Half-life
The time it takes for half of the atoms of a radioactive isotope in a sample to decay.
Radioactive atoms decay one at a time and at random, so nobody can predict when a single atom will change. But for huge numbers of atoms the behaviour is extremely regular. The half-life is the time needed for half of them to decay. After one half-life, half remain; after two, a quarter; after three, an eighth; and so on.
Half-lives range from tiny fractions of a second to far longer than the age of the Universe, and each radioactive isotope has its own. Carbon-14 has a half-life of about 5,730 years. Aluminium-26 has one of about 0.72 million years. Uranium-238 has one of about 4.5 billion years. Choosing an isotope with a suitable half-life is the key to dating: you want one that has decayed enough to measure, but not so much that almost none is left.
For example, the short half-life of carbon-14 makes it useful for working out how long a meteorite such as a desert or Antarctic find has been sitting on Earth, but useless for its 4.5-billion-year formation age, for which uranium and lead are used. A handy rule is that after about ten half-lives, less than a thousandth of the original isotope is left.
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Level: Explorer Ages 10 and up