neptunium

neptunium (Np), radioactive chemical element of the actinoid series of the periodic table that was the first transuranium element to be artificially produced, atomic number 93. Though traces of neptunium have subsequently been found in nature, where it is not primeval but produced by neutron-induced transmutation reactions in uranium ores, American physicist Edwin M. McMillan and chemist Philip H. Abelson first found neptunium in 1940 after uranium had been bombarded by neutrons from the cyclotron at Berkeley, California. The element was named after the planet Neptune, which is the first planet beyond Uranus.

Neptunium has been produced in weighable amounts in nuclear reactors. In breeder reactors it is a by-product of plutonium production from uranium-238 (about one part neptunium is produced for every 1,000 parts plutonium). All neptunium isotopes are radioactive; the stablest is neptunium-237, with a half-life of 2,144,000 years, and among the most unstable is neptunium-225, with a half-life of more than 2 microseconds. Neptunium-237 can be separated from used reactor fuel to study the physical and chemical properties of the element.

Neptunium, a silvery metal, exists in three crystalline modifications; the room-temperature form (alpha) is orthorhombic. Neptunium is chemically reactive and is more similar to plutonium than to uranium, with oxidation states from +3 to +7. Neptunium ions in aqueous solution possess characteristic colours: Np³⁺, pale purple; Np⁴⁺, pale yellow-green; NpO₂⁺, green-blue; NpO₂²⁺, varying from colourless to pink or yellow-green, depending on the anion present; and Np⁷⁺, dark green. Compounds of neptunium have been prepared in all oxidation states +3 to +7; they are generally similar to compounds of uranium and plutonium with the same oxidation state.