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NUCLEAR CHEMISTRY
Introduction
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The chemistry of the atom is
determined by the number and
arrangement of the electrons.
The properties of the nucleus do not
strongly affect the chemical behavior of
an atom.
Importance
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Security:
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Smoke detectors
Determining explosives in airline luggage
Monitoring nuclear technology in other
countries
Generating power
Medical applications – nuclear medicine
Dating artifacts
Review
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Atomic number (Z) – number of protons
Mass number (A) – sum of the protons and
the neutrons
Isotopes or Nuclides– atoms with the same
atomic number but different mass numbers,
different numbers of neutrons.
Nucleons – the particles that make up the
nucleus.
Facts about the nucleus
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Very small
Very dense: large density
Held together by the nuclear strong
force
Location of the protons and neutrons
Most of the mass of an atom is located
Mass Defect
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You might expect the mass of an atom
to be the same as the sum of it’s parts,
protons, neutrons, and electrons.
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Protons
Neutrons
Electrons
1.007276 amu
1.008665 amu
0.0005486 amu
The difference between the calculated
mass and the actual mass is known as
mass defect.
What causes the lost mass?
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According to Albert Einstein, mass and
energy can be converted into each
other.
Some of the mass is lost during the
formation of the nucleus.
The amount of energy can be caluclated
using Einstein’s famous equation.
Nuclear Binding Energy
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The energy released when a nucleus is
formed from nucleons.
E = mc2
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E is for energy unit: Joules (J)=kg.m2/s2
M is for mass
unit: kilograms (kg)
C is the speed of light (squared)
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3.00 x 108 m/s
Binding Energy per Nucleon
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The binding energy per nucleon is used
to compare the stability of different
nuclides.
It is the binding energy of the nucleus
divided by the number of nucleons that
are in the nucleus.
Binding Energy Con’t
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The higher the binding energy per
nucleon, the more tightly packed the
nucleons are held together, the more
stable the nuclide.
Elements with intermediate atomic
masses have the greatest binding
energies per nucleon and are therefore
the most stable. Iron is the most stable
isotope.
How does the nucleus stay
together?
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Relationship between the nuclear strong
force and the electrostatic forces
between protons.
Like charges repel each other through
electrostatic repulsion
The nuclear strong force allows protons
to attract each other at very short
distances.
Why do atoms want more
neutrons than protons?
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As protons increase in the nucleus so
does the electrostatic forces, faster than
nuclear forces.
More neutrons are required to increase
the nuclear force and stabilize the
nucleus.
> 83 the repulsive forces of protons is
so great that no stable nuclides exist.
Magic Numbers
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Stable nuclides tend to have even
numbers of nucleons.
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256 stable nuclides
159 have both even protons and neutrons
Only 4 have odd numbers of protons and
neutrons.
Nuclear Shell Model
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Nucleons exist in different energy
levels, or shells, in the nucleus.
The number of nucleons that represent
completed nuclear energy levels,
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2, 8, 20, 28, 50, 82, and 126
Called magic numbers
Nuclear Reactions
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Unstable nuclei undergo spontaneous
changes that change the number of
protons and/or neutrons.
Give off large amount of energy by
emitting radiation during the process of
radioactive decay.
Eventually unstable radioisotopes of one
element are transformed into stable,
non-radioactive, isotopes of a different
element.
Nuclear Reactions
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Total of mass number and atomic
number must be equal on both sides of
a reaction.
When the atomic number changes, the
identity of the element changes.
A transmutation is a change in the
identity of a nucleus as a result of a
change in the number of protons.
Types of Radiation
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Alpha Radiation
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Alpha radiation is a heavy, very short-range
particle and is actually an ejected helium
nucleus stripped of it’s electrons,
2 protons and 2 neutrons.
Charge +2
Large mass, 4 amu.
Low penetration power
Shielded by paper or clothing.
Alpha Radiation (Con’t)
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Occurs in unstable nuclei that has too
many protons and too many neutrons.
Effect on the nucleus:
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Mass number is reduced by 4 amu
Atomic number is reduced by 2
Beta Radiation
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A fast moving electron
Occurs in an unstable nuclei that has
too many neutrons
Converts a neutron into a proton and a
beta particle
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An electron that doesn’t belong in the
nucleus and therefore gets thrown out.
Beta radiation (Con’t)
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Charge -1
Mass = 1/1840 or 0.0005486 amu
Moderate penetration power (0.4 cm)
Shielded by metal foil
Effect on nucleus:
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Mass number remains the same
Atomic number increases by 1
Positron emission
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A positive electron or anti-electron
Has the same mass as an electron,
1/1840 or 0.0005486 amu
Charge +1
Occurs in unstable nuclei that has too
many protons
Positron emission (Con’t)
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Converts a proton into a neutron
Effect on the nucleus:
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Mass number remains the same
Atomic number decreases by 1
Electron Capture
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Occurs in unstable nuclei that has too
many protons: same as positron
emission
An inner orbiting electron gets captured
by the nucleus and is used to convert a
proton into a neutron.
The effect is the same as for positron
emission
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Mass number remains the same
Atomic number decreases by 1
Gamma radiation
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Is high-energy electromagnetic
radiation
No charge and no mass: no effect on
the nucleus
Penetration power is high and only lead
and several centimeters of concrete can
slow it down
Always accompanies another form of
radiation
Half-Life
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No two radioisotopes decay at the same
rate.
t1/2 is the symbol for half-life
Half-life is the time required for half the
atoms of a radioactive nuclide to decay.
