Download Chapter 9 Natural Radioactivity

Chapter 9
The Nucleus, Radioactivity and
Nuclear Medicine
26 November 2013
Natural Radioactivity
• Radioactivity – process by which atoms
emit energetic particles or rays
• Radiation – the particles or rays emitted
– comes from the nucleus
• Nuclear symbols – what we use to designate
the nucleus
– Atomic symbol
– Atomic number
– Mass number
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Nuclear Symbols
Recall the early chapters
mass number
number of
protons and
neutrons
11
5
B
atomic symbol
atomic number
number of protons
Writing Nuclear Symbols
11
5
B
• This defines an isotope of boron
• In nuclear chemistry often called a
nuclide
• This is not the only isotope of boron
– boron-10 also exists
– How many protons and neutrons does
boron-10 have?
• 5 protons, 5 neutrons
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Three Isotopes of Carbon
• Each nucleus contains the same number of protons
• Only the number of neutrons is different
• With different numbers of neutrons the mass of
each isotope is different
Unstable Isotopes
• Some isotopes are stable
• The unstable isotopes are the ones that produce
radioactivity
• To write nuclear equations we need to be able to
write the symbols for the isotopes and the
following:
– alpha particle
– beta particles
– positrons
– gamma rays
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Alpha Particles
• Alpha particle () – 2 protons, 2 neutrons
• Same as He nucleus (He2+)
• Slow moving, and stopped by small
barriers
• Symbolized in the following ways:
4
2
He2
4
2
He α
4
2
α
Beta Particles and Positrons
• Beta particles () – fast-moving electron
• Emitted from the nucleus as a neutron is
converted to a proton
• Higher speed particles, more penetrating
than alpha particles
• The symbol is…
0
1
e
0
-1
β
β
4
Gamma Rays
• Gamma Rays () – pure energy
(electromagnetic radiation)
• Highly energetic
• The most penetrating form of radiation
• Symbol is simply…

Half-Life
• Half-life (t1/2) – the time required for
one-half of a given quantity of a
substance to undergo change
• Each radioactive isotope has its own
half-life
– Ranges from a fraction of a second to a
billion years
– The shorter the half-life, the more unstable
the isotope
5
Half-Lives of Selected
Radioisotopes
Properties of Alpha, Beta, and
Gamma Radiation
• Ionizing radiation – produces a trail of ions
throughout the material that it penetrates
• The penetrating power of the radiation
determines the ionizing damage that can
be caused
• Alpha particle < beta particle < gamma rays
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