Atoms and Molecules
Radioactivity and ionising radiation
Radioactivity is concerned with reactions
that take place in the nucleus of atoms.
a (alpha) particles
are helium nuclei comprising two protons and
two neutrons.
They are easily stopped, for example by a sheet of paper.
The mass number is reduced by four and the atomic number by two.
b (beta)
particles are high energy electrons that come from the nucleus.
They are not so easily stopped requiring, for example, a thick sheet
of aluminium.
The atomic number is increased by one and the mass number is unchanged.
g (gamma)
rays are high energy electromagnetic radiation that can penetrate
several metres of lead or concrete.
Gamma rays do not change the atomic number or mass number.
The total mass number must be the same on both
sides of the equation; so too must be the atomic number. The rate
of decay is proportional to the mass of the radioactive isotope
present, i.e. a 1-g sample will produce only half the count rate
of a 2-g sample. The rate of decay is different for every radioactive
isotope.
The half-life (t½) is the time for
the count rate (or the mass of the original isotope to fall to half
its initial value. For carbon-14 (14C), in the diagram
below, the half-life is 5715 years. For a given isotope the half-life
is constant. The half-life is independent of the physical and chemical
state of the isotope.
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This page was last updated by
Martin
Chaplin
on
10 February, 2005
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