Other isotopes are used for dating the ages of rocks and minerals and identifying pathways that water follows in the ground. Isotopes are also extensively used in medical imaging. Half Life Half-life describes the time it takes for half the numbers of isotopes of an atom to undergo decay. The decay process is statistical so if an atom has a half-life of 20 minutes it means on average that half the number of atoms will have decayed after 20 minutes. Some of those atoms might have decayed after 30 seconds, some after 15 minutes, some at 20 minutes.
Some isotopes have half-lives of a billion years or more, others are a tiny fraction of a second.
Click here for more on half-life. Try a calculation of half-life for some isotopes.