A substance is radioactive
is the atoms of which it is made are unstable. The atoms may decay
and turn into atoms of another element or different isotopes of the
same element. The probability that a particular atom will decay in
each time period is a fixed constant. This means that in each time
period of a fixed length, the number of undecayed atoms remaining is
reduced by a fixed factor. An exponential decay curve is displayed
below.
In each period of 25 years
the number of undecayed atoms falls to one half of what it was at the
start of the time period. This factor of a half is significant. The
time period for the number of undecayed atoms, or the mass of
undecayed material, or the activity – the number of decays per
second of the material – is called the half life and labelled
To find the half life from
the graph we divided the initial mass by 2 then find the time
corresponding to the new mass.
Notice that the amount of
material decays by a factor of
every
25 years. We may draw up the table
every
25 years. We may draw up the table|
Tine (years)
|
Mass of Undecayed Substance (grams)
|
|
0
|
100
|
|
25
|
50
|
|
50
|
25
|
|
75
|
12.5
|
|
100
|
6.25
|
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