Krypton

Before Superman's writers borrowed its name for a fictional doomed planet, krypton was hiding quietly in the atmosphere, so scarce and inert that chemists had no reason to suspect its existence. The noble gases were late arrivals to the periodic table because they refuse to form compounds under ordinary conditions, leaving no obvious chemical footprint. Krypton, discovered in 1898 by William Ramsay and Morris Travers, was found by systematically evaporating liquid air and carefully studying what remained after the more abundant gases had been removed. Its name, from the Greek kryptos meaning hidden, perfectly captured the element's nature. Though krypton makes up only about one part per million of the atmosphere, that trace abundance still makes it far more accessible than many better-known elements, and its unique spectral properties have made it a surprisingly important precision standard.

Krypton's most commercially significant application is in lighting. Krypton-filled incandescent bulbs run hotter and more efficiently than argon-filled ones, allowing thinner filaments that produce whiter light with longer lifetimes — making them attractive for high-end photography and projection. Krypton arc lamps produce an intense, broad-spectrum white light used in high-speed photography and movie projectors. The krypton fluoride (KrF) excimer laser emits at 248 nanometers in the ultraviolet and has been central to semiconductor photolithography, enabling the printing of features smaller than the wavelength of visible light on microchips. Krypton-85, a radioactive isotope produced in nuclear reactors, has been used to detect leaks in sealed systems and to measure thickness of thin materials. Krypton gas is also used to fill double-pane insulating windows because its low thermal conductivity outperforms air.

William Ramsay and Morris Travers discovered krypton in May 1898, just weeks after finding neon, during a systematic examination of the residue left after boiling liquid air. They had previously identified argon and helium, and predicted that other noble gases should exist between them in the periodic table. Travers described the discovery as a moment of extraordinary excitement when spectroscopic analysis of the new gas revealed a striking spectrum of lines. Ramsay received the Nobel Prize in Chemistry in 1904 for his work discovering the noble gas series. For six decades krypton remained primarily a scientific curiosity, but in 1960 the International Bureau of Weights and Measures made a consequential decision: the meter was formally redefined as exactly 1,650,763.73 wavelengths of the orange-red spectral line of krypton-86, replacing the platinum-iridium physical artifact that had served as the standard since 1889. This definition held until 1983, when the speed of light replaced it.

Krypton constitutes approximately 1.14 parts per million of Earth's atmosphere by volume, making it the third rarest stable noble gas after xenon and radon. This small fraction still amounts to an enormous absolute quantity given the mass of the atmosphere, and krypton is commercially produced by fractional distillation of liquid air alongside neon, argon, and xenon. It does not occur in any minerals and has no stable chemistry that would concentrate it in rocks or soils. Small amounts of the radioactive isotope krypton-85 accumulate in the atmosphere from nuclear weapons testing and from the reprocessing of nuclear reactor fuel, and scientists have used atmospheric krypton-85 levels as a sensitive tracer for global nuclear fuel reprocessing activity. Krypton is also detected in the solar wind and in the spectra of certain stars.

For most of its known history, krypton was believed to form no chemical compounds at all — its full outer electron shell provides an almost impenetrable chemical barrier. The first true krypton compound, krypton difluoride (KrF2), was not synthesized until 1963, proving that even the most stubbornly inert noble gases can be coaxed into bonding under the right conditions. KrF2 is a powerful oxidizing agent and a strong fluorinating agent used in specialized chemical synthesis, but it is highly unstable and decomposes readily at room temperature. The KrF excimer laser, despite its name, does not contain stable KrF2 molecules; rather, it generates excited-state krypton fluoride species in a plasma discharge that emit UV photons as they relax. A handful of other krypton compounds, including krypton difluoride complexes, have been characterized at very low temperatures, but practical chemistry based on krypton compounds remains extremely limited.

• The krypton-86 spectral line served as the official definition of the meter from 1960 to 1983, making it briefly the most precisely characterized length standard in history.
• Krypton-filled double-pane windows provide about 30 percent better thermal insulation than argon-filled ones, though the higher cost limits their use to premium construction projects.
• The KrF excimer laser has been the workhorse of the semiconductor lithography industry since the 1980s, enabling the manufacture of microchips with features just 90 nanometers wide.
• Ramsay and Travers discovered three noble gases — neon, krypton, and xenon — within a single month in 1898, an astonishing run of elemental discovery with no modern parallel.
• Despite Superman's Krypton being uninhabitable due to its instability, the real element krypton is among the most chemically stable substances known, forming almost no compounds under ordinary conditions.