Chapter 20 – NUCLEAR CHEMSITRY
Symbol Examples
Symbol
Mass #
(A)
Atomic #
(B)
p+
n0
37
Cl
17
Mercury202
Radioactive decay
Nuclear Bombardment
Nuclear equations are balanced when the total mass
number and the atomic number on both reactant and
product sides are equal.
238
92
234
90
Th  24He
U
Symbols for other particles are given below:
Proton
1
1
H or 11P
Neutron
1
0
Electron
0
-1
Positron
0
1
Gamma photon
0
0
n
e or -01β
e or 01β
γ
1
Chapter 20 – NUCLEAR CHEMSITRY
Compare/Contrast
Chemical RXN
Reactions disturbs electrons Example: 2Na + Cl2  2NaCl
Nuclear Rxn
Electron disturbs protons
Examples:
222
86
14
7
Rn  42 He 
218
84
Po
N  42He  178 O  11H
Radon−222 is a radioac ve noble gas that is
sometimes present as an air pollutant in homes built
over soil with high uranium content (uranium−238
decays to radium−226, which in turn decays to
radon−222). A radon−222 nucleus decays to
polonium−218 by emi ng an alpha par cle. Write the
nuclear equation for this decay process.
2
3
4
5
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Chapter 20 – NUCLEAR CHEMSITRY
Nuclear Stability: An analogy
Nuclear “Magic #s”
• Analogy to filled electron shells (2 e-, 8 e-, 18e)
• Filled nuclear shells
• Proton magic #s are: 2,8,20, 28, 50, 82, 114
• Neutron magic #s are 2, 8, 20, 28, 50, 82, 126,
184
Even/odd rule
• Even protons and/or neutrons are more stable
than odd #’s
• Analogy to electron pair stability
Band of Stability
Atomic number
Stable ratio
Protons:
Neutrons
Small
1:1
Large
1:1.5
WHY: Proton-proton repulsion
GENERAL RULE: There are NO stable nuclides with
Z>83
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Chapter 20 – NUCLEAR CHEMSITRY
STRATEGY FOR FINDING STABLE NUCLIDES:
Look for :
• Magic Numbers
• Even proton
• Even Neutrons
• Lower atomic #’s (Above 83, there are no
stable nuclides)
Predict which nucleus in each pair should be
more stable and explain why.
a. astatine−210 OR lead−207
b. molybdenum−91 OR
molybdenum−92
c. calcium−37 OR calcium−42
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Chapter 20 – NUCLEAR CHEMSITRY
SIX TYPE OF RADIOACTIVE DECAY.
1. Alpha Emission: Unstable nuclei emits α
226
88
Ra 
222
86
Rn  42He
2. Beta emission: Equivalent to a neutron
converting to a proton
14
6
C  147N  01e
3. Positron Emission: Positron emission is
equivalent to a proton converting to a neutron
95
43
95
Tc  42
Mo  01e
4. Electron capture: Electron capture is
equivalent to a proton converting to a
40
19
K  01e 
40
18
Ar
neutron.
5. Gamma Emission: electromagnetic radiation
only
6. Spontaneous fission: heavy nucleus of mass
number greater than 89 splits into lighter
nuclei and energy is released.
236
96
136
1
92
39
53
0
U
Y
I4 n
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Chapter 20 – NUCLEAR CHEMSITRY
WHAT TYPE OF EMMISION AND WHY?
GOAL: Achieve a stable ratio protons / neutrons
Atomic number
Stable ratio
Protons:
Neutrons
Small
1:1
Large
1:1.5
IF N/Z ratio is too small THEN convert a proton to a
neutron
IF N/Z is too large THEN convert a neutron to a proton
Thallium−201 is a radioac ve isotope used in the
diagnosis of circulatory impairment and heart disease.
How do you expect it to decay?
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Chapter 20 – NUCLEAR CHEMSITRY
?
You have two samples of water, each made up of
different isotopes of hydrogen: one contains
hydrogen−1 and the other contains hydrogen−3.
a. Would you expect these two water samples to
be chemically similar?
b. Would you expect these two water samples to
be physically the same?
c. Which one of these water samples would you
expect to be radioactive?
