Neptunium

Neptunium was the first transuranium element to be made, synthesised in 1940 at Berkeley by Edwin McMillan and Philip Abelson, who produced it by bombarding uranium with neutrons. It was named after the planet Neptune. Neptunium is a dense, silvery, radioactive metal, the first element heavier than uranium and a bridge into the wholly synthetic reaches of the periodic table; small amounts arise in nuclear reactors. Neptunium is a by-product of the reactions that make plutonium, and although it can in principle sustain a chain reaction, its chief practical interest is as a precursor, since bombarding it with neutrons yields plutonium-238, the isotope that powers the generators of deep-space probes.

McMillan began the experiment but was pulled away to work on wartime radar research before it was finished, leaving the chemist Philip Abelson to complete the delicate work of separating the new element from uranium, a division of labour that meant the physicist who conceived the idea did not personally finish confirming it. Neptunium-237, with a half-life of over two million years, accumulates in spent nuclear fuel and is one of the longer-lived hazards nuclear waste management must plan for, since it persists across timescales that dwarf any human institution.

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.