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Radioactivity
• Spontaneous Process
• Occurs when unstable nuclei
spontaneously decompose to
more stable forms
• Process emits particles and
EM radiation
APCH 211
SUPPLEMENTAL MATERIAL
Radioactive Decay Via Loss of an Alpha
Particle
Radioactive Decay Via Loss of Beta
Particles
• Alpha decay is the loss of an α-particle (a
helium nucleus).
• Beta decay is the loss of a β-particle (a high
energy electron).
4
2
He
238
U
92
→
0
−1
234
U + 42He
90
131
I
53
β
→
or
0
e
−1
131
Xe + −10e
54
Stable Nuclei
Radioactive Decay Via Gamma
Emission
• This is the loss of a γ-ray, which is highenergy radiation that almost always
accompanies the loss of a nuclear particle.
0
0
γ
▫ The shaded region in the figure, the
so-called belt of stability, shows
which nuclides would be stable.
• Nuclei above this belt have too many
neutrons. They tend to decay by
emitting beta particles.
•
Nuclei below the belt have too many
protons. They tend to become more
stable by positron emission or electron
capture.
• There are no stable nuclei with an
atomic number greater than 83.
• Nuclei with such large atomic numbers
tend to decay by alpha emission.
Figure
19.1
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Radioactive Series
• Large radioactive
nuclei cannot stabilise
by undergoing only
one nuclear
transformation.
• They undergo a series
of decays until they
form a stable nuclide
(often a nuclide of
lead).
Energy in Nuclear Reactions
• There is a tremendous amount of energy stored in
nuclei.
• Einstein’s
Einstein s famous equation
equation, E = mc2, relates directly
to the calculation of this energy.
Measuring Radioactivity
• One can use a device like this Geiger counter to
measure the amount of activity present in a
radioactive sample.
• The ionising radiation creates ions, which
conduct a current that is detected by the
instrument.
Energy in Nuclear Reactions
ƒ For example, the mass change for the decay
of 1 mol of uranium-238 is −0.0046g.
ƒ The change in energy, ΔE, is then
• In the types of chemical reactions we have
encountered previously, the amount of mass
converted to energy has been minimal.
ΔE = (Δm) c2
ΔE = (−4.6 × 10−6 kg)(3.00 × 108 m/s)2
ΔE = −4.1 × 1011 J
• However, these energies are many thousands of
times greater in nuclear reactions.
Nuclear Fission
• How does one tap all that
energy?
• Nuclear fission is the type of
reaction carried out in
nuclear reactors.
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.
reaction
•
•
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.
Effects of Ionizing
Radiation
• Dependent on dose, exposure,
age and health
• High Dose can kill within
weeks (industrial accident,
nuclear bomb, etc)
• Can cause cancers
• Can cause sterility
• Can affect Offspring
• BUT – Used in Medicine!
• 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.
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
i l must b
be iin the
h
plasma state at several
million Kelvin.
• Tokamak apparatus, like the
one shown at the right,
demonstrate promise for
carrying out these reactions.
They use magnetic fields to heat
the material.
Some references
• http://www.iaea.org/Publications/Booklets/Rad
PeopleEnv/index.html
• Chapter 25 “General
General Chemistry
Chemistry…Petrucci
Petrucci et al
• Wikipedia
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