Nuclear Chemistry

1
Nuclear Chemistry

• Chapter 23

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n/p too large
n/p too small
23.2
3
Types Of Radioactive Decay

• An alpha (a) particle has the same composition as
  a helium nucleus two protons and two neutrons
  (positive charge)
• Beta (b-) particles are high-energy electrons
  (from the nucleus, not from the electron cloud).
• Gamma (g) rays are a highly penetrating form of
  electromagnetic radiation. No charge.
• Positrons are particles having the same mass as
  electrons but carrying a charge of 1.
• Electron capture (EC) is a process in which the
  nucleus absorbs an electron from an inner
  electron shell, usually the first or second, with
  the release of an X-ray.
• Most radionuclides undergo one specific type of
  decay, though some can undergo two or more types.

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How Far Will Radiation Penetrate?
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Summary Of Decay Types
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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
23.1
7
Balancing Nuclear Equations

• 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

• 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
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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
23.1
9

• (a) a-particle emission by radon-222
• 22288Rn ? 42He ??
• 22288Rn ? 42He 21886Po
• (b) ß-decay of phosphorus-32
• 3215P ? 01 ß ??
• 3215P ? 01 ß 3216S

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23.1
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Nuclear Stability and Radioactive Decay
Beta decay
Increase of protons by 1
Decrease of neutrons by 1
Positron decay
Decrease of protons by 1
Increase of neutrons by 1
23.2
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Nuclear Stability and Radioactive Decay
Electron capture decay
Decrease of protons by 1
Increase of neutrons by 1
Alpha decay
Decrease of neutrons by 2
Decrease of protons by 2
23.2
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n/p too large
beta decay
n/p too small
positron decay or electron capture
23.2
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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 are radioactive

23.2
15
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
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Nuclear binding energy per nucleon vs Mass number
23.2
17
Electron capture decay
- 36.966776amu - .000549amu 36.965903amu -
0.001422amu
0.001422amu 1.4924E-10 J/amu 2.1222E-13 J
E/h v 2.1222E-13 J / 6.626E-34 Js 3.2028E20
1/s
1amu 931.5MeV
0.001422amu 931.5 MeV/amu 1.3246 MeV
23.2
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Kinetics of Radioactive Decay
rate lN
N N0exp(-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
23.3
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Kinetics of Radioactive Decay
N N0exp(-lt)
lnN lnN0 - lt
23.3
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Selected Nuclide Half-Lives
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Radiocarbon Dating
t½ 5730 years
Uranium-238 Dating
t½ 4.51 x 109 years
23.3
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Radiocarbon Dating

• 14C is formed at a nearly constant rate in the
  upper atmosphere by the bombardment of 14N with
  neutrons from cosmic radiation.
• The 14C is eventually incorporated into
  atmospheric CO2.
• Since atmospheric CO2 eventually finds its way
  into living tissue, the level of 14C in living
  tissue is constant (about 15 disintegrations per
  minute per gram of carbon).
• When an organism dies, no more 14C is consumed.
• The level of 14C decreases as the dead tissue
  ages.
• The half-life for carbon-14 is 5,730 years. This
  dating method works well for objects up to 50,000
  years old.

23
EXAMPLE 19.4 A wooden object from an Egyptian
tomb is subjected to radiocarbon dating. The
decay rate observed for its carbon-14 content is
7.2 dis min1 per g carbon. What is the age of
the wood in the object (and, presumably, of the
object itself)? The half-life of carbon-14 is
5730 years, and the decay rate for carbon-14 in
living organisms is 15 dis min1 per g
carbon. SOLUTION First, we can determine the
decay constant, ? ? 0.693/ t1/2 0.693/5730
y 1.21 x 104 y1 Now we can use the decay
equation, substituting activity (disintegrations
per minute) for N Nt 7.2
ln ? t ln (1.21 x
104 y1) t No 15 t
6.1 x 103 y
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Nuclear Transmutation
23.4
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Nuclear Transmutation
23.4
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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
23.5
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Nuclear Fission
Representative fission reaction
23.5
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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.
23.5
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Nuclear Fission
Schematic diagram of a nuclear fission reactor
23.5
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Nuclear Fission
Annual Waste Production
23.5
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Nuclear Fission
Hazards of the radioactivities in spent fuel
compared to uranium ore
23.5
From Science, Society and Americas Nuclear
Waste, DOE/RW-0361 TG
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Chemistry In Action Natures Own Fission Reactor
Natural Uranium
0.7202 U-235 99.2798 U-238
Measured at Oklo
0.7171 U-235
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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
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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
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Geiger-Müller Counter
23.7
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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
23.8
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Chemistry In Action Food Irradiation