Nuclear Chemistry
• In a chemical reaction, the valence electrons
are important.
• But the nuclei of elements may undergo
changes as well.
• When the nuclei of elements change (either
spontaneously or when forced) a nuclear
reaction has occurred.
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Nuclear Chemistry
• Nuclear reactions give off enormous amounts
of energy for the very small masses involved.
E = mc2
• They are also dangerous to our bodies, but they
have medical applications which can save our
lives.
• Unlike chemical rxns, nuclear rxns are
unaffected by temperature, pressure, or by
catalysts.
• Nuclear rxns have 1st-order kinetics.
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Radioactivity
• The nucleus contains the nucleons: protons and
neutrons. So the Mass Number gives you the
number of nucleons.
• How many nucleons does C-14 contain?
• You should remember the above notation as an
isotopic notation for the isotope C-14.
• What are some of the other ways to write the
isotopic notations for C-14?
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Radioactivity
• Remember isotopes?
• Isotopes of an element have the same Atomic
Number, Z, but have different # of neutrons.
• How else are isotopes different?
• If an isotope emits radiation, then it is a
radioactive isotope, or a radioisotope or a
radionuclide.
• Every element has at least 1 radioisotope!
• What are the 3 common types of radiation and
what are they?
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Nuclear Decay Rxns or Nuclear Rxns
• Radionuclides aren’t stable.
• They spontaneously undergo nuclear reactions
and emit particles and/or electromagnetic
radiation.
• This process is called nuclear decay as the
nucleus is changing and decomposes.
• Eventually, the unstable radioisotope will
become a stable isotope!
• Here is a table showing the common types of
particles and their common notations:
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Nuclear Decay Rxns or Nuclear Rxns
• Here are the common types of nuclear decay:
 -decay or  -emission
-decay or -emission
positron emission
electron capture
gamma emission
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Patterns of Nuclear Stability
• What makes one isotope stable and
another radioactive?
• The neutron-to-proton ratio or the n/p
ratio!!
• Look at the nucleus: lots of + protons are
packed into an extremely dense, tiny
volume.
• Why don’t they repel and eject out of the
nucleus?
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Patterns of Nuclear Stability
• The neutrons help “glue” the nucleus together
and keep the protons from repelling too
strongly.
• This is part of the strong nuclear force, the
force which holds nucleons together.
• This is particle physics: gluons are the “strong
force” carrier particles, and the interaction
between neutrons and protons are actually
residual effects of the strong force.
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Patterns of Nuclear Stability
• So an atom with more than 1 proton (every
element but H) MUST have neutrons to
stabilize the nucleus.
• There is an optimum n/p ratio or else the
nucleus is unstable.
• For atoms with Z below 20, the ideal n/p ratio
is about 1.
• As the atomic number increases above 20, the
ideal n/p ratio increases till it reaches about
1.5/1 with Hg-200.
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Patterns of Nuclear Stability
• This is the Belt of Stability, or what the ideal
n/p ratio is for stable nonradioactive isotopes.
• Notice this graph stops at Z = 84. All nuclei
with 84 or more protons are radioactive!
• There is another graph showing nonradioactive
isotopes AND radioactive isotopes with
measurable half-lives.
• Radioisotopes with measurable half-lives may
be considered to be “stable” radioisotopes.
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Patterns of Nuclear Stability
• What can an unstable radionuclide do to decay
to reach the Belt of Stability?
• If the radionuclide is above the Belt, it has too
many neutrons (Yes, it can have too many
neutrons, ever used too much glue?). How can
it get rid of extra neutrons and lower the n/p
ratio?
• If the radionuclide is below the Belt, it has too
many protons. How can it lower its n/p ratio?
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Patterns of Nuclear Stability
• What if the radionuclide has Z > 84 and is
beyond the Belt?
• These heavy elements often have to undergo
many radioactive decays, or a series, in order to
reach a stable isotope. These series are called
radioactive series or nuclear disintegration
series.
