neptunium

Neptunium is the chemical element with the symbol Np and atomic number 93. It is the first synthetic transuranium element and a radioactive actinide metal. Neptunium was discovered in 1940 by Edwin M. McMillan and Philip H. Abelson at the Lawrence Berkeley National Laboratory while studying the irradiation of uranium with neutrons. The element was named after the planet Neptune, in keeping with the planetary naming scheme used for other heavy elements such as uranium and plutonium.

Occurrence and production: Neptunium is not found in meaningful quantities in nature; trace amounts arise from uranium and thorium decay chains and certain neutron-catalyzed processes. It is produced primarily in nuclear reactors by neutron capture in uranium-238, via a decay sequence that passes through uranium-239 and neptunium-239 to plutonium-239. The most long-lived isotope is neptunium-237, with a half-life of about 2.14 million years. Shorter-lived isotopes such as neptunium-239 (half-life about 2.3 days) are important in reactor contexts.

Properties: Neptunium is a dense, silvery metal with a density around 20.5 g/cm3 and a melting point near 639 °C. It tarnishes in air, forming surface oxides. It exhibits several oxidation states, commonly +4 and +5 in aqueous solutions, and forms oxometalate species such as neptunyl ions (NpO2+) and related complexes. Neptunium compounds may be studied in various oxidation states from +3 to +7.

Applications and safety: Neptunium has limited commercial uses, primarily in basic actinide chemistry research and in the production of plutonium in nuclear fuel cycles. Neptunium-237 has been used as a target material to produce Pu-238 for space power systems. Handling of neptunium requires licensed facilities because it is highly radioactive and chemically toxic, with radiological hazards primarily from internal exposure if ingested or inhaled.