Neon

A full shell

A neon atom has ten electrons, and the outer eight fill its second shell exactly. That is the whole of its chemistry: with nothing to gain and nothing to give, it bonds neither with itself nor with anything else, and the gas is made of single atoms. Its first ionisation energy, 2081 kJ per mole, is the highest of any element after helium, and no neutral compound of neon has ever been made — fluorine, which combines with xenon and krypton, cannot touch it.

The atoms attract one another so little that neon liquefies only at −246 °C and freezes two and a half degrees lower, the narrowest liquid range of any element. Litre for litre, liquid neon is about forty times as effective a refrigerant as liquid helium.

Common in the stars, rare on Earth

Neon is made inside massive stars, when two carbon nuclei fuse, and scattered when they explode: by mass it is the fifth most abundant element in the universe, after hydrogen, helium, oxygen and carbon. The Earth, small and warm and forming near the Sun, kept almost none of its share; neon forms no minerals to trap it and was too light for a young planet to hold. What remains is a trace in the air, 18 parts per million by volume.

That trace is the only source. Neon comes from the plants that liquefy air for oxygen and nitrogen: the gases that will not condense — neon, helium, hydrogen — are drawn off the top of the column and separated by cooling and adsorption. Roughly eighty tonnes of air yield a kilogram. The gas is correspondingly dear, and its largest use today is not in signs but as the buffer gas of the excimer lasers that print microchips.

Red light

Ramsay and Travers knew they had a new element when a spark through the tube gave a blaze of crimson. Every gas glows its own colour under a discharge, and neon's is the purest red-orange. Georges Claude, a French engineer whose air liquefaction plants produced neon with no market for it, sealed the gas in glass tubes with an electrode at each end and showed two of them, twelve metres long, at the Paris Motor Show in December 1910. The first sign went to a barber's on the boulevard Montmartre in 1912; in 1923 a Packard dealer in Los Angeles bought two and stopped the traffic. Neon lit the twentieth century's streets, and the word came to mean any glowing tube, although the blues and greens were argon, mercury and phosphors.

In 1913 J. J. Thomson, bending a beam of neon ions in electric and magnetic fields, found two spots where he expected one, at masses 20 and 22: the first evidence that a stable element could have isotopes. The helium–neon laser, the first gas laser, built at Bell Laboratories in 1960, later gave laboratories and barcode scanners their red beam, neon's line at 632.8 nm.

Discovery and name

After argon in 1894 and helium in 1895, William Ramsay had two noble gases and room in the periodic table for more. In 1898 at University College London he and Morris Travers took liquid air, newly available, and distilled it in fractions, examining each in a spectroscope. Krypton came first, from the heavy residue, on 30 May; neon in June, from the lightest fraction; xenon in July. Ramsay's thirteen-year-old son proposed novum for the new gas; his father kept the meaning and changed the language — neon, Greek for new. Ramsay received the Nobel Prize in Chemistry in 1904 for the whole family.