1Nuclear Nuclear ChemistryChemistry

Chemistry IH – Chapter 25Chemistry IH – Chapter 25

Chemistry I – Chapter 21Chemistry I – Chapter 21

2

RadioactivityRadioactivity

• Much of the early important Much of the early important research about radioactivity was research about radioactivity was completed by completed by Marie CurieMarie Curie (1876-1934).(1876-1934).

• She discoveredShe discovered radioactive radioactive decaydecay, the spontaneous , the spontaneous disintegration of some elements into disintegration of some elements into smaller pieces.smaller pieces.

3Nuclear Reactions vs. Nuclear Reactions vs. Normal Chemical Normal Chemical

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

neutrons are rearrangedneutrons are rearranged• The opening of the nucleus releases a The opening of the nucleus releases a

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

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

4

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

5

Why decay occurs

• Nuclei of radioactive isotopes are unstable.

• There are various types of instability and radioactive decay

6

Types of RadiationTypes of Radiation

e01

He42

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

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

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

00

7

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

8

Penetrating AbilityPenetrating Ability

9

Balancing Nuclear ReactionsBalancing Nuclear Reactions•In the reactants (starting materials – on the left side of an equation) and products (final products – on the right side of an equation)Atomic numbers must balance

andMass numbers must balance

•Use a particle or isotope to fill in the missing protons and neutrons

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Nuclear ReactionsNuclear Reactions• Alpha emissionAlpha emission

Note that mass number (A) goes down by 4 and atomic number (Z) goes down by 2.

Nucleons (nuclear particles… protons and neutrons) are rearranged but conserved

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Nuclear ReactionsNuclear Reactions• Beta emissionBeta emission

Note that mass number (A) is unchanged and atomic number (Z) goes up by 1.

12Other Types of Nuclear Other Types of Nuclear ReactionsReactions

Positron (Positron (00+1+1): a positive electron): a positive electron

Electron capture: Electron capture: the capture of an electron

207 207

13

Learning Check

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

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

14

Write Nuclear Equations!

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

15Artificial 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.

16Artificial 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 +

17

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

starting with a starting with a 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

18

Nuclear FissionNuclear Fission

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

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

Fission chain has two general steps:Fission chain has two general steps:

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

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

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

initiate other fissionsinitiate other fissions

20Stability 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

21Band of Stability Band of Stability and Radioactive and Radioactive DecayDecay

22Representation of a fission process.

23Nuclear 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.

24Figure 19.6: Diagram of a nuclear power plant.

25

Nuclear Fusion

Fusion small nuclei combine

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

Occurs in the sun and other stars

Energy

26

Nuclear Fusion

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

FAILED.• “Hot” fusion is difficult to contain

27

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.

• Symbol for half-life is tSymbol for half-life is t1/21/2

28Half-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?

29Kinetics 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

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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?

ANSWER: 8 mg of I-123 remainANSWER: 8 mg of I-123 remainANALYSIS: ANALYSIS: •39/13 = 3 half-lives39/13 = 3 half-lives•After 1 half-life = 32 mg remainAfter 1 half-life = 32 mg remain•After 2 half-lives = 16 mg remainAfter 2 half-lives = 16 mg remain•After 3 half-lives = 8 mg remainAfter 3 half-lives = 8 mg remainSUMMARY: 64g I-123 x ½ x ½ x ½ = 8 g I-123 SUMMARY: 64g I-123 x ½ x ½ x ½ = 8 g I-123

31

Effects of RadiationEffects of Radiation

32

Geiger Counter• Used to detect radioactive substances

33

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Radiocarbon DatingRadiocarbon DatingRadioactive C-14 is formed in the upper atmosphere Radioactive C-14 is formed in the upper atmosphere

by nuclear reactions initiated by neutrons in cosmic by nuclear reactions initiated by neutrons in cosmic radiationradiation

1414N + N + 11oon ---> n ---> 1414C + C + 11HH

The C-14 is oxidized to COThe C-14 is oxidized to CO22, which circulates through , which circulates through the biosphere.the biosphere.

When a plant dies, the C-14 is not replenished.When a plant dies, the C-14 is not replenished.But the C-14 continues to decay with tBut the C-14 continues to decay with t1/21/2 = 5730 years. = 5730 years.

Activity of a sample can be used to date the sample.Activity of a sample can be used to date the sample.

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Sample Problem 25.1p 806

• Carbon-14 emits beta radiation & decays with a t1/2=5730 years. If you start with a mass of 2.00 x 10-12 g of carbon-14

a. How long is three half-lives?b.How many g of the isotope remain at the

end of three half-lives?

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Answer

a. t1/2 = 5730 years x 3 half-lives = 17,190years

half-life

b. 2.00 x 10-12 g x ½ x ½ x ½ = 2.5 x 10-11 g

37Nuclear Medicine: Nuclear Medicine: ImagingImaging

Thyroid imaging using Tc-99mThyroid imaging using Tc-99m

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Food Food IrradiationIrradiation

•Food can be irradiated with Food can be irradiated with rays from rays from 6060Co or Co or 137137Cs.Cs.•Irradiated milk has a shelf life of 3 mo. Irradiated milk has a shelf life of 3 mo. without refrigeration.without refrigeration.•USDA has approved irradiation of meats USDA has approved irradiation of meats and eggs.and eggs.