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1 Chapter 21: Chapter 21: Nuclear Chemistry Nuclear Chemistry

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Chapter 21:Chapter 21:Nuclear ChemistryNuclear Chemistry

2Mass Defect & Nuclear Mass Defect & Nuclear StabilityStability

• Because an atom is made of protons, neutronsBecause an atom is made of protons, neutrons and electrons, you would expect the mass of and electrons, you would expect the mass of an atom to be equal to the sum of the massesan atom to be equal to the sum of the masses of the eof the e--, p, p++, and n, and n00, right??, right??

Neutrons + protons + electrons =Neutrons + protons + electrons = atomic massatomic mass

……right?right?

3Mass Defect & Nuclear Mass Defect & Nuclear StabilityStability

Nay.Nay.The measured mass will be The measured mass will be lessless than the combined than the combinedmasses. This difference is called the masses. This difference is called the Mass DefectMass Defect..

Let’s look at Let’s look at heliumhelium ( He) as an example: ( He) as an example:

2 protons: (2 x 1.007276 amu) = 2.014552 amu2 protons: (2 x 1.007276 amu) = 2.014552 amu2 neutrons: (2 x 1.008665 amu) = 2.017330 amu2 neutrons: (2 x 1.008665 amu) = 2.017330 amu2 electrons: (2 x 0.005486 amu) = 0.001097 amu2 electrons: (2 x 0.005486 amu) = 0.001097 amu

Total combined mass = 4.032979 amuTotal combined mass = 4.032979 amu

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4Mass Defect & Nuclear Mass Defect & Nuclear StabilityStability

But wait…But wait…The atomic mass of a He atom has been measuredThe atomic mass of a He atom has been measured

to be 4.002602 amu, to be 4.002602 amu, NOTNOT 4.032979 amu. 4.032979 amu.

That’s 0.030377 amu LESS than the combined mass.That’s 0.030377 amu LESS than the combined mass.

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Just what the heckJust what the heckis going on?is going on?

5Mass Defect & Nuclear Mass Defect & Nuclear StabilityStability

So what causes the difference in mass?So what causes the difference in mass?

Well, according to Einstein’s famous equation E = mcWell, according to Einstein’s famous equation E = mc22,,mass can be converted to energy, and energy to mass.mass can be converted to energy, and energy to mass.So the mass defect is caused by the So the mass defect is caused by the conversion of massconversion of mass

to energyto energy upon formation of the nucleus. upon formation of the nucleus.

This is called This is called nuclear binding energynuclear binding energy..Remember, the nucleus is like a mousetrap, and the Remember, the nucleus is like a mousetrap, and the nuclear binding energy is needed to “set” the trap. nuclear binding energy is needed to “set” the trap.

It is TREMENDOUS potential energy.It is TREMENDOUS potential energy.

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• 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.

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• 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.

8Nuclear 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

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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 massmassspeed of speed of light light squaredsquared

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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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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

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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 balanceand

Mass 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.

16Other 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

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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.

19Artificial 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.

20Artificial 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 +

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Transuranium ElementsTransuranium Elements

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

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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 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. ___________ ___________ . .

24Stability 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

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DecayDecay

26Representation of a fission process.

27Nuclear Fission & Nuclear Fission & POWERPOWER

nuclear power plants in nuclear power plants in

worldwide.worldwide.

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

energy comes from energy comes from

nuclear.nuclear.

28Figure 19.6: Diagram of a nuclear power plant.

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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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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?

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

decomposes.

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

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?

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Geiger Counter

• Used to detect radioactive substances

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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.

39Nuclear Medicine: Nuclear Medicine: ImagingImaging

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

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