[ 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.

Nearby words

  1. nepos, cornelius,
  2. nepotic,
  3. nepotism,
  4. neptune,
  5. neptunian,
  6. neptunium series,
  7. ner tamid,
  8. neral,
  9. nerc,
  10. nerd

Origin of neptunium

First recorded in 1940–45; Neptune + -ium Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019

British Dictionary definitions for neptunium


/ (nɛpˈtjuːnɪəm) /


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, 237 Np: 2.14 × 10 6 years; valency: 3, 4, 5, or 6; relative density: 20.25; melting pt: 639±1°C; boiling pt: 3902°C (est)

Word Origin for neptunium

C20: from Neptune ², 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 Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012

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

Medicine definitions for neptunium


[ nĕp-tōōnē-əm ]

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 Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.

Science definitions for neptunium


[ nĕp-tōōnē-əm ]


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 © 2011. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.