Chemical element of the actinide series (with many transition element properties), chemical symbol U, atomic number 92.
A dense, hard, silvery white metal that tarnishes in air, it is isolated from such ores as pitchblende . Until the discovery of the first transuranium element in 1940, uranium was believed to be the heaviest element. A.-H. Becquerel . All its isotopes are radioactive; several have half-lives long enough to permit determination of the age of the Earth by uranium-thorium-lead dating and uranium-234–uranium-238 dating . Nuclear fission was discovered in 1938 in uranium bombarded with {{link=neutron">neutron s, and the self-sustaining nuclear chain reaction , the atomic bomb , and the generation of nuclear power followed. Uranium has various valence s in compounds, some of which have been used as colours in ceramic glazes, in lightbulb filaments, in photography, and as dyes and mordants.