Radioactive Decay
The spontaneous transformation of unstable atomic nuclei into other nuclei, emitting alpha, beta, or gamma radiation with characteristic half-lives.
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Radioactive decay is the spontaneous process by which an unstable atomic nucleus transforms into a different nucleus, releasing energy as radiation. It was discovered by Henri Becquerel in 1896 and characterized by Marie Curie and Ernest Rutherford. Decay is a quantum process: it is unpredictable for any single nucleus, but a large population decays with a precise statistical rate described by the half-life — the time after which half of the original nuclei remain. Half-lives range from fractions of a second to billions of years.
Three common decay modes exist. Alpha decay emits a helium-4 nucleus (2 protons, 2 neutrons), which is strongly ionizing but easily stopped; it typically occurs in heavy nuclei such as uranium and radium. Beta decay converts a neutron into a proton (or vice versa) while emitting an electron or positron together with a neutrino, shifting the element one place in the periodic table. Gamma decay emits a high-energy photon when a nucleus drops from an excited state, usually accompanying alpha or beta decay; gamma rays penetrate deeply and require dense shielding.
The mathematics is exponential: N(t) = N₀(½)^(t/T), where T is the half-life. The activity — decays per second — is measured in becquerels (1 Bq = 1 decay/s). Decay chains, such as uranium-238's series of fourteen steps ending in lead-206, trace how heavy elements transmute. In stars, nucleosynthesis builds elements through nuclear reactions, and radioactive decay shapes which isotopes survive on Earth.
Applications depend on predictable decay. Radiocarbon dating uses the 5730-year half-life of carbon-14 to date organic material; potassium-argon and uranium-lead dating cover much longer spans. Nuclear medicine uses short-lived isotopes for imaging and therapy, and nuclear reactors and weapons release energy from induced fission, while radioactive decay provides a steady power source for spacecraft. The same process is a radiation hazard: ionizing radiation damages DNA, which is why exposure is regulated and shielding matters.
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half-life isotopes nuclear physics radiation