Transmutation is the change of one element into
another by bombarding the nucleus of the element
with nuclear particles or nuclei.
A neutron is produced when lithium−7 is bombarded
with a proton. What product nucleus is obtained in this
reaction?
Biological Effects and Radiation Dosage:
Risk Assessment:
Depends on
•
Exposure
• How much many Joules/kg of tissue?
• How bad is it? (Not all radiation is
equal)
•
Length of Time
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Chapter 20 – NUCLEAR CHEMSITRY
The activity of a radioactive source is the number of
nuclear disintegrations per unit time occurring in a
radioactive material.
The curie (Ci) is a unit of activity equal to 3.700 × 1010
disintegrations per second.
Rates are reported as “Activity” in units of Ci. They
must be converted
Ci  disintegrations/sec
The thorium−234 isotope decays by emi ng a beta
particle. The kinetics are first order. A 50.0−μg sample
of thorium−234 has an ac vity of 1.16 Ci. What is the
decay constant (aka rate constant) for thorium−234?
Rate = kNt
Half−life is the time it takes for one−half of the nuclei
in a sample to decay.
Kinetics are first order:
ln[A] = −
+ ln[ ]
Half−life is related to the decay constant by the
following equation:
After one half−life, half of the sample (0.5)
remains.
After two half−lives, one−fourth of the
sample (0.25) remains.
After three half−lives, one−eighth of the
sample remains.
n
 1
Fraction remaining    ,
2
where n  number of half - lives
Thallium−201 is used in the diagnosis of heart
disease. This isotope decays by electron capture; the
decay constant is 2.63 × 10−6/s. What is the half−life
of thallium−201 in days?
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Chapter 20 – NUCLEAR CHEMSITRY
A sample of wheat recovered from a cave was
analyzed and gave 12.8 disintegrations of carbon−14
per minute per gram of carbon. What is the age of the
grain?
Carbon from living material decays at a rate of 15.3
disintegrations per minute per gram of carbon. The
half−life of carbon−14 is 5730 years.
Applications of Radioisotopes:
 Chemical Analysis: A radioactive tracer is a
very small amount of radioactive isotope that
is added to a chemical, biological, or physical
system so as to study the system.
 Medical Therapy and Diagnosis:
Radioisotopes are used for diagnosis of many
medical conditions. For example, they are
used to develop images of internal body
organs so those organs’ functioning can be
examined. More than 100 different
radioactive isotopes have been used in
medicine.
We can compute the change in energy for a nuclear
reaction by calculating the change in mass. The change
in mass must be given in kilograms to satisfy Einstein’s
equation.
ΔE = (Δm)c2
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Chapter 20 – NUCLEAR CHEMSITRY
Consider the following nuclear reaction, in which a
lithium−7 nucleus is bombarded with a hydrogen
nucleus to produce two alpha particles:
7
3
7
3
1
1
H, 1.00728 amu
4
2
Li  11H  2 42 He
What is the energy change of this reaction per gram of
lithium?
Nuclear Binding Energy
The equivalence of mass and energy explains the mass
defect—that is, the difference between the total mass
of the nucleons that make up an atom and the mass of
the atom. The difference in mass is the energy holding
the nucleus together.
The binding energy of a nucleus is the energy needed
to break a nucleus into its individual protons and
neutrons.
Both the binding energy and the mass defect are
indications of the stability of the nucleus.
14
Li, 7.01436 amu
He, 4.00150 amu
Chapter 20 – NUCLEAR CHEMSITRY
Nuclear fission is a nuclear reaction in which a heavy
nucleus splits into lighter nuclei and releases energy.
This process sometimes occurs spontaneously, as with
californium−252.
252
98
Cf 
142
56
Ba 
106
42
Mo  4 01 n
Nuclear fusion is a nuclear reaction in which light
nuclei combine to give a more stable, heavier nucleus
plus possibly several neutrons. This process releases
energy.
2
1
H  31H  42 He  01n
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