Krypton

Krypton (chemical symbol Kr, atomic number 36) is a colorless, odorless, tasteless noble gas belonging to Group 18 of the periodic table, occurring in trace quantities in Earth's atmosphere and best known for its uses in high-performance lighting, gas lasers, precision metrology, and its historical role in providing the first atomic definition of the metre.

Discovery and Historical Background

Krypton was discovered on 30 May 1898 by the Scottish chemist Sir William Ramsay and his assistant Morris Travers at University College London. Working shortly after their 1894 discovery of argon (with Lord Rayleigh), the two researchers liquefied a sample of air and allowed its components to evaporate, then examined the small residue that remained after nearly everything else had boiled away. Spectroscopic analysis of this residue revealed new green and yellow lines unlike those of any known element. Ramsay named the gas "krypton," derived from the Greek kryptos, meaning "the hidden one." Within the following weeks, the same research programme yielded the further noble gases neon and xenon, completing the family of heavier noble gases and forcing a significant revision of the periodic table to accommodate an entire new group of chemically inert elements.

The element achieved renewed historical prominence in 1960, when the 11th General Conference on Weights and Measures redefined the metre as exactly 1,650,763.73 wavelengths in vacuum of the orange-red spectral radiation corresponding to a specific transition of the krypton-86 atom. This marked the first time an SI base unit was defined by reference to an atomic property, replacing the platinum–iridium prototype bar that had served since 1889. The krypton-86 definition remained in force until 1983, when the metre was redefined in terms of the speed of light.

The first confirmed chemical compound of krypton, krypton difluoride (KrF₂), was synthesized in 1963, only a year after Neil Bartlett's landmark synthesis of the first noble gas compound, xenon hexafluoroplatinate, which overturned the long-standing belief that noble gases were wholly unreactive.

Physical Properties

Krypton is a monatomic gas under standard conditions, with an electron configuration of [Ar] 3d¹⁰ 4s² 4p⁶, meaning its outermost electron shell is completely filled. Its key physical constants include:

• Atomic mass: 83.798 u
• Melting point: −157.37 °C (115.78 K)
• Boiling point: −153.415 °C (119.735 K)
• Density: approximately 3.75 g/L at standard temperature and pressure, roughly 2.8 times that of air

Krypton is denser than air and, like other noble gases, is characterized by very low chemical reactivity, low thermal conductivity, and a low rate of diffusion. When subjected to an electrical discharge at low pressure, it emits a characteristic pale, whitish-blue glow; its emission spectrum contains prominent lines in the green and yellow regions, together with important lines in the red and violet. Krypton is only sparingly soluble in water, though more soluble than nitrogen, and it is readily adsorbed onto activated charcoal and other porous solids.

Chemical Properties

Krypton is one of the least reactive elements, and its compounds are few, fragile, and formed only under aggressive conditions. The best characterized is krypton difluoride (KrF₂), a colorless crystalline solid prepared at low temperatures by electrical discharge or photochemical methods involving fluorine. KrF₂ is thermally delicate and decomposes readily, but it is a powerful fluorinating and oxidizing agent—stronger even than elemental fluorine. Derivatives containing the KrF⁺ cation in salts such as [KrF][AsF₆] are also known, and traces of the protonated species HKrF⁺ have been generated in matrix-isolation experiments.

Under ambient conditions krypton forms no ordinary covalent compounds, but it does form inclusion compounds (clathrates) in which krypton atoms are physically trapped within crystal lattices of hosts such as hydroquinone and phenol. Van der Waals molecules such as ArKr and KrH₂ have been observed spectroscopically at very low temperatures, and theoretical and experimental work has suggested that under extreme pressures krypton may form additional compounds with hydrogen and other elements. Because it is chemically inert, krypton plays no known biological role and is essentially non-toxic.

Isotopes

Naturally occurring krypton is a mixture of six stable isotopes: krypton-78, 80, 82, 83, 84, and 86. Krypton-84 is the most abundant, accounting for roughly 57 percent of natural krypton; krypton-78 is technically radioactive as well, decaying by double electron capture with a half-life of approximately 10²¹–10²² years, vastly longer than the age of the universe and therefore of no practical radiological consequence.

