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neptunium (nĕpto͞oˈnēəm), radioactive chemical element; symbol Np; at. no. 93; mass number of most stable isotope 237; m.p. about 640℃; b.p. 3,902℃ (estimated); sp. gr. 20.25 at 20℃; valence +3, +4, +5, or +6. Neptunium is a ductile, silvery radioactive metal. It is a member of the actinide series in Group 3 of the periodic table.

Neptunium has three distinct forms (see allotropy); the orthorhombic crystalline structure occurs at room temperature. There are 20 known isotopes of neptunium. Neptunium-237, the most stable, has a half-life of 2.14 million years and is used in neutron-detection equipment. Neptunium forms numerous chemical compounds, and is found in very small quantities in nature in association with uranium ores.

The element was discovered in 1940 by Edwin M. McMillan and Philip H. Abelson, who produced neptunium-239 (half-life 2.3 days) by bombarding uranium with neutrons from a cyclotron at the Univ. of California at Berkeley. Neptunium, the first transuranium element, was named for the planet Neptune, which is beyond Uranus in the solar system.

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The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.



Np, an artificially produced radioactive chemical element of the actinide group. Atomic number, 93; atomic weight, 237.0482. It was discovered in 1940 by the American scientists E. M. McMillan and P. H. Abelson, who determined that the uranium isotope 239U, which forms during neutron bombardment of 238U, decays rapidly, emitting a β-particle, and is converted into an isotope of the element with atomic number 93. The element was named after the planet Neptune.

By 1973, 15 neptunium isotopes had been obtained; 237Np has the longest half-life (α-emitter; T½ = 2.14 × 106 yr). The β- radioactive isotope 239 Np (T½ = 2.346 days) is widely used in research. The successive transformations of the isotope 237Np yield the stable isotope 209Bi; this chain of transformations is called the neptunium radioactive series. Negligible quantities of 237Np and 239Np are found in uranium ores, where they are continuously formed as a result of nuclear interactions of uranium atoms and neutrons.

Elementary neptunium is a malleable, relatively soft metal with a silver luster. Density, about 20 g/cm3; melting point, 640°C

The three outer electron shells of a neptunium atom have the configurations 5s25p6, 5d105f4, and 6s26d1 7s2; the 5f-, 6d-, and 7s-electrons take part in the formation of neptunium compounds. The chemical properties of neptunium are very similar to those of uranium and plutonium. The oxidation numbers of neptunium compounds range from +2 to +7. In solution, neptunium produces the ions Np3+, Np4+, NpO2+ (the most stable), NpO22+, and NpO53-; all neptunium ions tend toward hydrolysis and complexing.

Ponderable quantities of isotope 237Np are formed as a byproduct during the production of plutonium in nuclear reactors through nuclear reactions of uranium and neutrons. Neptunium is mainly used in scientific research.


Mikhailov, V. A. Analiticheskaia khimiia neptuniia. Moscow, 1971. (See also References under ACTINIDES.)


The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.


A chemical element, symbol Np, atomic number 93, atomic weight 237.0482; a member of the actinide series of elements.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.


a silvery metallic transuranic element synthesized in the production of plutonium and occurring in trace amounts in uranium ores. Symbol: Np; atomic no.: 93; half-life of most stable isotope, 237Np: 2.14 × 106 years; valency: 3, 4, 5, or 6; relative density: 20.25; melting pt.: 639?1?C; boiling pt.: 3902?C (est.)
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