Krypton

Krypton was discovered in 1898 in London by William Ramsay and Morris Travers, who found it as a residue in liquefied air after the more volatile gases had boiled away. Its name comes from the Greek kryptos, hidden, marking how well it had escaped detection. Krypton is a colourless, odourless noble gas present in the atmosphere only in traces, and chemically almost wholly unreactive. Obtained in small amounts from liquefied air, krypton fills some high-performance light bulbs and fluorescent lamps, is used in certain lasers and in energy-efficient double glazing, and forms a few compounds with fluorine only with difficulty.

Krypton's supposed total inertness was revised in 1963, a year after Neil Bartlett's landmark synthesis of a xenon compound broke the noble gases' assumed unreactivity, when researchers produced krypton difluoride, a pale solid that remains one of the few well characterised krypton compounds. Krypton fluoride is also the active medium of an excimer laser widely used in advanced semiconductor photolithography, so an element first noticed only as a faint residue in liquefied air now plays a role in etching the very silicon chips that depend on far more abundant elements.

The name kryptos, the hidden one, suited a gas that made up less than a millionth of the air and had eluded chemists until the liquefaction of air laid it bare. Krypton has no mythology or alchemical past, being a discovery of the 1890s, but it earned a precise scientific dignity: between 1960 and 1983 the metre was officially defined in terms of a specific orange-red wavelength of light emitted by the krypton-86 atom, so that for a generation the world's fundamental unit of length was fixed by this hidden gas. The name is also, coincidentally, familiar far beyond chemistry through the fictional planet Krypton, an association the element itself neither earned nor influenced.

The metre's krypton-86 definition was retired in 1983 in favour of a definition based on the fixed speed of light, closing a twenty-three-year chapter in which the world's basic unit of length rested on this one obscure gas. Krypton's cultural fame owes nothing to that metrology and everything to fiction: when Superman's creators, Jerry Siegel and Joe Shuster, needed a suitably exotic-sounding home planet for their hero in the late 1930s, they settled on Krypton for its scientific ring, unaware that the real element would later hold, however briefly, the definition of the metre itself.