Neptunium

Neptunium is a silvery, radioactive actinide metal and the first transuranium element, with one more proton than uranium. Its configuration is 5f4 6d1 7s2, and with seven valence electrons available it shows every oxidation state from +3 to +7. In solution these states have distinct colours, from purple Np3+ to green NpO2^{+}, and several can coexist, which makes neptunium chemistry complicated. Metallic neptunium is dense (about 20.4 g/cm3) and reactive, tarnishing in air.

Neptunium-237, with a half-life of 2.1 million years, is the longest-lived isotope and builds up in spent nuclear fuel, where it is one of the more troublesome long-term wastes because it is mobile in groundwater. It is also useful: irradiating neptunium-237 with neutrons produces plutonium-238, the heat source for spacecraft power supplies. Trace amounts of neptunium occur naturally in uranium ores, formed when uranium atoms capture stray neutrons.

Discovery

Edwin McMillan and Philip Abelson made neptunium in 1940 at the Berkeley Radiation Laboratory by bombarding uranium with neutrons. Uranium-238 captured a neutron to form uranium-239, which beta-decayed into a new element with a 2.3-day half-life. Because uranium was named after Uranus, they named element 93 after Neptune, the next planet outward. McMillan shared the 1951 Nobel Prize in Chemistry with Glenn Seaborg for work on transuranium elements.