For a radioactive element, the moment when each individual atom will decay is completely random. But when there are very many atoms together, almost exactly half remain after each fixed interval of time (the half-life). Each atom is unpredictable, yet the whole population becomes predictable.
Rule: Each time one half-life passes, every remaining atom has an independent 50% chance of decaying. With a small number of atoms, the result may not be exactly half, but as the number of atoms increases, it gets very close to "exactly half."
Remaining atoms 300Half-lives elapsed 0 times
Substance
Half-life
Carbon-14
about 5,730 years
Iodine-131
about 8 days
Uranium-238
about 4.5 billion years
Caffeine (in the body)
about 5 hours, depending on the person
💡 Radiocarbon dating, used to determine the age of artifacts, works on this very principle. By measuring the proportion of carbon-14 remaining in an artifact and working backward through the half-life formula, we can determine "how many half-lives have passed."