Uranium

Uranium is a dense (19.1 g/cm3), silvery actinide metal with the configuration 5f3 6d1 7s2. Its six outer electrons can be involved in bonding, so it shows oxidation states from +3 to +6. The +6 state is most stable in air and water, usually as the linear uranyl ion UO2^{2+}, which gives uranium minerals and salts their yellow-green colours and makes uranium glass glow green under ultraviolet light. Finely divided uranium metal is pyrophoric, and bulk metal slowly tarnishes in air.

Natural uranium is 99.3% uranium-238 and 0.7% uranium-235. Both are radioactive with very long half-lives (4.5 billion and 704 million years), which is why uranium is still present from Earth's formation. Uranium-235 matters because it can split, or fission, when it absorbs a slow neutron, releasing about 200 MeV and two or three new neutrons that can sustain a chain reaction. Reactor fuel is enriched to 3–5% U-235 by spinning uranium hexafluoride gas in centrifuges.

Discovery

Martin Heinrich Klaproth identified a new element in pitchblende in 1789 and named it after the planet Uranus, discovered eight years earlier. What he isolated was actually an oxide; Eugène-Melchior Péligot produced the metal in 1841. In 1896 Henri Becquerel found that uranium salts fogged photographic plates in the dark, the discovery of radioactivity. In 1938 Otto Hahn and Fritz Strassmann showed that uranium splits under neutron bombardment.