Argon

A full shell

Argon has eighteen electrons and its outer shell, 3s² 3p⁶, is complete: no vacancy for another electron and no loose one to give away. So an argon atom neither takes nor shares. It forms no compounds under ordinary conditions, and the gas consists of single atoms rather than molecules. It is colourless, odourless, tasteless and non-toxic; in a closed space it kills only by displacing the oxygen.

Chemists have coaxed it into one real compound, argon fluorohydride, HArF, made in Helsinki in 2000 by shining ultraviolet light on hydrogen fluoride frozen in solid argon; it falls apart on warming. Argon atoms also let themselves be caged by water molecules or inside fullerenes, but that is confinement, not bonding.

The third gas in the air

Dry air is 0.934 % argon by volume, 1.29 % by mass: far more than carbon dioxide, and the third gas in the atmosphere after nitrogen and oxygen. The air holds some 66 trillion tonnes of it, and every breath takes in about a hundredth of a lungful.

Almost all of it is argon-40, which is not the argon of the universe. In the Sun and the stars the element is mostly argon-36; the Earth's is the daughter of potassium-40, a radioactive isotope with a half-life of 1.25 billion years, decaying in the rocks since the planet formed. The ratio of the two in a mineral tells geologists when it last cooled: potassium–argon dating gives the ages of lavas and of the strata that hold the early human fossils.

Industry takes argon from liquefied air by distillation. Its boiling point, −185.85 °C, lies between those of nitrogen and oxygen, so it comes off the middle of the column, a by-product of the oxygen trade and the cheapest of the noble gases.

Where nothing must react

Argon's use is its uselessness. Welders blanket the arc with it so that molten metal cannot take up oxygen or nitrogen from the air, and this shielding — for aluminium, stainless steel and titanium — is its largest market. Titanium, zirconium and the silicon crystals for microchips are melted and grown under argon. It has filled incandescent bulbs since soon after Irving Langmuir's gas-filled lamp of 1913, because a hot tungsten filament evaporates more slowly in it than in a vacuum; it sits between the panes of double glazing, conducting heat worse than air; it keeps oxygen off opened wine, old documents and, since 2003, the Declaration of Independence in Washington. Blue-green argon-ion lasers weld detached retinas, and liquid argon fills the detectors that hunt for dark matter and neutrinos.

Discovery and name

A bubble Cavendish could not absorb

Henry Cavendish nearly found argon in 1785. Sparking air with oxygen over alkali to use up the nitrogen, he was left with a bubble, not more than a hundred-and-twentieth of the original, that nothing would absorb. A century later Lord Rayleigh found nitrogen taken from the air about half a per cent denser than nitrogen made from ammonia, and in 1892 asked the readers of Nature why. William Ramsay, at University College London, took up the question. He passed the air's nitrogen over hot magnesium until nothing more would combine, Rayleigh repeated Cavendish's sparking, and both were left with a heavier gas that had a spectrum of its own and reacted with nothing at all. They announced it at Oxford on 13 August 1894 and named it argon, from the Greek argos, idle. It embarrassed the periodic table — heavier than potassium — until Ramsay gave it a group of its own and, within four years, filled that group with helium, neon, krypton and xenon. In 1904 they took the Nobel prizes for physics and for chemistry.