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Definition: neptunium from Collins English Dictionary

n

1 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)

[C20: from Neptune2, the planet beyond Uranus, because neptunium is the element beyond uranium in the periodic table]


Summary Article: neptunium
From The Columbia Encyclopedia

(nĕptō'nēӘm), radioactive chemical element; symbol Np; at. no. 93; mass number of most stable isotope 237; m.p. about 640 degrees Celsius; b.p. 3,902 degrees Celsius (estimated); sp. gr. 20.25 at 20 degrees Celsius; 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.

The Columbia Encyclopedia, © Columbia University Press 2018

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