25 gramsAfter 3.6 days > 25 grams

After 7.2 days > 12.5 gramsAfter 7.2 days > 12.5 grams

After 10.8 days > 6.25 gramsAfter 10.8 days > 6.25 grams

Radiocarbon Dating14N + 1n 14C + 1H7 160

14C 14N + 06 7 -1 t½ = 5730 years

Uranium-238 Dating238U 206Pb + 8 4 + 6 092 -182 2 t½ = 4.51 x 109 years

Learning Check!

The half life of I-123 is 13 hr. How much of a 64 mg sample of I-123 is left after 39 hours?

Nuclear FissionNuclear FissionFission is the splitting of atomsFission is the splitting of atoms

These atoms are usually very large, so that they are not as stableThese atoms are usually very large, so that they are not as stable

Fission chain has three general steps:Fission chain has three general steps:

1.1. Initiation.Initiation. Reaction of a single atom starts the chain Reaction of a single atom starts the chain

(e.g., (e.g., 235235U + neutron)U + neutron)

2.2. PropagationPropagation. . 236236U fission releases neutrons that initiate U fission releases neutrons that initiate

other fissionsother fissions

3. 3. TerminationTermination. .

Nuclear FissionNuclear Fission

Nuclear Fission

235U + 1n 90Sr + 143Xe + 31n + Energy92 54380 0

Energy = [mass 235U + mass n – (mass 90Sr + mass 143Xe + 3 x mass n )] x c2

Energy = 3.3 x 10-11J per 235U

= 2.0 x 1013 J per mole 235U

Combustion of 1 ton of coal = 5 x 107 J

Representation of a fission process.

Mass DefectMass Defect• Some of the mass can be converted into Some of the mass can be converted into

energyenergy• Shown by a very famous equation!Shown by a very famous equation!

E=mcE=mc22

EnergyEnergy

MassMass

Speed of lightSpeed of light

Nuclear Fission & Nuclear Fission & POWERPOWER

• Currently about 103 Currently about 103

nuclear power plants in nuclear power plants in

the U.S. and about 435 the U.S. and about 435

worldwide.worldwide.

• 17% of the world’s 17% of the world’s

energy comes from energy comes from

nuclear.nuclear.

Diagram of a nuclear power plant

Annual Waste Production35,000 tons SO2

4.5 x 106 tons CO2

1,000 MW coal-firedpower plant

3.5 x 106

ft3 ash

1,000 MW nuclearpower plant

70 ft3 waste

Nuclear Fission

Nuclear Fusion

Fusion small nuclei combine

2H + 3H 4He + 1n + 1 1 2 0

Occurs in the sun and other stars

Energy

Nuclear Fusion

2H + 2H 3H + 1H1 1 1 1

Fusion Reaction Energy Released

2H + 3H 4He + 1n1 1 2 0

6Li + 2H 2 4He3 1 2

6.3 x 10-13 J

2.8 x 10-12 J

3.6 x 10-12 J

Tokamak magnetic plasma

confinement

Nuclear Fusion

Fusion • Excessive heat can not be contained• Attempts at “cold” fusion have

FAILED.• “Hot” fusion is difficult to contain

Radioisotopes in Medicine• 1 out of every 3 hospital patients will undergo a nuclear

medicine procedure

• 24Na, t½ = 14.8 hr, emitter, blood-flow tracer

• 131I, t½ = 14.8 hr, emitter, thyroid gland activity

• 123I, t½ = 13.3 hr, ray emitter, brain imaging

• 18F, t½ = 1.8 hr, emitter, positron emission tomography

• 99mTc, t½ = 6 hr, ray emitter, imaging agent

Brain images with 123I-labeled compound

Chemistry In Action: Food Irradiation

Dosage Effect

Up to 100 kiloradInhibits sprouting of potatoes, onions, garlics. Inactivates trichinae in pork. Kills or prevents insects from reproducing in grains, fruits, and vegetables.

100 – 1000 kilorads Delays spoilage of meat poultry and fish. Reduces salmonella. Extends shelf life of some fruit.

1000 to 10,000 kilorads Sterilizes meat, poultry and fish. Kills insects and microorganisms in spices and seasoning.