The concept of "half-life" is basic to an understanding of radioactive
decay of unstable nuclei.
Unlike physical "systems"--bacteria, animals, men and stars--unstable
isotopes do not individually have a predictable life span. There is no
way of forecasting when a single unstable nucleus will decay.
Nevertheless, it is possible to get around the random behavior of an
individual nucleus by dealing statistically with large numbers of
nuclei of a particular radioactive isotope. In the case of
thorium-232, for example, radioactive decay proceeds so slowly that 14
billion years must elapse before one-half of an initial quantity
decayed to a more stable configuration. Thus the half-life of this
isotope is 14 billion years. After the elapse of second half-life
(another 14 billion years), only one-fourth of the original quantity of
thorium-232 would remain, one eighth after the third half-life, and so
on.
Most manmade radioactive isotopes have much shorter half-lives, ranging
from seconds or days up to thousands of years. Plutonium-239 (a
manmade isotope) has a half-life of 24,000 years.
For the most common uranium isotope, U-238, the half-life is 4.5
billion years, about the age of the solar system. The much scarcer,
fissionable isotope of uranium, U-235, has a half-life of 700 million
years, indicating that its present abundance is only about 1 percent of
the amount present when the solar system was born.