Nuclear chemistry
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Isotopes
Atomic number Z = number of protons
Mass number A = # of protons + # of
neutrons
mass number (A )
atomic number (Z )
12
C
6
number of protons 6
number of neutrons 12 – 6 = 6
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Isotopes are atoms of the same element
having a different number of neutrons
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A radioactive isotope, called a radioisotope, is
unstable and spontaneously emits energy to form
a more stable nucleus
Radioactivity is the nuclear radiation emitted by a
radioactive isotope
Of the known isotopes of all elements, 264 are
stable and 300 are naturally occurring but
unstable
An even larger number of radioactive isotopes,
called artificial isotopes, have been produced in
the laboratory
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Types of radiation: alpha particles, beta
particles, positrons, and gamma radiation
An alpha (α) particle is a high-energy particle
that contains 2 protons and 2 neutrons
It has a +2 charge and a mass number of 4
alpha particle:
a
or
4
He
2
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A beta (β) particle is a high-energy electron
It has a −1 charge and a negligible mass
compared to a proton
beta particle:

β
0
e
or
−1
A β particle is formed when a neutron (n) is
converted to a proton (p) and an electron (e)
1
n
0
1
p
1
neutron
proton
+
0
e
−1
 particle
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A positron is called an antiparticle of a β
particle
Their charges are opposite, but their masses
are the same (i.e., effectively zero)
A positron has a +1 charge and is called a
“positive electron.”
positron:

β+
0
e
or
+1
A positron is formed when a proton is
converted to a neutron
1
p
1
proton
1
0
n
e
+
0
+1
neutron
positron
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Gamma rays are high-energy radiation
released from a radioactive nucleus
They are a form of energy, so they have no
mass and no charge
gamma ray:
g
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Radioactivity cannot be detected by the
senses, yet it can have a powerful effect
Nuclear radiation will damage or kill rapidly
dividing cells such as bone marrow, skin, and
the reproductive and intestinal systems
Cancer cells divide rapidly as well, making
radiation an effective treatment for cancer
Food is irradiated, exposed to gamma
radiation, to kill any living organism in the
food
Afterwards, the food is not radioactive, and
has a considerably longer shelf life
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Radioactive decay is the process by which an
unstable radioactive nucleus emits radiation
A nuclear equation can be written for this
process
original
nucleus

new
nucleus
+
radiation
emitted
The following must be equal on both sides of
a nuclear equation
◦ The sum of the mass numbers (A)
◦ The sum of the atomic numbers (Z)
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Alpha emission is the decay of a nucleus by
emitting an a particle
HOW TO Balance an Equation for a Nuclear Reaction
Example
Write a balanced nuclear equation showing
how americium-241 decays to form an
a particle.
Step [1]
Write an incomplete equation with the
original nucleus on the left and the particle
emitted on the right.
241
95 Am
4
2
He
+
?
HOW TO Balance an Equation for a Nuclear Reaction
Calculate the mass number and atomic
number of the newly formed nucleus on
the right.
4
241
237
+
Np
He
Am
2
95
93
Step [2]
mass number
atomic number
241 − 4 = 237 95 − 2 = 93
Step [3]
Use the atomic number to identify the new
nucleus and complete the equation.
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Beta emission is the decay of a nucleus by
emitting a β particle; 1 neutron is lost and 1
proton is gained
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Positron emission is the decay of a nucleus by
emitting a positron, β+; 1 proton is lost and 1
neutron is gained
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Gamma emission is the decay of a nucleus by
emitting g radiation
◦ The g rays are a form of energy only
◦ Their emission causes no change in the atomic number
or the mass number
99m
43 Tc

