plutonium n.

Roman god of the underworld, early 14c., from Latin Pluto, Pluton, from Greek Ploutōn "god of wealth," from ploutos "wealth, riches," probably originally "overflowing," from PIE root *pleu- "to flow." The alternative Greek name or epithet of Hades in his function as the god of wealth (precious metals and gems, coming from beneath the earth, form part of his realm). The planet (since downgraded) was discovered 1930 by U.S. astronomer Clyde W. Tombaugh; Minerva also was suggested as a name for it. The cartoon dog first appeared in Walt Disney's "Moose Hunt," released April 1931.

word-forming element in chemistry, used to coin element names, from Latin adjectival suffix -ium (neuter of -ius), which formed metal names in Latin (ferrum "iron," aurum "gold," etc.). In late 18c chemists began to pay attention to the naming of their substances with words that indicate their chemical properties. Berzelius in 1811 proposed forming all element names in Modern Latin. As the names of some recently discovered metallic elements already were in Latin form (uranium, chromium, borium, etc.), the pattern of naming metallic elements in -ium or -um was maintained (in cadmium, lithium, plutonium, etc.; helium is an anomaly).

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.