chapter 21 nuclear chemistry - dhouts.com · isotopes • not all atoms of the same element have...
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Chapter 21
Nuclear Chemistry
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The Nucleus
• Remember that the nucleus is comprised of
the two nucleons, protons and neutrons.
• The number of protons is the atomic number.
• The number of protons and neutrons together
is effectively the mass of the atom.
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Isotopes
• Not all atoms of the same element have
the same mass due to different
numbers of neutrons in those atoms.
• There are three naturally occurring
isotopes of uranium:
Uranium-234
Uranium-235
Uranium-238
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Radioactivity
• It is not uncommon for some nuclides of
an element to be unstable, or
radioactive.
• We refer to these as radionuclides.
• There are several ways radionuclides
can decay into a different nuclide.
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Types of Radioactive Decay
Alpha Decay
Loss of an -particle (a helium nucleus)
He 4
2
U 238
92 Th
234
90 He 4
2 +
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Types of Radioactive Decay
Beta Decay
Loss of a -particle (a high energy electron)
0
−1 e 0
−1 or
I 131
53 Xe 131
54 + e
0
−1
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Types of Radioactive Decay
Gamma Emission
Loss of a -ray (high-energy radiation that
almost always accompanies the loss of a
nuclear particle)
0
0
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Types of Radioactive Decay
Electron Capture (K-Capture)
Addition of an electron to a proton in the
nucleus
As a result, a proton is transformed into a
neutron.
p 1
1 + e
0
−1 n
1
0
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Neutron-Proton Ratios • Any element with more than
one proton (i.e., anything but hydrogen) will have repulsions between the protons in the nucleus.
• A strong nuclear force helps keep the nucleus from flying apart.
• Neutrons play a key role stabilizing the nucleus.
• Therefore, the ratio of neutrons to protons is an important factor.
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Neutron-Proton Ratios
For smaller nuclei
(Z 20) stable
nuclei have a
neutron-to-proton
ratio close to 1:1.
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Neutron-Proton Ratios
As nuclei get
larger, it takes a
greater number of
neutrons to
stabilize the
nucleus.
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Stable Nuclei
The shaded region in
the figure shows
what nuclides would
be stable, the so-
called belt of stability.
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Stable Nuclei
• Nuclei above this
belt have too many
neutrons.
• They tend to decay
by emitting beta
particles.
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Stable Nuclei
• Nuclei below the belt
have too many protons.
• They tend to become
more stable by positron
emission or electron
capture.
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Stable Nuclei
• There are no stable nuclei with an
atomic number greater than 83.
• These nuclei tend to decay by alpha
emission.
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Radioactive Series
• Large radioactive
nuclei cannot stabilize
by undergoing only
one nuclear
transformation.
• They undergo a series
of decays until they
form a stable nuclide
(often a nuclide of
lead).
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Nuclear Transformations Nuclear transformations can be
induced by accelerating a particle
and colliding it with the nuclide.
These particle accelerators are enormous, having
circular tracks with radii that are miles long.
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Measuring Radioactivity
• One can use a device like this Geiger counter to
measure the amount of activity present in a
radioactive sample.
• The ionizing radiation creates ions, which conduct
a current that is detected by the instrument.
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Energy in Nuclear Reactions
• There is a tremendous amount of
energy stored in nuclei.
• Einstein’s famous equation, E = mc2,
relates directly to the calculation of this
energy.
• In chemical reactions the amount of
mass converted to energy is minimal.
• However, these energies are many
thousands of times greater in nuclear
reactions.
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Energy in Nuclear Reactions
For example, the mass change for the decay
of 1 mol of uranium-238 is −0.0046 g.
The change in energy, E, is then
E = (m) c2
E = (−4.6 10−6 kg)(3.00 108 m/s)2
E = −4.1 1011 J
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Nuclear Fission
• How does one tap all that energy?
• Nuclear fission is the type of reaction carried
out in nuclear reactors.
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Nuclear Fission
• Bombardment of the radioactive nuclide with
a neutron starts the process.
• Neutrons released in the transmutation strike
other nuclei, causing their decay and the
production of more neutrons.
• This process continues in what we call a
nuclear chain reaction.
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Nuclear Fission
• If there are not enough radioactive nuclides in the
path of the ejected neutrons, the chain reaction
will die out.
• Therefore, there must be a certain minimum
amount of fissionable material present for the
chain reaction to be sustained: Critical Mass.
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Nuclear Reactors In nuclear reactors the heat generated by the
reaction is used to produce steam that turns a
turbine connected to a generator.
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Nuclear Reactors
• The reaction is kept in
check by the use of
control rods.
• These block the paths of
some neutrons, keeping
the system from reaching
a dangerous supercritical
mass.
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Nuclear Fusion
• Fusion would be a superior
method of generating power.
The good news is that the
products of the reaction are
not radioactive.
The bad news is that in order to achieve fusion, the
material must be in the plasma state at several million
kelvins.
Tokamak apparati like the one shown at the right show
promise for carrying out these reactions.
They use magnetic fields to heat the material.
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Half-Life
• Amount of time it takes for
one half of a sample of
radioactive atoms to decay
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Summary of Radioactive
Isotopes
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Radioactive by-products of Uranium
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Medical Applications of Half-
Life
Nuclide Half-Life Area of Body
I–131 8.1 days Thyroid
Fe–59 45.1 days Red Blood Cells
Sr–87 2.8 hours Bones
Tc–99 6.0 hours Heart
Na–24 14.8 hours Circulatory System
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Half-Life Calculation #1
• You have 400 mg of a
radioisotope with a half-life of
5 minutes. How much will be
left after 30 minutes?
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Half-Life Calculation #2
• Suppose you have a 800 mg
sample of U-238, which has
a half-life of 4.5 billion years.
How long will it take to have
100mg left?
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Half-Life Calculation # 3
• Cobalt-60 is a radioactive isotope used in cancer treatment. Co-60 has a half-life of 5 years. If a hospital starts with a 1000 mg supply, how many mg will need to be purchased after 10 years to replenish the original supply?
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Half-Life Calculation # 4
• A radioisotope has a half-life
of 1 hour. If you began with
a 100 g sample of the
element at noon, how much
remains at 3 PM? At 6 PM?
At 10 PM?
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Half-Life Calculation # 5
• How many half-lives have
passed if 255 g of Co-60
remain from a sample of
8160 g?
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Half-Life Calculation # 6
• Suppose you have a sample
containing 400 nuclei of a
radioisotope. If only 25
nuclei remain after one hour,
what is the half-life of the
isotope?
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Half-Life Calculation # 7
• If a radioactive element has
diminished by 7/8 of its
original amount in 30
seconds, what is its half-life?
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Answers to Half-Life
Calculations
• Half-Life Calculation #1
6.25 mg
• Half-Life Calculation #2
12.5 mg
• Half-Life Calculation #3
750 mg
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Answers to Half-Life
Calculations
• Half-Life Calculation #4
12.5 g, 1.5625 g,
0.09765625 g
• Half-Life Calculation #5
5 half-lives
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Answers to Half-Life
Calculations
• Half-Life Calculation #6
15 minutes
• Half-Life Calculation #7
10 seconds