Beyond Uranus, the Sea God

Neptunium's name continues one of the periodic table's most elegant sequences. Uranium had been named after Uranus, the planet discovered in 1781; when the next element beyond it was created, McMillan and Abelson named it for Neptune, the next planet beyond Uranus in the solar system, and the following element would be named for Pluto. Neptune the planet is named in turn for the Roman god of the sea, brother of Jupiter and Pluto, ruler of the oceans and wielder of the trident. The naming is astronomical rather than mythological in intent, extending the planetary series into the transuranium elements, but through the planet it seats neptunium among the great gods of Roman religion. It has no ancient tradition of its own, being a creation of 1940.

The planetary-naming sequence stopped with plutonium simply because the supply of outer planets ran out: after Uranus, Neptune and Pluto, no further planet remained to name element 95, so Seaborg's team turned instead to the Americas, breaking a pattern that had held for exactly three elements in a row. Pluto's later 2006 reclassification as a dwarf planet came six decades after plutonium was named, leaving the metal as a permanent reminder of the ninth planet's original status.