Nuclear Chemistry

1
Nuclear Chemistry

• Chapter 18

2
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.

3
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

4
(No Transcript)
5
Types of Radiation

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

6
Other Nuclear Particles

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

7
Penetrating Ability
8
Geiger Counter

• Used to detect radioactive substances

9
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
10
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
11
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
12
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
13
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
14
Learning Check

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

15
Learning Check

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

16
Write Nuclear Equations!

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

60Co 0e 60Ni27 -1 28
17
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.

18
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

19
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

20
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

21
Band of Stability and Radioactive Decay
22
Stability of Nuclei

• Out of gt 300 stable isotopes

N
Even
Odd
Z
157
52
Even
Odd
50
5
23
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.

24
Half-Life
Decay of 20.0 mg of 15O. What remains after 3
half-lives? After 5 half-lives?
25
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 ? 25 grams After 7.2 days ? 12.5
grams After 10.8 days? 6.25 grams
26
Kinetics of Radioactive Decay
X X0e(-kt)
lnX lnX0 - kt
X the amount of atoms at time t
X0 the amount of atoms at time t 0
k is the decay constant (sometimes called l)
0.693
t½

t½
k
27
Radiocarbon Dating
t½ 5730 years
Uranium-238 Dating
t½ 4.51 x 109 years
28
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?

29
Biological Effects of Radiation
Radiation absorbed dose (rad)
1 rad 1 x 10-5 J/g of material
Roentgen equivalent for man (rem)
1 rem 1 rad x Q
Quality Factor
g-ray 1
b 1
a 20
30
Effects of Radiation
31
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.

32
Nuclear Fission
33
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
34
Representation of a fission process.
35
Mass Defect

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

Energy Mass Speed of light
36
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
37
Nuclear binding energy per nucleon vs. Mass number
38
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.
39
Nuclear Fission POWER

• Currently about 103 nuclear power plants in the
  U.S. and about 435 worldwide.
• 17 of the worlds energy comes from nuclear.

40
Diagram of a nuclear power plant
41
Nuclear Fission
Annual Waste Production
42
Nuclear Fission
Hazards of the radioactivity in spent fuel
compared to uranium ore
From Science, Society and Americas Nuclear
Waste, DOE/RW-0361 TG
43
Chemistry In Action Natures Own Fission Reactor
Natural Uranium
0.7202 U-235 99.2798 U-238
Measured at Oklo
0.7171 U-235
44
Nuclear Fusion

• Fusion
• small nuclei combine
• 2H 3H 4He 1n
• 1 1
  2 0
• Occurs in the sun and other stars

Energy
45
Nuclear Fusion
Fusion Reaction
Energy Released
6.3 x 10-13 J
2.8 x 10-12 J
3.6 x 10-12 J
Tokamak magnetic plasma confinement
23.6
46
Nuclear Fusion

• Fusion
• Excessive heat can not be contained
• Attempts at cold fusion have FAILED.
• Hot fusion is difficult to contain

47
Radioisotopes in Medicine

• 1 out of every 3 hospital patients will undergo a
  nuclear medicine procedure
• 24Na, t½ 14.8 hr, b emitter, blood-flow tracer
• 131I, t½ 14.8 hr, b emitter, thyroid gland
  activity
• 123I, t½ 13.3 hr, g-ray emitter, brain imaging
• 18F, t½ 1.8 hr, b emitter, positron emission
  tomography
• 99mTc, t½ 6 hr, g-ray emitter, imaging agent

Brain images with 123I-labeled compound
23.7
48
Chemistry In Action Food Irradiation
Dosage Effect
Up to 100 kilorad Inhibits 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.