Xenon

• 1. Introduction

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  Xenon is a chemical element with the symbol Xe, and in November 1989 it became the medium for the first deliberate act of atomic arrangement in history. IBM scientists used a scanning tunneling microscope to position 35 individual xenon atoms on a chilled crystal of nickel, spelling out the company's three-letter name. It was an unlikely debut for a gas that chemists had spent decades insisting could do almost nothing at all. Xenon is dense, colorless, and odorless, and it turns up only in trace amounts in Earth's atmosphere. For most of its history it was defined by refusal: no burning, no bonding, no visible chemistry with anything else. That reputation has since collapsed. How did an element assumed to be permanently inert end up forming real chemical compounds, sedating surgical patients, lighting movie projectors, and steering spacecraft across the solar system? And why, more than a century after it was first isolated, is xenon still missing from Earth's atmosphere in quantities nobody can fully account for?

• 2. Discovery and Early Experiments

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  On the 12th of July 1898, Scottish chemist William Ramsay and English chemist Morris Travers isolated xenon in England, finding it in the leftover residue after boiling off the other gases from liquefied air. The pair had identified krypton and neon only days before. Ramsay named the new gas xenon, from the Greek word xenos, meaning stranger or guest. By 1902 he estimated it made up roughly one part in 20 million of the atmosphere.

  American engineer Harold Edgerton began experimenting with strobe lighting for high-speed photography during the 1930s, work that led him to build the first xenon flash lamp, passing electric current through a tube of the gas to produce light. By 1934 he was generating flashes as brief as one microsecond.

  In 1939, American physician Albert R. Behnke Jr. set out to explain why deep-sea divers experienced a strange form of drunkenness underwater, and found that breathing different gas mixtures changed his subjects' sense of depth. He concluded that xenon itself could act as an anesthetic. Russian toxicologist Nikolay V. Lazarev reportedly studied xenon anesthesia in 1941, though the first published confirmation came in 1946, when American researcher John H. Lawrence tested it on mice. In 1951, American anesthesiologist Stuart C. Cullen became the first to use xenon as a surgical anesthetic on human patients, successfully treating two of them.

• 3. Breaking the Noble Gas Rule

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  At the University of British Columbia, chemist Neil Bartlett found that platinum hexafluoride, PtF6, was a powerful enough oxidizer to strip an electron from oxygen gas, producing a compound called dioxygenyl hexafluoroplatinate. Because oxygen and xenon share almost the same first ionization energy, Bartlett suspected platinum hexafluoride might do the same to xenon. On the 23rd of March 1962, he mixed the two gases and produced xenon hexafluoroplatinate, the first known compound of a noble gas.

  Bartlett originally proposed the compound's formula as Xe+PtF6-, though later analysis suggested it was actually a mixture of several xenon-containing salts. The floodgates opened from there. Chemists went on to synthesize compounds of argon, krypton, and radon too, including argon fluorohydride, krypton difluoride, and a radon fluoride, and by 1971 more than 80 distinct xenon compounds were known. Three xenon fluorides became the starting point for nearly all of them, formed by exposing a mixture of fluorine and xenon gas to ultraviolet light, even the ordinary ultraviolet in daylight.

  One compound, the tetraxenonogold cation, bonds xenon directly to gold, linking two elements normally considered chemically unreactive. Another, made of two bonded xenon atoms, holds the longest element-to-element bond ever measured, at 308.71 picometres. In 1995, chemist M. Rasanen and co-workers at the University of Helsinki in Finland prepared xenon dihydride, known as HXeH, and later produced related compounds such as HXeOH and HXeCCH. In 2008, researcher Khriachtchev and colleagues went further, creating HXeOXeH by shining light on water trapped in frozen xenon.

