NUCLEAR CHEMISTRY

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NUCLEAR CHEMISTRY

• By Stephanie Chen
• and
• Stephanie Ng

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Radioactivity

• One of the pieces of evidence for the fact that
  atoms are made of smaller particles came from the
  work of Marie Curie (1876-1934).
• She discovered radioactivity, the spontaneous
  disintegration of some elements into smaller
  pieces.

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Nuclear Reactions vs. Normal Chemical Changes

• Nuclear reactions involve the nucleus
• The nucleus opens, and protons and neutrons are
  rearranged
• The opening of the nucleus releases a tremendous
  amount of energy that holds the nucleus together
  called binding energy
• Normal Chemical Reactions involve electrons,
  not protons and neutrons

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Types of Radiation

• Alpha (?) a positively charged (2) helium
  isotope - we usually ignore the charge because
  it involves electrons, not protons and neutrons
• Beta (ß) an electron
• Gamma (?) pure energy called a ray rather than
  a particle

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Other Nuclear Particles

• Neutron
• Positron a positive electron
• Proton usually referred to as hydrogen-1
• Any other elemental isotope

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Penetrating Ability
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Atomic number (Z) number of protons in nucleus
Mass number (A) number of protons number of
neutrons
atomic number (Z) number of neutrons
A
1
1
0
0
4
Z
1
0
-1
1
2
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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.
235 1 138 96 2x1

1. 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.
92 0 55 37 2x0
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212Po decays by alpha emission. Write the
balanced nuclear equation for the decay of 212Po.
212 4 A
A 208
84 2 Z
Z 82
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Nuclear Stability and Radioactive Decay
Beta decay
Decrease of neutrons by 1
Increase of protons by 1
Positron decay
Increase of neutrons by 1
Decrease of protons by 1
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Nuclear Stability and Radioactive Decay
Electron capture decay
Increase of neutrons by 1
Decrease of protons by 1
Alpha decay
Decrease of neutrons by 2
Decrease of protons by 2
Spontaneous fission
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Learning Check

• What radioactive isotope is produced in the
  following bombardment of boron?
• 10B 4He 13N 1n
• 5 2 7
  0

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Write Nuclear Equations!

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

60Co 0e 60Ni27 -1 28
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Artificial Nuclear Reactions

• New elements or new isotopes of known elements
  are produced by bombarding an atom with a
  subatomic particle such as a proton or neutron --
  or even a much heavier particle such as 4He and
  11B.
• Reactions using neutrons are called g reactions
  because a g ray is usually emitted.
• Radioisotopes used in medicine are often made by
  g reactions.

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Artificial Nuclear Reactions

• Example of a g reaction is production of
  radioactive 31P for use in studies of P uptake in
  the body.
• 3115P 10n ---gt 3215P g

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Transuranium Elements

• Elements beyond 92 (transuranium) made starting
  with an g reaction
• 23892U 10n ---gt 23992U g
• 23992U ---gt 23993Np 0-1b
• 23993Np ---gt 23994Pu 0-1b

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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

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Band of Stability and Radioactive Decay
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Stability of Nuclei

• Out of gt 300 stable isotopes

N
Even
Odd
Z
157
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Even
Odd
50
5
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Half-Life

• HALF-LIFE is the time that it takes for 1/2 a
  sample to decompose.
• The rate of a nuclear transformation depends only
  on the reactant concentration.

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Half-Life
Decay of 20.0 mg of 15O. What remains after 3
half-lives? After 5 half-lives?
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Kinetics of Radioactive Decay

• For each duration (half-life), one half of the
  substance decomposes.
• For example Ra-234 has a half-life of 3.6
  daysIf you start with 50 grams of Ra-234

After 3.6 days gt 25 grams After 7.2 days gt 12.5
grams After 10.8 days gt 6.25 grams
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Kinetics of Radioactive Decay
rate lN
N N0e(-lt)
lnN lnN0 - lt
N the number of atoms at time t
N0 the number of atoms at time t 0
l is the decay constant (sometimes called k)
k
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Kinetics of Radioactive Decay
N N0exp(-lt)
lnN lnN0 - lt
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Radiocarbon Dating
t½ 5730 years
Uranium-238 Dating
t½ 4.51 x 109 years
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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 31 hours?

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Nuclear Fission

• Fission is the splitting of atoms
• These are usually very large, so that they are
  not as stable
• Fission chain has three general steps
• 1. Initiation. Reaction of a single atom
  starts the chain (e.g., 235U neutron)
• 2. Propagation. 236U fission releases neutrons
  that initiate other fissions
• 3. Termination.

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Nuclear Fission
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Nuclear Fission
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
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Representation of a fission process.
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Mass Defect

• Some of the mass can be converted into energy
• Shown by a very famous equation!
• Emc2

Energy Mass Speed of light
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Nuclear binding energy (BE) is the energy
required to break up a nucleus into its component
protons and neutrons.
E mc2
BE 9 x (p mass) 10 x (n mass) 19F mass
BE (amu) 9 x 1.007825 10 x 1.008665 18.9984
BE 0.1587 amu
1 amu 1.49 x 10-10 J
BE 2.37 x 10-11J
1.25 x 10-12 J
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Nuclear binding energy per nucleon vs Mass number
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Nuclear Fission
Nuclear chain reaction is a self-sustaining
sequence of nuclear fission reactions.
The minimum mass of fissionable material required
to generate a self-sustaining nuclear chain
reaction is the critical mass.
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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