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NUCLEAR CHEMISTRY

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Title: NUCLEAR CHEMISTRY


1
NUCLEAR CHEMISTRY
  • By Stephanie Chen
  • and
  • Stephanie Ng

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
23.1
5
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

6
Other Nuclear Particles
  • Neutron
  • Positron a positive electron
  • Proton usually referred to as hydrogen-1
  • Any other elemental isotope

7
Penetrating Ability
8
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
23.1
9
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
23.1
10
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
23.1
11
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
23.2
12
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
23.2
13
Learning Check
  • What radioactive isotope is produced in the
    following bombardment of boron?
  • 10B 4He 13N 1n
  • 5 2 7
    0

14
Write Nuclear Equations!
  • Write the nuclear equation for the beta emitter
    Co-60.

60Co 0e 60Ni27 -1 28
15
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.

16
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

17
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

18
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

23.2
19
Band of Stability and Radioactive Decay
20
Stability of Nuclei
  • Out of gt 300 stable isotopes

N
Even
Odd
Z
157
52
Even
Odd
50
5
21
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.

22
Half-Life
Decay of 20.0 mg of 15O. What remains after 3
half-lives? After 5 half-lives?
23
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
24
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
23.3
25
Kinetics of Radioactive Decay
N N0exp(-lt)
lnN lnN0 - lt
23.3
26
Radiocarbon Dating
t½ 5730 years
Uranium-238 Dating
t½ 4.51 x 109 years
23.3
27
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?

28
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.

29
Nuclear Fission
30
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
23.5
31
Representation of a fission process.
32
Mass Defect
  • Some of the mass can be converted into energy
  • Shown by a very famous equation!
  • Emc2

Energy Mass Speed of light
33
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
23.2
34
Nuclear binding energy per nucleon vs Mass number
23.2
35
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.
23.5
36
Nuclear Fusion
  • Fusion
  • small nuclei combine
  • 2H 3H 4He 1n
  • 1 1
    2 0
  • Occurs in the sun and other stars

Energy
37
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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