neptunium

[nep-too-nee-uhm, -tyoo-]
noun Chemistry, Physics.
a transuranic element produced in nuclear reactors by the neutron bombardment of U-238: decays rapidly to plutonium and then to U-235. Symbol: Np; atomic number: 93.

Origin:
1940–45; Neptune + -ium

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World English Dictionary
neptunium (nɛpˈtjuːnɪəm)
 
n
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]

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American Heritage
Medical Dictionary

neptunium nep·tu·ni·um (něp-tōō'nē-əm, -tyōō'-)
n.
Symbol Np
A metallic radioactive element found in trace quantities in uranium ores or synthesized; its longest-lived isotope is Np 237 with a half-life of 2.1 million years. Atomic number 93.

The American Heritage® Stedman's Medical Dictionary
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American Heritage
Science Dictionary
neptunium   (něp-t'nē-əm)  Pronunciation Key 
Symbol Np
A silvery, radioactive metallic element of the actinide series. It occurs naturally in minute amounts in uranium ores and is produced artificially as a byproduct of plutonium production. Its longest-lived isotope is Np 237 with a half-life of 2.1 million years. Atomic number 93. See Periodic Table.
The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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Encyclopedia Britannica
Encyclopedia

neptunium

radioactive chemical element of the actinoid series of the periodic table, 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, Edwin M. McMillan and Philip H. Abelson first found neptunium in 1940 after uranium had been bombarded by neutrons from the cyclotron at Berkeley, Calif. Neptunium has been produced in weighable amounts in breeder reactors as 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 2,140,000 years, and among the most unstable is neptunium-232, with a half-life of 13 minutes.

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Encyclopedia Britannica, 2008. Encyclopedia Britannica Online.
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Example sentences
Neptunium oxide was prepared by precipitating neptunium oxalate and calcining.
The system has also been applied to the characterization of neptunium standard solutions with a comparable reliability.
It is formed in much the same manner as neptunium: by irradiation of natural uranium with the neutrons which are present.
His discovery of neptunium is an example of this style.
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