Chemistry Reference
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14
HALF-LIFE
The concept of
half-life
applies to processes involving changes in which the
rate of change of some quantity in a system depends on this quantity. The
half-life is the time required, under specific conditions, for a quantity
to decrease by one-half. After one half-life has elapsed, only 50% of the
original quantity remains. During the second half-life, one-half of the
remaining quantity vanishes, leaving one-quarter (25%) of the original.
One-eighth (12.5%) is left after the third half-life, and so on. The decrease
in the quantity follows a geometric regression, being continuously reduced
by one-half, as illustrated in Figure 10. The half-life of specific materials is
of use for dating the materials.
FIGURE 10
The half-life.
It is impossible to predict when a radioisotope or an
unstable substance (molecule) will decay or be decomposed. On an average,
however, only half of any type of radioisotope or unstable substance (molecule)
remains after one half-life (A/2); one-quarter will remain after two half-lives (A/4),
one-eighth after three half-lives (A/8), and so on. The half-life is characteristic of
every radioisotope and unstable molecule; that of radioisotopes is not affected in
any way by the physical or chemical conditions to which the radioisotope
may be subjected. Not so the half-life of chemically unstable molecules, which is
altered by changes in temperature and by other physical and chemical conditions.
