Glossary
half-life
A half-life is the time it takes for half of the atoms in a sample to decay; the remaining amount halves again every half-life and never reaches zero.
A half-life is the time it takes for half of a sample to decay. Which atom goes is random; how many go in a half-life is not, which is why a large sample behaves like a clock. The arithmetic is a count, not a formula to memorize: find how many half-lives have passed, then halve the amount that many times. amount remaining = starting amount × (1/2)ⁿ, where n counts whole half-lives.
A worked case. A sample of 80.0 mg has a half-life of 6.0 hours and 18.0 hours pass. 18.0 ÷ 6.0 = 3 half-lives, so the amount is 80.0 × (1/2)³ = 80.0 ÷ 8 = 10.0 mg. Check it forwards: 80.0, 40.0, 20.0, 10.0 mg, one halving per six hours. Half-lives differ across substances: carbon-14 runs about 5,730 years, which is what makes it useful for dating once-living material, and a radiopharmaceutical may run in hours.
Two things the term does not mean. It is not a shelf life — nothing becomes zero at two half-lives, and a quarter of the original is still there. And it describes the nucleus, not the electron shell: a half-life belongs to nuclear change, which is not chemistry.
Taught in
Source: NIST periodic table and reference data for the atomic masses and half-life figures, NIH PubChem for the substance data, and NIST Special Publication 811 for the unit conventions — the same sources the chemistry guide cites. All three are US federal government material, not subject to copyright in the United States. Every definition, example and calculation above was written for this course; no dictionary or textbook text is reproduced. (NIST and PubChem accessed 2026-10-26; definitions written 2026-10-27)