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.
late 14c., "Roman god of the sea," from Latin Neptunus, the Roman god of the sea (son of Saturn, brother of Jupiter, later identified with Greek Poseidon), probably from PIE root *nebh- "cloud" (source of Latin nebula "fog, mist, cloud"), via a sense of "moist, wet."
The planet so named was discovered by German astronomer Johann Gottfried Galle (1812-1910) on the night of Sept. 23-24, 1846 and named by French astronomer Urbain Le Verrier (1811-1877), who had predicted its position based on anomalies in the motion of Uranus and sent the coordinates to Galle. It is too dim to be seen with the naked eye, but it had been seen by observers using telescopes as far back as Galileo, but they did not recognize and identify it as a planet. Until the identification of Pluto in 1930 (and since that planet's demotion), it was the most distant known planet of the solar system.
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).