Key ideas1 of 3
Half-lives cover a wide range
Different radioactive isotopes have half-lives ranging from tiny fractions of a second to many billions of years. Half-life is the time for the number of unstable nuclei, or the activity, to fall to half its starting value. A short half-life means rapid decay of a population. It does not mean every nucleus survives for exactly that length of time: the decay of any individual nucleus is random.
