from The American Heritage® Dictionary of the English Language, 4th Edition

  • n. 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.

from Wiktionary, Creative Commons Attribution/Share-Alike License

  • n. The transuranic chemical element with atomic number 93 and symbol Np.

from the GNU version of the Collaborative International Dictionary of English

  • n. A radioactive metallic element of atomic number 93, produced in nuclear reactors from Plutonium or Uranium. Symbol Np; The atomic weight of the most stable isotope is 237.0482.

from The Century Dictionary and Cyclopedia

  • n. In chem., a supposed new element announced by Hermann in 1877 as present in columbite and ferro-ilmenite. Its existence has not been confirmed.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

  • n. a radioactive transuranic metallic element; found in trace amounts in uranium ores; a by-product of the production of plutonium


from The American Heritage® Dictionary of the English Language, 4th Edition

After the planet Neptune (from the fact that it follows uranium in the periodic table).

from Wiktionary, Creative Commons Attribution/Share-Alike License

After Neptune (the planet).


  • In this way there was discovered an entirely new radioactive series which, from its most long-lived member, is now called the neptunium family.

    Nobel Prize in Chemistry 1951 - Presentation Speech

  • There are no easy intermediate stages, like the neptunium which is a stage between uranium and plutonium.

    The Earth Book of Stormgate

  • Scientists have been creating new elements since 1940, when neptunium and plutonium were first forged at the University of California, Berkeley.

    Party of Two Is Seated at the Periodic Table

  • Between those planets sits Neptune, and the gap between the two elements leaves a space for their relatively unsung cousin, neptunium – element number 93 in the periodic table.

    Archive 2010-01-01

  • And Ireta, a satellite of a suspected third generation sun, ought to be rich in the heavier elements, rich in the neptunium, plutonium and the more esoteric of the rare transuranics and actinites above uranium on the periodic table, so urgently and constantly required by the Federation of Sentient Populations the search for which was one of the primary tasks of the EEC.

    Cattle Town

  • Although all of these goals were demonstrated, the program was aborted before recycling of neptunium and americium was properly evaluated.

    Fast neutron reactors (FBR)

  • General Atomics say that the MHR has a neutron spectrum is such and the TRISO fuel so stable that the reactor can be powered fully with separated transuranic wastes (neptunium, plutonium, americium and curium) from light water reactor used fuel.

    Small nuclear power reactors

  • More recently, there has been interest in transmuting the long-lived transuranic radionuclides (the actinides neptunium, americium and curium particularly) formed by neutron capture in a conventional reactor and reporting with the high-level waste.

    Accelerator-driven nuclear energy

  • Fast reactors are able to unlock energy in waste because they can burn plutonium, neptunium and other materials that Generation II and Generation III reactors leave behind.

    The New Nukes

  • This description is consistent with the claim that it is essential to neptunium that nuclei of neptunium atoms have 93 protons whereas it is essential to plutonium that its nuclei have 94 protons.

    Natural Kinds


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  • Np.

    December 16, 2007