• 4. A Heavy, Reluctant Gas

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  At standard temperature and pressure, xenon gas has a density of 5.894 kilograms per cubic metre, about four and a half times denser than Earth's atmosphere at sea level. As a liquid it reaches a density of up to 3.100 grams per millilitre, and its high polarizability makes it an unusually good solvent, capable of dissolving hydrocarbons, biological molecules, and even water. Solid xenon is denser still, at 3.640 grams per cubic centimetre, heavier than granite.

  Squeezed above roughly 140 gigapascals, xenon's solid crystal structure shifts from face-centered cubic to hexagonal close-packed, and the substance begins turning metallic, becoming fully metallic at 155 gigapascals. In this state xenon appears sky blue, absorbing red light while letting other visible wavelengths pass through, an unusual trait for a metal caused by the narrow width of its electron bands.

  Xenon belongs to the noble gases because its outer shell holds eight electrons, a tightly bound, minimum-energy configuration that resists ordinary chemical reactions such as combustion. Run an electric discharge through a tube of the gas, however, and it glows blue or lavender, the result of emission lines concentrated in the blue region of the visible spectrum.

• 5. Rarer Than It Should Be

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  Xenon occurs in Earth's atmosphere at a volume fraction of just 87 parts per billion, roughly one part in 11.5 million, and also turns up in gases emitted from certain mineral springs. Across the entire atmosphere, that adds up to an estimated 2.03 gigatonnes of xenon in total.

  Xenon's atmospheric concentration sits far below what would be expected relative to argon and krypton, a puzzle scientists call the missing xenon problem. Proposed explanations include xenon binding with iron inside Earth's lower mantle, xenon dioxide forming within silica minerals, and reactions between xenon and iron or nickel deep in Earth's core.

  Commercially, xenon is recovered as a by-product of separating air into oxygen and nitrogen, then concentrated through fractional distillation until the liquid oxygen fraction holds 0.1 to 0.2 percent of a krypton-xenon mixture, which is pulled out by adsorption onto silica gel or further distillation and finally split into its two component gases. Because it is so scarce, xenon costs far more than its lighter relatives. In Europe in 1999, small quantities sold for about 10 euros a litre, compared with roughly 1 euro a litre for krypton and 0.20 euros a litre for neon, while argon, which makes up over 1 percent of the atmosphere by volume, cost less than a cent a litre.

• 6. A Tracer for Cosmic History

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  Within the Solar System as a whole, xenon makes up roughly one part in 630,000 of the total mass, and it stays rare in the Sun's atmosphere, on Earth, and among asteroids and comets. Jupiter breaks that pattern: its atmosphere holds about 2.6 times the Sun's proportion of xenon, an excess still unexplained, though one theory points to an early, rapid buildup of small planetesimals before the surrounding disk of gas around the young Sun heated up and let the trapped xenon escape.

  Building elements heavier than iron-56 costs a star energy rather than releasing it, which is why ordinary stellar nucleosynthesis cannot manufacture xenon at all. Instead xenon forms during supernova explosions through rapid neutron capture, inside red giant stars through slower neutron capture after they exhaust their core hydrogen, and through the radioactive decay of elements including extinct iodine-129 and the spontaneous fission of thorium, uranium, and plutonium.

  In 1960, physicist John H. Reynolds discovered that certain meteorites held far more xenon-129 than expected, an anomaly he traced to the decay of radioactive iodine-129. Because iodine-129 has a half-life of only about 16 million years, short on a cosmic timescale, its presence showed that only a brief interval separated a nearby supernova from the solidifying of the meteorites, both events dated to the earliest history of the Solar System.

  Mars's atmosphere holds about the same overall xenon abundance as Earth's, roughly 0.08 parts per million, but a much greater share of xenon-129, suggesting Mars lost most of its original atmosphere within the first 100 million years after the planet formed. In New Mexico, xenon-129 found in carbon dioxide well gases is believed to trace back to mantle gases released soon after Earth itself formed.

• 7. The Reactor's Poison

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  Naturally occurring xenon is built from seven stable isotopes and two nearly stable ones, 124Xe and 136Xe, whose half-lives stretch to trillions of times the age of the universe. More than 40 unstable xenon isotopes are also known, produced through radioactive decay processes including the beta decay of iodine-129, which has a half-life of 16.1 million years.

