Chapter 4
Nuclear Chemistry: The
Heart of Matter
Radioisotopes
• Radioactive decay
– Many isotopes are unstable
– Nuclei that undergo radioactive decay
– May produce one or more types of
radiation
Natural Radioactivity
• Background
radiation
– What occurs from
natural sources
– Accounts for >80%
of radioactivity
exposure
Types of Radiation
• Ionizing radiation – knocks electrons out of
atoms or groups of atoms
– Produces charged species – ions
– Charged species that cause damage
Alpha Decay
4
2
He
• Nucleus loses  particle
– Mass decreases by 4 and atomic number decreases by 2
Beta Decay
• Nucleus loses  particle
– No change in mass but atomic number increases
0
1
•
e
Positron
Emission
Loses a positron
– Equal mass but opposite charge of an electron
– Decrease in atomic number and no change in mass
– +
Electron Capture
• Nucleus absorbs an
electron and then
releases an X-ray
• Mass number stays the
same and atomic
number decreases
Gamma Radiation
• Release of high-energy photon
– 
• Typically occurs after another radioactive decay
• No change in mass number or atomic number
Nuclear Equations
• Elements may change in nuclear reactions
• Total mass and sum of atomic numbers must be
the same
• MUST specify isotope
222
86
Rn  He 
4
2
218
84
Po
Differences Between Chemical and Nuclear
Reactions
Example 4.1 Balancing Nuclear Equations
Write balanced nuclear equations for each of the
following processes. In each case, indicate what new
element is formed.
a. Plutonium-239 emits an alpha particle when it
decays.
b. Protactinium-234 undergoes beta decay.
c. Carbon-11 emits a positron when it decays.
d. Carbon-11 undergoes electron capture.
Solution
a. We start by writing the symbol for plutonium-239 and a partial equation showing that
one of the products is an alpha particle (helium nucleus):
239
Pu
94
4
He + ?
2
Mass and charge are conserved. The new element must have a mass of 239 – 4 = 235
and a charge of 94 – 2 = 92. The nuclear charge (atomic number) of 92 identifies the
element as uranium (U):
239
Pu
94
4
235
U
He +
2
92
Half-Life
• Period for one-half of the
original elements to undergo
radioactive decay
• Characteristic for each isotope
1
• Fraction remaining =
2n
n = number of half-lives
Example 4.2 Half-Lives
You obtain a new sample of cobalt-60, half-life 5.25 years, with a
mass of 400 mg. How much cobalt-60 remains after 15.75 years
(three half-lives)?
Solution
The fraction remaining after three half-lives is
1
2n
=
1
=
23
1
2x2x2
=
1
8
1
The amount of cobalt-60 remaining is ( 8 ) (400 mg) = 50 mg.
Exercise 4.2A
You have 1.224 mg of freshly prepared gold-189, half-life 30
min. How much of the gold-189 sample remains after five
half-lives?
Exercise 4.2B
What percentage of the original radioactivity remains
after five half-lives?
Example 4.3
You obtain a 20.0-mg sample of mercury-190, halflife 20 min. How much of the mercury-190 sample
remains after 2 hr?
Solution
There are 120 min in 2 hr. There are (
after six half-lives is
1
2n
=
1
26
=
120
20)
= 6 half-lives in 2 hr. The fraction remaining
1
2x2x2x2x2x2
=
1
64
The amount of mercury-190 remaining is ( 641) (20.0 mg) = 0.313 mg.
Exercise 4.3A
A sample of 16.0 mg of nickel-57, half-life 36.0 hr, is
produced in a nuclear reactor. How much of the
nickel-57 sample remains after 7.5 days?
Exercise 4.3B
Tc-99 decays to Ru-99 with a half-life of 210,000
years. Starting with 1.0 mg of Tc-99, how long will it
take for 0.75 mg of Ru-99 to form?
Radioisotopic Dating
• Use certain isotopes to estimate the age of various
items
235U
•
half-life = 4.5 billion years
– Determine age of rock
3H
•
half-life = 12.3 years
– Used to date aged wines
•
•
•
•
Carbon-14 Dating
98.9% 12C
Produce 14C in upper atmosphere
Half-life of 5730 years
~50,000 y maximum age for dating
14
7
N n  C  H
1
0
14
6
1
1
Example 4.4
A piece of fossilized wood has carbon-14 activity one-eighth
that of new wood. How old is the artifact? The half-life of
carbon-14 is 5730 years.
