Geoscience Reference
In-Depth Information
of radon-220, we could not predict when that atom would decay. It might happen
within the first millisecond of observation, or take hours, days, weeks, or years.
We can speak only of the probability that the atom will decay. But if the number of
atoms observed is large enough, the laws of probability dictate that in one half-life,
exactly one-half that number will decay.
A law of probability also governs the outcome of tossing a coin, with its 50-50
chance of landing on heads. Toss an honest coin only a few times, and you might
get several heads in a row, or several tails, or any combination: the exact outcome
is unpredictable. But as you toss the coin over and over, the percentage of heads
approaches and finally reaches 50 percent to whatever number of significant fig-
ures you have the patience to achieve. Toss the coin 10
23
times, for example, and
the number of heads will be 50 percent to a mind-numbing number of significant
figures. Ten raised to the twenty-third power is approximately the number of mo-
lecules in only a single gram-mole of any chemical element (Avogadro's Number:
6.02 × 10
23
). Even minute traces of a radioactive element contain so many atoms
that they obey the law of radioactive decay with complete fealty.
