Neon (Ne)

Neon is somewhere around the fifth most abundant element in the universe, after hydrogen, helium, oxygen and carbon. It is also, for practical purposes, absent from this planet. The atmosphere holds about eighteen parts per million of it and the rocks hold essentially none. Few elements show such a violent disagreement between their cosmic and terrestrial abundances, and the reason for the gap explains almost everything about how neon is obtained and what it costs.

The most depleted element on Earth

Planets are assembled by chemistry. Oxygen, silicon, magnesium and iron are abundant on Earth because they form solid compounds that condensed out of the protoplanetary disc and stayed. Neon forms no compounds at all, so there was nothing to condense: it remained a gas, and the young Earth had no way to hold onto it.

The scale of the loss is extraordinary. Measured against solar composition, Earth is depleted in neon by something on the order of ten orders of magnitude — a far larger discrepancy than for any reactive element. What little neon the atmosphere has is mostly leakage from the interior over billions of years.

Three elements in six weeks

By 1898 William Ramsay had helium and argon in hand and a conspicuous hole in his new column of the periodic table. Working with Morris Travers at University College London, he set about distilling liquefied air and examining every fraction spectroscopically.

Krypton came out of the least volatile residue in late May. Neon followed in June, from the fraction that boiled off first. Xenon arrived in July. Three elements in about six weeks, from one apparatus, by systematic search rather than accident.

Travers left an account of holding the discharge tube up and seeing a blaze of crimson light that he said he would never forget, and which told him at once that this was something new. The card above credits Travers alone, which is less common than the joint attribution; he was the junior partner and did much of the cryogenic work, and the division of credit between the two men has never been entirely settled.

The name came from Ramsay's thirteen-year-old son Willie, who suggested calling it novum for its newness. His father kept the idea and gave it a Greek form.

Only one colour, and the signs are lying

Neon's discharge spectrum is dominated by a cluster of strong emission lines in the orange-red, between roughly 585 and 700 nanometres. That is fixed by the structure of the atom. A tube of pure neon glows orange-red and can glow no other colour, ever.

Which means that almost every "neon" sign is not neon. Blues, greens, whites and pinks come from argon with a trace of mercury, whose ultraviolet output excites a phosphor coating on the inside of the glass, and the phosphor decides the colour. Coloured glass tubing supplies further variations. Only the classic orange-red is the element the industry is named after.

Georges Claude demonstrated the first neon lighting at the Paris Motor Show in December 1910, having developed it as an outlet for the neon his air-liquefaction business produced as a by-product. He franchised the technology internationally and made a fortune, and the signs defined the look of twentieth-century urban night. His later career is generally omitted from the story: Claude collaborated with the Vichy regime, was convicted after the liberation of France and imprisoned, and was expelled from the Académie des Sciences.

A supply chain nobody had looked at

Neon cannot be mined, synthesised or recovered from a mineral. The only source is the atmosphere, and at eighteen parts per million a plant must process an enormous volume of air to obtain a small volume of gas. In practice it arrives as a by-product wherever an air separation plant is already enormous for other reasons — the ones feeding a steelworks its oxygen and its nitrogen — with crude neon collecting in a side stream that is purified somewhere else entirely.

That structure concentrated the industry in an unexpected place. Soviet-era steel plants in Ukraine were fitted with air separation capacity of the required scale, and two companies — Ingas in Mariupol and Cryoin in Odesa — purified the output to semiconductor grade. Estimates of Ukraine's share of the world's high-purity neon in early 2022 ranged from around half to well over two thirds, with the United States importing nearly all of its supply from there.

The reason the semiconductor industry cared is specific. Deep-ultraviolet photolithography uses excimer lasers — krypton fluoride at 248 nanometres, argon fluoride at 193 — and neon is the buffer gas that makes up the great bulk of the laser's gas mixture. Both Ukrainian producers halted after the Russian invasion in February 2022. Prices rose by multiples, chipmakers drew down stockpiles, and the industry moved quickly to qualify Chinese supply and to install on-tool recycling. Nothing about neon had looked like a strategic vulnerability until it was one.

Nothing has ever taken hold of a neon atom

Neon has less chemistry than any other element, and it is worth being exact about what that means, because "most inert" is decided by two different measures that give two different winners.

Judged by whether anything will bond to it, neon is the extreme case. There is no neon fluoride, no neon salt, no neon compound stable at any temperature or pressure that has been reached. The nearest things to exceptions are a few adducts caught in solid neon at around 4 kelvin — NeBeS is the best characterised of them — held together by a couple of kilojoules per mole, which is a species leaning on a neighbour rather than joined to it. Helium, for all its reputation, does have entries in the catalogue: HeH⁺ is a stable molecular ion and has been identified in a planetary nebula, and Na₂He has been made in a diamond anvil cell above a hundred gigapascals.

Judged instead by how hard the atom is to disturb at all, helium wins and neon is second. Neon's first ionisation energy is the highest of any element bar helium's. Its static dipole polarisability — how far the electron cloud will deform in an applied field — is 2.66 in atomic units against helium's 1.38, so helium, not neon, is the most rigid atom there is. Neon is nonetheless rigid enough that it loses even the weak attraction which lets the heavier noble gases be caught in cages and clathrates: it will not bond, and it barely sticks.

The element on which isotopes were discovered

Stable isotopes were not deduced; they were seen, and they were seen in neon.

J.J. Thomson, working with positive ray analysis at the Cavendish in 1913, passed neon ions through electric and magnetic fields and photographed the parabolic traces they left. There were two, not one, corresponding to masses 20 and 22. Radioactive elements were already known to come in variants of different mass, but neon is not radioactive, and the result meant that an ordinary stable element could be a mixture. Francis Aston confirmed and extended it with the mass spectrograph he built after the war, and went on to survey the whole table.

Neon's three stable isotopes have since become one of the sharpest tools in geochemistry. Volcanic gases from Iceland and from the Hawaiian seamount Loihi carry neon with a ratio of neon-20 to neon-22 significantly higher than the atmosphere's, and closer to the composition of the solar wind. The interpretation is that a reservoir deep in the mantle still holds gas trapped during Earth's accretion which has never mixed with the atmosphere — direct evidence that the planet's interior is not thoroughly stirred, from an element too inert to have been altered by anything in between.

Neon-22 also has a role in stellar nucleosynthesis: in evolved stars it reacts with helium to release free neutrons, one of the principal neutron sources driving the slow capture process that builds elements heavier than iron.

Cold, and not flammable

Liquid neon is a specialist cryogen for the range between roughly 25 and 40 kelvin. Per unit volume it carries a refrigerating capacity more than forty times that of liquid helium and around three times that of liquid hydrogen.

Hydrogen would otherwise be the obvious coolant in that band and is not always acceptable, because it burns. Neon does the same job with no flammability whatsoever, at considerably greater cost — a trade that makes sense for superconducting equipment, cryopumps and certain detector systems and almost nowhere else.