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Magic Numbers
• If you study tables of stable and unstable
isotopes, you might notice something strange.
• Nuclei with 2, 8, 28, 50, or 82 protons tend to be
more stable.
• Nuclei with 2, 8, 20, 28, 50, 82, or 126 neutrons
tend to be more stable.
• These are the “magic numbers” of protons and
neutrons.
• Also, nuclei with even numbers of both protons
and neutrons are more stable than those with
odd numbers of nucleons.
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Kinetics of Nuclear Decay Rxns
• Normal nuclear decay reactions are first order,
and so are solved using the first-order
equations that you already learned.
• Problems:
– The half-life of Co-60 is 5.3 years. How much of a
1.000 mg sample is left after 15.9 years? How much
is left after 17.0 years?
– A rock contains 0.257 mg Pb-206 per mg U-238. The
half-life for the decay of U-238 to Pb-206 is 4.5x109
years. How old is this rock?
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Nuclear Binding Energy
• It takes energy to bind the nucleus
together (the strong force).
• The nuclear binding energy is defined as
the energy required to hold or “bind” the
nucleus together.
• It is also the energy required to break
apart the nucleus into the nucleons.
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Nuclear Binding Energy
• But where does this binding energy come from?
• From MASS!
E = mc2
• Units?
• But where does this mass come from?
• If you add up the mass of all the protons,
electrons, and neutrons in an isotope and
compared it to the actual isotopic mass, they
AREN’T THE SAME!
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Nuclear Binding Energy
• The actual mass of an isotope IS NOT
equal to the sum of the parts!
• This mass difference is called the mass
defect.
• It is the amount of mass which has been
converted into the nuclear binding
energy.
• The mass defect is usually given in amu
or in kg/atom or in kg/mol.
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Nuclear Binding Energy
• Nuclear binding energies are usually
given in kJ/atom or kJ/nucleon or kJ/mol
nucleon.
• You can see from the following slide that
nuclear binding energies per nucleon
don’t vary that much.
• But intermediate elements are the most
stable.
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Nuclear Binding Energy per Nucleon
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Energy in Nuclear Reactions
• During nuclear rxns, nuclides either gain
or lose mass, and so release or require
energy.
• In spontaneous rxns, mass is lost so
energy is released.
• This is why most nuclear rxns give off
gamma rays: this is the mass which has
been “lost” or converted to energy.
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Fission vs. Fusion
• During Fission, a large nuclei is split into
several smaller nuclei.
• In Fusion (which is what stars do), small
nuclei are fused together to make a larger
one.
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Fission of U-235
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Nuclear Chain Reaction
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Little Boy Schematic
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Fat Man Schematic
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Dangers of Radioactivity
• Although radioactivity is used in cancer
therapy and in medical diagnostics, it is
harmful to our cells.
• It can damage cells, damage the DNA in cells,
and kill cells.
• Is there any difference between the 3 types of
radiation?
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Dangers of Radioactivity
• There is a difference in penetration and
ionization damage (but they are opposing).
• The more penetrating the radiation is, the less
damaging it is.
• Alpha particles may be stopped by paper or
our skin.
• Beta particles are stopped by ¼” of plexiglass.
• Gamma rays are stopped by several inches of
lead or several feet of steel or concrete.
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Dangers of Radioactivity
• So any type of radioactivity can be dangerous.
• We also talk about the intensity of emitted
radiation.
• The intensity of emitted radiation is related to:
– the amount of radioactive material in a sample;
– the half-life of the radioactive isotope
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Dangers of Radioactivity
• How can you protect yourself when working
with radioactive isotopes?
• Proper clothing
• Proper lead shielding
• Distance!
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Sources of Radioactivity
• We are exposed to a variety of radioactivity in
many ways.
• Do you think that your exposure is from
synthetic sources (like dental X-rays, etc.) or
from natural sources?
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