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Nuclear ChemistryNuclear ChemistryChemistry I – Chapter 25Chemistry I – Chapter 25

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RadioactivityRadioactivity

• One of the pieces of evidence for the One of the pieces of evidence for the fact that atoms are made of smaller fact that atoms are made of smaller particles came from the work ofparticles came from the work of

________ ________ (1876-1934).(1876-1934). • She discoveredShe discovered ________________, the , the

spontaneous disintegration of some spontaneous disintegration of some elements into smaller pieces.elements into smaller pieces.

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

ReactionsReactions• Nuclear reactions involve the nucleusNuclear reactions involve the nucleus• The nucleus opens, and protons and neutrons are The nucleus opens, and protons and neutrons are

rearrangedrearranged• The opening of the nucleus releases a tremendous The opening of the nucleus releases a tremendous

amount of energy that holds the nucleus together – amount of energy that holds the nucleus together – called called binding energybinding energy

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

• Nuclear reactions are not affected by surrounding Nuclear reactions are not affected by surrounding conditions such as pressure or temperature while conditions such as pressure or temperature while chemical reactions are affected.chemical reactions are affected.

• Nuclear reactions cannot be speeded up or slowed Nuclear reactions cannot be speeded up or slowed down or turned off.down or turned off.

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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

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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

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Penetrating AbilityPenetrating Ability

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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 balanceAtomic numbers must balanceandand

Mass numbers must balanceMass numbers must balanceUse a particle or isotope to fill in the missing protons and neutrons

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Nuclear ReactionsNuclear Reactions

• Alpha emissionAlpha emission

92U238 90Th234 + 2He4 (α emission)

Note that mass number goes down by 4 and atomic number goes down by 2.

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

Uranium 238Uranium 238 Thorium 234Thorium 234

Radioactive Radioactive decaydecay

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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.

66CC14 14 77NN1414 + + -1-1e e 00 ( (ββ emission) emission)

Carbon-14 Carbon-14 Nitrogen Nitrogen

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Nuclear ReactionsNuclear Reactions

Gamma emissionGamma emission

• A high energy photons is emitted by the A high energy photons is emitted by the radioisotope is called gamma rays.radioisotope is called gamma rays.

• The high-energy photons are The high-energy photons are electromagnetic radiations.electromagnetic radiations.

• Gamma rays are often released along Gamma rays are often released along with alpha or beta particles.with alpha or beta particles.

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Nuclear ReactionsNuclear Reactions

Gamma emissionGamma emission• Since gamma rays have no mass and

no electrical charge the emission does not alter either the mass number of the atomic number.

90Th230 88Ra226 + 2He4 + γ

Thorium -230 Radon-226 Alpha Gamma

particle rays

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Transmutation Reactions

• The conversion of an atom of an element to an atom of another element is called transmutation.

• Transmutations occur by spontaneous radioactive decay or by bombarding the nucleus of a radioactive element with a projectile.

• Transmutation can occur naturally.

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Examples of Transmutation Reactions

• 7N14 + 2He4

9F18

Nitrogen Alpha particle Flourine

• The fluorine quickly decomposes to a stable isotope of oxygen and a proton.

•9F18

8O17 + 1H1

Fluorine -18 Oxygen-17 Proton

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Learning Check

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

10B + 4He ? + 1n

5 2 0

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Write Nuclear Equations!

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

16Kinetics 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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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.

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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?

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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?

20Artificial Nuclear Artificial Nuclear ReactionsReactions

New elements or new isotopes of known elements New elements or new isotopes of known elements are produced by bombarding an atom with a are 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.

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Nuclear FissionNuclear Fission

http://library.thinkquest.org/17940/texts/fission/fission.html

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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 three general steps:Fission chain has three 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

3. 3. __Termination_________ __Termination_________ . .

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Representation of a fission process.

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Nuclear FissionNuclear Fission• In a chain reaction some of the neutrons produced

react with other fissionable atoms producing more neutrons which react with still more atoms.

• For a chain fission reaction to occur the mass of the fissionable material must not be less than the critical mass.

• Critical mass is the smallest amount of fissionable material needed for a sustained chain nuclear reaction.

25Nuclear 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.

26Figure 19.6: Diagram of a nuclear power plant.

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Fission Reactor

• The water around the reactor acts both as a coolant and also to heat the water and change it to steam.

• Neutron moderation, it is a process that slows down the neutrons to make sure that the neutrons are absorbed by the Uranium-235 nucleus which in turn breaks down in the fission reaction.

• Neutron Absorption, is a process that decreases the number of slow moving neutrons to prevent the chain reaction from going too fast.

• http://www.visionlearning.com/library/flash_viewer.php?oid=3602&mid=59

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Nuclear Fusion

Fusion small nuclei combine

2H + 3H 4He + 1n +

1 1 2 0

Occurs in the sun and other stars

Energy

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Nuclear Fusion

Fusion

• Excessive heat can not be contained

• Attempts at “cold” fusion have FAILED.

• “Hot” fusion is difficult to contain

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Effects of RadiationEffects of Radiation

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Geiger Counter

• Used to detect radioactive substances

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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.

33Nuclear 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.