Solution
The carbon-14 has gone through three half-lives:
1
8
=
()
1
2
3
=
1
2
x
1
2
x
1
2
It is therefore about 3 x 5730 = 17,190 years old.
Exercise 4.4
How old is a piece of cloth that has carbon-14 activity
that of new cloth fibers? The half-life of carbon-14 is
5730 years.
1
16
Shroud of Turin
• Alleged burial shroud of Jesus Christ
– Contains faint human likeness
– First documented in Middle Ages
• Carbon-14 dating done in 1988
– Three separate labs
– Shroud ~800 years old
– Unlikely to be burial shroud
Artificial Transmutation
• Transmutation changes one element into another
– Middle Ages: change lead to gold
• In 1919 Rutherford established protons as fundamental particles
– Basic building blocks of nuclei
14
7
N  42 He 178 O 11 H
Uses of Radioisotopes
• Tracers
– Easy to detect
– Different isotopes have similar chemical and physical properties
– Physical, chemical, or biological processes
• Agriculture
– Induce heritable genetic alterations – mutations
– Preservative
– Destroys microorganisms with little change to taste or appearance
of the food
Nuclear Medicine
• Used for two purposes
• Therapeutic – treat or cure disease using radiation
• Diagnostic – obtain information about patient’s health
Radiation Therapy
• Radiation most lethal to dividing cells
• Makes some forms of cancer susceptible
• Try to destroy cancer cells before too much damage to healthy cells
– Direct radiation at cancer cells
– Gives rise to side effects
Diagnostic Uses
• Many different isotopes used
– See Table 4.6
• Can measure specific things
– Iodine-131 to locate tumors in thyroid
– Selenium-75 to look at pancreas
– Gadolinium-153 to determine bone mineralization
Imaging
• Positron emission tomography (PET)
• Uses an isotope that emits a positron
• Observe amount of radiation released
11
6
C B e
11
5
0
1
Penetrating Power of Radiation
• The more mass the particle has, the less penetrating it is
• The faster the particle is, the more penetrating it is
Prevent Radiation Damage
• To minimize
damage
– Stay a distance
from radioactive
sources
– Use shielding;
need more with
more penetrating
forms of radiation
Example 4.6
One of the isotopes used for PET scans is oxygen15, a positron emitter. What new element is formed
when oxygen-15 decays?
Solution
First write the nuclear equation
15
O
8
0
e+?
+1
The nucleon number does not change, but the atomic number becomes 8 – 1, or 7; and so
the new product is nitrogen-15:
15
O
8
0
15
N
e+
+1
7
Exercise 4.6
Phosphorus-30 is a positron-emitting radioisotope
suitable for use in PET scans. What new element is
formed when phosphorus-30 decays?
Energy from Nucleus
• E = mc2
• Lose mass, gain
energy
– For chemical
reactions, mass
changes are not
measurable
– For nuclear reactions,
mass changes may
be measurable
Binding Energy
• Holds protons and neutrons together in
the nucleus
• The higher the binding energy, the more
stable the element
Nuclear Fission
• “Splitting the atom”
• Break a large
nucleus into
smaller nuclei
Nuclear Chain Reaction
• Neutrons from one
fission event split
further atoms
• Only certain
isotopes, fissile
isotopes, undergo
nuclear chain
reactions
Manhattan Project
• How to sustain the nuclear reaction?
• How to enrich uranium to >90% 235U?
– Only 0.7% natural abundance
• How to make 239Pu (another fissile isotope)?
• How to make a nuclear fission bomb?
Radioactive Fallout
• Nuclear bomb detonated; radioactive materials may rain down
miles away and days later
– Some may be unreacted U or Pu
– Radioactive isotopes produced during the explosion
Nuclear Power Plants
• Provide ~20% U.S. electricity
– France >70%
• Slow controlled release of energy
• Need 2.5–3.5% 235U
• Problem with disposal of radioactive waste
Nuclear Fusion
• Reaction takes smaller nuclei and builds larger ones
– Also called thermonuclear reactions
•
4 H  He  2 e
1
4
0
1
2
1
Releases tremendous amounts of energy
–1 g of H would release same as 20 tons of coal