Advertisement - Continue reading below

Definition of "neptunium" [nep•tu•ni•um]

  • A silvery, metallic, naturally radioactive element, atomic number 93, the first of the transuranium elements. Its longest-lived isotope is Np-237 with a half-life of 2.1 million years. Found in trace quantities in uranium ores, it is produced synthetically by nuclear reactions. See Table at element. (noun)

American Heritage(R) Dictionary of the English Language, Fifth Edition. Copyright (c) 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.

Use "neptunium" in a sentence
  • "In this way there was discovered an entirely new radioactive series which, from its most long-lived member, is now called the neptunium family."
  • "There are no easy intermediate stages, like the neptunium which is a stage between uranium and plutonium."
  • "Scientists have been creating new elements since 1940, when neptunium and plutonium were first forged at the University of California, Berkeley."