The longer the half-life the more stable
the nuclide.
Half-life Variables
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Variables
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Ao = original amount
A = final amount
T = total time elapsed
t1/2 = half-life
n = number of half-lives
Half-life Equations
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n=T
t1/2
Ao = A * 2n
Half-life Calculations
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To solve half-life problems first write
down all of the data in the problem.
Determine which formula you’re going
to use.
Plug in the values and calculate
Half-life problem
Phosphorus-32 has a half-life of 14.3
days. How many milligrams of
phosphorus-32 remains after 57.2 days
if you start with 4.0 mg of the isotope?
A0 = 4.0 mg
A=?
T = 57.2 days
n = T / t1/2
t1/2 = 14.3 days
A = A0 / 2n
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Problem (Con’t)
n = T / t1/2
n = 57.2 days / 14.3 days
n = 4 half-lives
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A = A0 / 2n
A = 4.0 mg / 24
A = 4.0 mg / 16
A = 0.25 mg
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Half-life graphic
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Picture representation of half-life
½ remain
½ decay
¼ remain
¾ decay
1/8 remain
7/8 decay
Total time problem
The half-life of radon-222 is 3.824 days.
After what time will one-fourth of a
given amount of radon remain?
A = ¼ remain
n = T / t1/2
T=?
t1/2 = 3.824 days
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Total time problem (Con’t)
* We don’t need to know the beginning
amount. Looking at the picture representation
we see that it needs to go through 2 halflives in order to have ¼ remaining.
 n = T / t1/2
 2 = T / 3.824 days
 T = 2 x 3.824 days
 T = 7.648 days
Decay Series
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One nuclear reaction is not always enough to
produce a stable nuclide.
A decay series is a series of radioactive
nuclides produced by successive radioactive
decay until a stable nuclide is reached.
The heaviest nuclide of each decay series is
the parent nuclide and the nuclides produced
by the decay is called the daughter nuclide.
Artificial Transmutations
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Artificial radioactive nuclides are
radioactive nuclides not found naturally
on Earth.
They are made by artificial
transmutations, bombardment of nuclei
with charged and uncharged particles.
Neutrons have no charge and no mass
and can easily penetrate the nucleus of
an atom.
Artificial Transmutations
(Con’t)
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Positively charged alpha particles,
protons, and other ions are repelled by
the nucleus.
A great deal of energy is needed to
bombard nuclei with these particles.
Energy may be supplied by accelerating
these particles in the magnetic or
electric field of a particle accelerator.
Artificial Radioactive Nuclides
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Radioactive isotopes of all the natural
elements have been produced.
Technetium and Promethium are not
natural elements and have been
artificially produced and have filled gaps
in the periodic table.
Transuranium elements are elements
with more than 92 protons in their
nucleus.
All are radioactive and man-made.
Nuclear Radiation
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Nuclear Radiation can transfer energy form
nuclear decay to the electrons of atoms or
molecules and cause ionization.
A roentgen (R) is a unit used to measure
nuclear radiation exposure.
A rem (roentgen equivalent, man) is a unit
used to measure the dose of any type of
ionizing radiation that factors in the effect
that the radiation has on human tissue.
Nuclear Exposure
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Long term exposure to radiation can
cause DNA mutations that result in
cancer and other genetic defects.
Average background radiation exposure
in the U.S. is ~0.1 rem per year.
The maximum permissible dose of
radiation exposure for a person in the
general population is 0.5 rem/year.
Radiation Detection
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Film badges use exposure of film to
measure the approximate radiation
exposure of people working with
radiation.
Geiger-Muller Counters are instruments
that detect radiation by counting
electric pulses carried by gas ionized by
radiation.
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Used to detect beta, x-rays, and gamma
radiation
Radiation Detection
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Radiation can also be detected when it
transfers its energy to substances that
scintillate, or absorb ionizing radiation
and emit visible light.
Scintillation counters are instruments
that convert scintillations light to an
electric signal for detecting radiation.
Nuclear Fission
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A very heavy nucleus splits into morestable nuclei of intermediate mass.
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Releases enormous amounts of energy
Occurs spontaneously or when bombarded
by particles.
Nuclear Chain Reactions
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A chain reaction is a reaction in which the
material that starts the reaction is also one of
the products and can start another reaction.
When there isn’t enough starting material left
or when the neutrons escape without hitting
the nucleus, the reaction stops.
A critical mass is needed to sustain the chain
reaction.
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The minimum amount of nuclide that provides the
number of neutrons needed to sustain a chain
reaction.
Nuclear Reactors
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Use controlled-fission chain reactions to
produce energy and radioactive
nuclides.
Nuclear power plants convert heat
produced by nuclear fission into
electrical energy.
Nuclear power plants
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There are five main components: sheilding,
fuel, control rods, moderator, and coolant.
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Shielding – radiation-absorbing material used to
decrease the emission of radiation, especially
gamma rays, from nuclear reactors.
Control rods – neutron-absorbing rods that help
control the reaction by limiting free neutrons.
Moderator – used to slow down the fast neutrons
produced by fission
Uranium-235 is usually the fuel
Coolant is simply water which can absorb neutrons
to become heavy water, the H2O becomes D2O.
Nuclear Fusion
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Low-mass nuclei combine to form a heavier,
more stable nucleus.
Releases more energy per gram of fuel than
fission.
Occurs at extremely high temperature and
pressure.
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Occurs in our sun and stars that are similar to our
sun.
Researchers are currently studying ways to
contain the reacting plasma that is required for
fusion.