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[nep-too-nee-uh m, -tyoo-] /nɛpˈtu ni əm, -ˈtyu-/
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 of neptunium
1940-45; Neptune + -ium Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2015.
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British Dictionary definitions for neptunium


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)
Word Origin
C20: from Neptune², the planet beyond Uranus, because neptunium is the element beyond uranium in the periodic table
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition
© William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
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Word Origin and History for neptunium

1941, from Neptune + element ending -ium. Named for its relative position in the periodic table, next after Uranium, as the planet Neptune is one beyond Uranus. Cf. also plutonium.

Online Etymology Dictionary, © 2010 Douglas Harper
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neptunium in Medicine

neptunium nep·tu·ni·um (něp-tōō'nē-əm, -tyōō'-)
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
Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.
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neptunium in Science
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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