  Two of xenon's isotopes, 129Xe and 131Xe, carry a property called nuclear spin that makes them useful for magnetic resonance work. Shining circularly polarized light through rubidium vapor can align these spins far beyond normal levels, boosting polarization past 50 percent of its theoretical maximum, compared with roughly 0.001 percent under ordinary conditions even inside the strongest magnets. This artificially boosted alignment, called hyperpolarization, can persist for seconds when the xenon is dissolved in blood, for hours in gas form, and for days in xenon frozen solid.

  Xenon-135 has an enormous appetite for neutrons, a capture cross-section of 2.6 million barns, letting it act as a neutron poison that can stall a chain reaction inside a nuclear reactor. Scientists on the American Manhattan Project first discovered this effect in their earliest reactors built for plutonium production, and had to design in extra reactivity to compensate. Decades later, xenon-135 poisoning was a major contributing factor in the Chernobyl disaster: cutting or reducing a reactor's power lets xenon-135 build up faster than it decays, a condition called the iodine pit.

  Isotope ratios of xenon recovered from Oklo, in Gabon, revealed evidence of a natural nuclear reactor that had sustained a fission chain reaction underground roughly 2 billion years ago.

• 8. Light, Medicine and Deep Space

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  Introduced during the 1940s, high-pressure xenon arc lamps produce a color temperature close to noon sunlight, used in solar simulators and to replace the shorter-lived carbon arc lamps that once lit movie projectors; they still run today's 35mm, IMAX, and digital cinema projectors, plus automotive headlights and high-end tactical flashlights. In 1962, researchers at Bell Laboratories discovered that xenon could generate laser action, work that led to the first excimer laser, which used a xenon dimer molecule energized by an electron beam to produce ultraviolet light at a wavelength of 176 nanometres.

  Xenon blocks the glycine site of the NMDA receptor with high affinity, yet unlike some other NMDA blockers it is not neurotoxic; it even suppresses the neurotoxic effects of ketamine and nitrous oxide while providing its own neuroprotection. At a concentration of 72 percent in a 40-year-old patient, xenon reaches full surgical anesthesia, making it 44 percent more potent than nitrous oxide, so it can be delivered alongside oxygen at concentrations that carry a lower risk of hypoxia. Unlike nitrous oxide, it is not a greenhouse gas, and modern systems recycle it rather than venting it away.

  Inhaling a xenon-oxygen mixture activates a transcription factor called HIF-1-alpha, which can raise production of the hormone erythropoietin and, in theory, boost athletic performance, a practice reportedly used for doping in Russia since 2004. The World Anti-Doping Agency added xenon, alongside argon, to its list of banned substances on the 31st of August 2014, even though no reliable test for either gas yet exists. In 2025, four UK mountaineers, including Alistair Carns, climbed Mount Everest in a single week, crediting xenon inhalation with letting them skip the usual weeks of altitude acclimatization. The International Climbing and Mountaineering Federation criticized the claim, noting no evidence that xenon improves high-altitude performance, and warned that used without monitoring, the gas can impair brain function, compromise breathing, and kill.

  The radioactive isotope 133Xe emits gamma rays that can image the heart, lungs, and brain and measure blood flow, while hyperpolarized xenon-129, which is non-ionizing, images lung alveoli and gas flow directly during an MRI scan. In space, xenon serves as fuel: its low ionization potential per unit weight, easy storage as a pressurized liquid, and inert, non-corrosive nature made it the propellant for JPL's Deep Space 1 probe, Europe's SMART-1 spacecraft, and all three ion engines on NASA's Dawn mission. On Earth, liquid xenon's density and self-shielding properties make it useful in calorimeters built to catch the faint signals of hypothetical dark matter particles, should one ever collide with a xenon nucleus and release a flash of light.