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Symbol Np. A radioactive metallic transuranic element belonging to the actinoids; a.n. 93; r.a.m. 237.0482. The most stable isotope, neptunium–237, has a half-life of 2.2 × 106 years and is produced in small quantities as a by-product by nuclear reactors. Other isotopes have mass numbers 229–236 and 238–241. The only other relatively long-lived isotope is neptunium–236 (half-life 5 × 103 years). The element was first produced by Edwin McMillan (1907–91) and Philip Abelson (1913–2004) in 1940.

Subjects: Chemistry — Physics.

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