99
43 Tc
+
g
Technetium-99m is a metastable isotope; it
decays by gamma emission to the more stable
(but still radioactive) technetium-99
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Commonly,
emission
g emission accompanies a or β
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The half-life (t1/2) of a radioactive isotope is the
time it takes for one-half of the sample to decay
The half-life of a radioactive isotope is a property
of a given isotope and is independent of the
amount of sample, temperature, and pressure
HOW TO Use a Half-Life to Determine the Amount of
Radioisotope Present
Example If the half-life of iodine-131 is 8.0 days, how
much of a 100. mg sample remains after
32 days?
Step [1]
Determine how many half-lives occur in the
given amount of time.
32 days x 1 half-life =
8.0 days
4.0 half-lives
HOW TO Use a Half-Life to Determine the Amount of
Radioisotope Present
Step [2]
For each half-life, multiply the initial mass
by one-half to obtain the final mass.
100. mg x
initial mass
1
2
x
1
2
x
1
2
x
1
2
=
The mass is halved four times.
6.25 mg
final mass
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Radiocarbon dating uses the half-life of carbon14 to determine the age of carbon-containing
materials
The ratio of radioactive carbon-14 to stable
carbon-12 is a constant value in a living
organism
Once the organism dies, the carbon-14 decays
without being replenished
By comparing the ratio of C-14 to C-12 in an
artifact to the same ratio present in organisms
today, the age of the artifact can be determined
The half-life of C-14 is 5,730 years
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The amount of radioactivity in a sample is
measured by the number of nuclei that decay
per unit time - disintegrations per second
Common units include
◦
◦
◦
◦
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1
1
1
1
Curie (Ci)
Curie (Ci)
Curie (Ci)
becquerel
= 3.7 x 1010 disintegrations/second
= 1,000 millicuries (mCi)
= 1,000,000 microcuries (mCi)
(Bq) = 1 disintegration/second
Thus, 1 Ci = 3.7 x 1010 Bq
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Several units are used to measure the amount
of radiation absorbed by an organism
The rad - radiation absorbed dose—is the
amount of radiation absorbed by one gram of
a substance
The rem - radiation equivalent for humans—
is the amount of radiation that also factors in
its energy and potential to damage tissue
1 rem of any type of radiation produces the
same amount of tissue damage
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The average radiation dose per year for a person
is about 0.27 rem
Generally, no detectable biological effects are
noticed for a radiation dose less than 25 rem
A single dose of 25–100 rem causes a temporary
decrease in white blood cell count
A dose of more than 100 rem causes radiation
sickness - nausea, vomiting, fatigue, etc
The LD50 - the lethal dose that kills 50% of a
population - is 500 rem in humans, while 600
rem is fatal for an entire population
http://www.epa.gov/radiation/unders
tand/perspective.html
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Radioisotopes can be injected or ingested to
determine if an organ is functioning properly
or to detect the presence of a tumor
Technetium-99m is used to evaluate the gall
bladder and bile ducts and to detect internal
bleeding
Thallium-201 is used in stress tests to
diagnose coronary artery disease
Using a scan, normal organs are clearly
visible, while malfunctioning or obstructed
organs are not
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Positron emission tomography (PET) scans
use radioisotopes which emit positrons which
enable scanning of an organ
PET scans can detect tumors, coronary artery
disease, Alzheimer’s disease, and track the
progress of cancer
A PET scan is a noninvasive method of
monitoring cancer treatment
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Nuclear fission is the splitting apart of a heavy
nucleus into lighter nuclei and neutrons. It can begin
when a neutron bombards a uranium-235 nucleus
235 + 1
92 U
0n
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91
142
1
+
36 Kr
56 Ba + 30 n
The bombarded U-235 nucleus splits apart into
krypton-91, barium-142, and three high-energy
neutrons, while releasing a great deal of energy
The released neutrons can then bombard other
uranium nuclei, creating a chain reaction
Critical mass: The minimum amount of U-235 needed
to sustain a chain reaction
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A nuclear power plant uses the large amount of
energy released in fission
This energy is used to boil water and create
steam, which turns a turbine and generates
electricity
The dangers of generating nuclear power are
possible radiation leaks and the disposal of
nuclear waste
Radiation leaks can be minimized by containment
facilities within the power plant itself
Nuclear waste is currently buried, but it is unclear
whether this is the best method
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Nuclear fusion is the joining together of two light
nuclei to form a larger nucleus
Hydrogen-2 (deuterium) and hydrogen-3 (tritium)
undergo fusion to create a helium nucleus
2
1
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H
3
+
1
H
4
2
1
He + 0
n
A neutron and a large amount of energy are also
produced
Fusion is not currently useable (on Earth) as an
energy source because it can only occur at extremely
high temperatures and pressures