noun. 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. cross-reference: Periodic Table.

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

noun. 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.

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

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

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

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