Discovery and meaning
In 1903 Ernest Rutherford and Frederick Soddy at McGill University showed that radioactive elements transform into other elements (decay) and that their activity falls at a fixed rate; by around 1907 Rutherford was calling this period the half-life. No one can predict when a single atom will decay, but across huge numbers of atoms the amount drops by exactly half in each interval. After two half-lives a quarter remains, after three an eighth and after ten about a thousandth. Iodine-131, with an 8-day half-life, falls to one-eighth after 24 days.
Half-lives of common isotopes
Half-lives range from fractions of a second to longer than the age of the universe. Short ones suit medical tests where nothing should linger in the body; long ones serve as clocks for deep time.
| Isotope | Half-life | Main use |
|---|---|---|
| Technetium-99m | About 6 hours | Nuclear medicine imaging |
| Iodine-131 | About 8 days | Thyroid treatment |
| Cesium-137 | About 30 years | Marker of reactor-accident fallout |
| Carbon-14 | About 5,730 years | Radiocarbon dating |
| Uranium-238 | About 4.5 billion years | Dating the age of Earth |
Cesium-137 became the key measure of soil and food contamination after the 2011 Fukushima accident. A longer half-life means weaker radiation that lingers; a shorter one means stronger radiation that fades fast.
Dating and drug half-life
In 1949 Willard Libby developed radiocarbon dating, based on carbon-14 that stays constant in living things and decays with a 5,730-year half-life after death, winning the 1960 Nobel Prize in Chemistry. It works for material up to about 50,000 years old; older rocks are dated with long-lived isotopes such as uranium and potassium. In 1956 Clair Patterson used lead isotopes in meteorites to put Earth's age at about 4.55 billion years. In medicine, a drug's half-life is the time for its blood level to halve; it guides dosing intervals, and a drug is usually considered cleared after four to five half-lives.
FAQ
- Is a radioactive substance gone after two half-lives?
- No. It halves each time, so a quarter remains after two. In theory it never reaches zero; after ten half-lives (about 1/1000) it is usually considered greatly reduced.
- Does heating a substance shorten its half-life?
- Radioactive decay happens inside the nucleus, so temperature, pressure and chemistry have essentially no effect. That stability is what makes it a reliable dating clock.
Sources reviewed: 2026-10-26
References
- Rutherford, E., & Soddy, F. (1903). Radioactive change. Philosophical Magazine, 5(29), 576–591.
- Libby, W. F. (1955). Radiocarbon Dating (2nd ed.). University of Chicago Press.
- Patterson, C. (1956). Age of meteorites and the earth. Geochimica et Cosmochimica Acta, 10(4), 230–237.