Xenon (Xe)

For sixty years after the noble gases were found, chemistry taught that they formed no compounds at all. The full outer shell was the explanation and the closed case. Then in 1962 a young chemist at the University of British Columbia noticed a number.

Neil Bartlett had been working with platinum hexafluoride, an aggressively oxidising red gas, and had found that it reacted with ordinary oxygen to form a salt containing the O₂⁺ ion. That meant platinum hexafluoride was strong enough to strip an electron from a molecule with an ionisation energy of about 1175 kilojoules per mole. Bartlett knew that xenon's first ionisation energy is about 1170 — very slightly lower. If platinum hexafluoride could take an electron from oxygen, it should be able to take one from xenon.

He asked a colleague for some xenon, mixed the two gases in a glass apparatus in March 1962, and watched the red vapour turn instantly to an orange-yellow solid. The exact composition of that product is still argued over and is probably a mixture rather than the clean Xe⁺[PtF₆]⁻ he proposed. It did not matter. The point had been made, and within months laboratories elsewhere had made xenon tetrafluoride from nothing but xenon and fluorine and a hot nickel can.

Xenon now has by far the largest chemistry of any noble gas — difluoride, tetrafluoride and hexafluoride, the explosively unstable trioxide and tetroxide, perxenate salts, and compounds with bonds to nitrogen and carbon. The reason it and not its lighter siblings is straightforward: xenon's outer electrons are further from the nucleus and more weakly held, so an aggressive enough partner can reach them.

The stranger in the last fraction

Xenon was the third element William Ramsay and Morris Travers pulled from liquid air in 1898, after krypton and neon. It was the least volatile and the scarcest — about one part in eleven million of the atmosphere — and they named it from Greek xenos, the stranger. Its scarcity is why it costs perhaps a hundred times as much as krypton and why nothing uses it in bulk.

Two point six million barns

Xenon-135 is the most voracious absorber of thermal neutrons known: its capture cross-section is around 2.6 million barns, which is roughly a hundred times cadmium's and about ten million times that of a typical structural metal. It is also a fission product, formed both directly and by the decay of iodine-135.

The consequence for reactor operation is a genuine hazard, and it was discovered the hard way. In September 1944 the B Reactor at Hanford, the first full-scale plutonium production reactor, was brought to power, ran for a few hours, and then steadily died. Nobody could find a fault. John Wheeler worked out overnight that a fission product with an enormous appetite for neutrons was accumulating, and that as it decayed away between runs the reactor would recover — which it duly did.

Hanford survived that discovery because of a disagreement over engineering margin. DuPont's engineers, over the objections of the physicists, had built the reactor with far more fuel tubes than the design called for. Loading the spare tubes provided enough extra reactivity to override the xenon, and the plutonium programme continued on schedule.

The same physics has a darker entry in the record. A reactor that has been running at high power and is then throttled back builds up xenon faster than it burns it, producing a deep dip in reactivity known as the iodine pit. At Chernobyl in April 1986 the reactor was held at low power for hours before the test, went deeply into xenon poisoning, and the operators withdrew control rods far beyond the permitted limit to keep it running. The reactor they then tested was in a configuration nobody had analysed.

Pushing spacecraft with a heavy gas

Electric propulsion works by accelerating ions electrostatically, and the thrust per unit of electrical power rises with the mass of the ion. Xenon is the heaviest gas that is inert, easily ionised, and storable at high density — it can be carried as a supercritical fluid at a few hundred atmospheres, occupying a fraction of the volume of a lighter propellant.

That combination made it standard. Deep Space 1 demonstrated it; the Dawn mission used xenon ion thrusters to enter and then leave orbit around Vesta and travel on to Ceres, a manoeuvre no chemical rocket of that mass could perform; Hayabusa returned an asteroid sample with it, and BepiColombo is using it to spiral in to Mercury. Commercial satellite operators adopted Hall thrusters for station-keeping because the propellant mass saving translates directly into revenue payload.

Xenon's cost eventually became the constraint. Large satellite constellations consume propellant in quantities that would move the world market, which is why some operators have shifted to krypton and then argon, accepting worse performance for a supply that exists.

An anaesthetic too expensive to use, and one that was banned

Xenon is a genuine general anaesthetic, which is remarkable for an element that forms no bonds under physiological conditions. It appears to act by blocking NMDA receptors rather than through the mechanism of conventional agents, and it produces very little cardiovascular depression, has almost no metabolic footprint, and allows fast emergence. It is licensed for anaesthesia in several European countries.

It is also priced out of routine use. Xenon anaesthesia requires closed-circuit machines that recycle the gas, and even then the cost per case is many times that of the alternatives, so it remains confined to patients whose circulation cannot tolerate anything else.

The World Anti-Doping Agency added xenon to its prohibited list in 2014, after reports that inhaling it had been used to stimulate erythropoietin production by mimicking the cellular response to low oxygen. Very few banned substances are chemically inert.

Tonnes of it, underground

The largest single stockpiles of xenon in the world are in caverns beneath mountains, waiting for something that may not exist. Liquid xenon is dense, transparent to its own scintillation light, can be purified to extraordinary levels, and can be instrumented as a time projection chamber that reconstructs the position and energy of every interaction inside it. Crucially, the outer layers of xenon shield the inner volume from external radiation, so the detector protects itself.

XENONnT and LUX-ZEPLIN, and PandaX in China, each hold several tonnes of liquid xenon and constitute the most sensitive searches ever mounted for weakly interacting dark matter particles. They have found no dark matter. What they have caught instead is the slowest process anyone has ever observed in the act: the two-neutrino double electron capture of xenon-124, which XENON1T picked out of a year of data in 2019 at a half-life of order 10²² years. Slower decays are on record — tellurium-128 is about a hundred times slower still — but those are inferred from decay products that piled up in rock over geological time, not counted event by event, and the distinction is worth keeping straight. The same detectors have since registered the coherent scattering of solar neutrinos off xenon nuclei, which is the irreducible background that will eventually limit the technique.

The missing xenon

Xenon has nine stable isotopes, more than any element except tin, and they carry two of the better puzzles in planetary science.

Xenon-129 in meteorites is present in excess, and the excess correlates with iodine content. Iodine-129 decays to xenon-129 with a half-life of 15.7 million years and is long extinct, so the excess is a record of how quickly solids formed and trapped iodine before the parent isotope died away. It gives one of the sharpest chronometers available for the first few million years of the solar system.

The second puzzle is unresolved. Earth's atmosphere holds far less xenon than it should — around 90% less than the chondritic pattern predicts, once krypton is used as the yardstick, and the xenon that is here is isotopically fractionated towards the heavy end. Something removed most of our xenon and preferentially removed the lighter atoms. Proposals include escape during a hydrogen-rich early atmosphere, sequestration in ice, and incorporation into deep mantle minerals or the core, where xenon may form compounds with iron and nickel at extreme pressure. No explanation is agreed, and the missing xenon remains a live problem.