Etymology
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neptunium (n.)

transuranic element, 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. See also plutonium.

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Definitions of neptunium

neptunium (n.)
a radioactive transuranic metallic element; found in trace amounts in uranium ores; a by-product of the production of plutonium;
Synonyms: Np / atomic number "
From wordnet.princeton.edu