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plutonium (n.)

transuranic metallic element, 1942, from Pluto, the planet, + element ending -ium. Discovered at University of California, Berkeley, in 1941, the element was named on suggestion of Seaborg and Wahl because it follows neptunium in the periodic table as Pluto follows (or followed) Neptune in the Solar System. The name plutonium earlier had been proposed for barium and was used sometimes in this sense early 19c.

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Definitions of plutonium from WordNet

plutonium (n.)
a solid silvery grey radioactive transuranic element whose atoms can be split when bombarded with neutrons; found in minute quantities in uranium ores but is usually synthesized in nuclear reactors; 13 isotopes are known with the most important being plutonium 239;
Synonyms: Pu / atomic number "
From wordnet.princeton.edu