Plutonium

google

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1940s: from Greek‘Pluto’, on the pattern of neptunium, being the next planet beyond Neptune.

wiktionary

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After Pluto (the entity formerly considered to be a planet) +‎ -ium.

etymonline

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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.