Krypton (Kr)

Krypton is present in the air you are breathing at about one part per million, which makes it roughly a thousand times scarcer than argon and a hundred times more plentiful than xenon. It is too inert to have any chemistry worth speaking of and too dilute to be cheap, and yet for twenty-three years the length of everything on Earth was defined by the colour of light it emits.

Hidden in the last drop of liquid air

William Ramsay and Morris Travers spent 1898 systematically boiling off liquid air at University College London, hunting for elements that argon's discovery had implied should exist. In late May they evaporated fifteen litres of liquid argon almost to dryness and examined the stubborn residue in a spectroscope, where two bright lines — one yellow, one green — belonged to nothing known.

They named it krypton, from Greek kryptos, hidden, because it had been sitting in every sample of air anyone had ever analysed without leaving a trace. Within weeks the same apparatus gave them neon, the new one, and xenon, the stranger. Three elements in a summer from a single technique is an unusual return, and it is why Ramsay's name attaches to the whole group even though argon and helium arrived by other routes.

The Superman comics borrowed the name for a doomed planet in 1938. The direction of influence runs that way and not the other, which is worth stating because a surprising number of people assume the opposite.

The metre, held in an orange line

Until 1960 the metre was a scratch on a bar of platinum-iridium kept outside Paris. Physical artefacts are unsatisfactory standards — they can be damaged, they must be copied by comparison, and nobody can verify one without travelling to it. The eleventh General Conference on Weights and Measures replaced the bar with a wavelength: the metre became 1,650,763.73 wavelengths in vacuum of the orange-red line emitted by krypton-86 in an electrical discharge, with the lamp held at the triple point of nitrogen.

Krypton-86 was chosen because it is an even-mass isotope with no nuclear spin, so its emission line has no hyperfine structure to blur it, and because a single-isotope lamp removes the broadening caused by a mixture of masses. Any competent laboratory anywhere could now build the standard from scratch instead of borrowing it.

The definition lasted until 1983, when it was superseded by fixing the speed of light and letting the metre follow from the second. Krypton's tenure as the ruler of the world is short by the standards of metrology and it produced the first standard that was genuinely reproducible from first principles.

Where a scarce gas earns its price

Krypton is expensive enough that it is only used where nothing else will do.

• Excimer lasers. Krypton fluoride lases at 248 nanometres in the deep ultraviolet. KrF steppers were the workhorse of semiconductor photolithography through the 1990s and are still used for less demanding layers, and the NIKE laser at the US Naval Research Laboratory uses krypton fluoride for inertial confinement fusion research.
• Insulating glazing. Krypton conducts heat about half as well as argon, and unlike argon it still performs in a narrow cavity. Triple-glazed units with thin gaps are filled with it where the specification demands a low U-value without a thick, heavy window.
• Lighting. Krypton-filled incandescent lamps ran hotter and lasted longer than argon-filled ones, and high-intensity flashtubes for photography and airfield approach lights still exploit its output. This market shrank with the incandescent bulb.

Krypton-85 gives reprocessing away

Nuclear fission produces krypton-85, a beta emitter with a half-life of about 10.8 years. Inside intact fuel it stays trapped in the ceramic. It escapes when the cladding is cut open and the fuel dissolved — that is, during reprocessing — and once released it is chemically inert, so nothing scrubs it out of the atmosphere. It simply mixes and decays.

The consequence is that atmospheric krypton-85 is a global ledger of how much spent fuel the world has taken apart. Concentrations were negligible before 1945 and have risen by orders of magnitude since, and downwind measurements have been used to estimate the throughput of reprocessing plants whose operators published nothing. It is one of very few nuclear activities with a signature that cannot be contained, which is why it appears repeatedly in non-proliferation monitoring proposals.

Dating water that fell a million years ago

A second isotope does the opposite job. Krypton-81 is made in the upper atmosphere by cosmic rays at a steady trickle, has a half-life of about 229,000 years, and is chemically inert, so once groundwater is sealed away from the atmosphere its krypton-81 clock starts running and nothing disturbs it. That covers a range — roughly 40,000 to 1.5 million years — that radiocarbon cannot reach and other tracers handle badly.

The obstacle was always counting. Krypton-81 makes up about one atom in 10^13 of atmospheric krypton, and no mass spectrometer could pick it out. Atom trap trace analysis, developed at Argonne, uses laser cooling to capture individual krypton-81 atoms in a magneto-optical trap and count their fluorescence one at a time. It has since dated the deep Nubian Sandstone aquifer under the Sahara, and ancient ice recovered from Antarctic blue-ice areas.

A chemistry of one compound

Xenon has a substantial chemistry. Krypton essentially has one compound, krypton difluoride, which decomposes above about −30 °C and must be made in a discharge at cryogenic temperatures. The reason is straightforward: krypton's outer electrons are held more tightly than xenon's, so only fluorine is aggressive enough to take a share of them, and the resulting bonds are weak. The group's reactivity is a gradient, and krypton sits just on the far side of the point where it becomes practical.

Made only because somebody wants oxygen

There is no krypton production in its own right. It is separated in the tail of large air separation units built to supply oxygen and nitrogen to steelworks, chemical plants and hospitals, and world output is therefore set by industrial gas demand rather than by anyone's interest in krypton. That coupling made the supply unexpectedly fragile: a substantial share of the noble gases used by the semiconductor industry came from air separation plants attached to Ukrainian steelworks, and the 2022 invasion took much of that capacity offline at once.