Two radioactive isotopes are of particular scientific importance:

• Krypton-81 (half-life about 229,000 years) is produced cosmogenically in the upper atmosphere by cosmic-ray spallation. It is used for radiometric dating of ancient groundwater and polar ice in the range of roughly 50,000 to 1.5 million years, a window beyond the reach of radiocarbon dating. The development of Atom Trap Trace Analysis (ATTA), a laser-based technique capable of counting individual krypton-81 atoms in environmental samples, made such dating practical.
• Krypton-85 (half-life about 10.76 years) is a fission product of uranium and plutonium released into the atmosphere by nuclear weapons testing, reactor operations, and—most significantly—the reprocessing of spent nuclear fuel. Atmospheric krypton-85 concentrations rose sharply after the mid-twentieth century, and the isotope serves as a tracer for monitoring atmospheric circulation and for detecting undeclared nuclear activities, including covert reprocessing.

Krypton-83, one of the few noble gas isotopes with a nuclear magnetic moment, is used in research on hyperpolarized magnetic resonance imaging, particularly experimental imaging of lung spaces.

Occurrence and Production

Krypton constitutes about 1.14 parts per million by volume (roughly 3 parts per million by mass) of Earth's atmosphere, making it one of the rarest stable components of air, though the sheer mass of the atmosphere means the total atmospheric inventory is substantial. Minute quantities also occur dissolved in seawater and trapped in some natural gas deposits. Like the other noble gases heavier than helium, krypton is obtained commercially as a by-product of the fractional distillation of liquefied air. It concentrates in air separation plants together with xenon, from which it is separated by further distillation and adsorption processes.

Because of the enormous volumes of air that must be processed to obtain small amounts of the gas, krypton is considerably more expensive than argon, and global production is modest—on the order of a few tens of tonnes per year. A practical complication for some precision applications is that commercially produced krypton carries trace contamination from anthropogenic krypton-85, which must be removed for experiments demanding extremely low radioactivity backgrounds.

Applications

Lighting and Photography

Krypton's principal historical application has been in lighting. Filling incandescent lamp bulbs with krypton rather than argon or a vacuum reduces heat conduction from the filament and slows filament evaporation, permitting higher operating temperatures and improved luminous efficiency; the gas's high cost restricted this to premium long-life and low-voltage lamps. Krypton-filled fluorescent tubes and discharge lamps emit a bright white light. The gas is also used in flash lamps for high-speed photography, where it produces intense white flashes lasting only microseconds, and in certain airport and runway signal lighting.

Lasers

Krypton is central to two laser technologies. Krypton ion lasers generate strong red, orange, and yellow lines (most notably at 647.1 nm) and have been used in ophthalmology for retinal photocoagulation, in laser light displays, and in scientific spectroscopy, though they have largely been superseded by solid-state and diode lasers. The krypton fluoride (KrF) excimer laser, emitting ultraviolet light at 248 nm, remains industrially important: it has been widely used in photolithography for semiconductor manufacturing, in refractive eye surgery, in micromachining, and in research programs on inertial confinement fusion.

Metrology

The orange-red spectral line of krypton-86 defined the international standard of length from 1960 to 1983, as described above. This definition made length measurements reproducible in any sufficiently equipped laboratory worldwide and represented a milestone in the shift of metrology from artefact-based to atomic and physical standards.

Thermal Insulation

Owing to its low thermal conductivity, krypton is used as a fill gas in high-performance double- and triple-glazed windows. Because it performs well in narrower gaps than argon, it enables slimmer window units with excellent insulating properties, though its high price limits use to premium architectural applications.

Space Propulsion and Scientific Research

Krypton has attracted attention as a lower-cost alternative to xenon for electric spacecraft propulsion. Hall-effect ion thrusters fueled by krypton have been employed on large satellite constellations, where propellant cost is a significant factor, although krypton offers somewhat lower specific impulse than xenon. In scientific research, krypton isotopes underpin groundwater and ice dating (krypton-81), environmental nuclear monitoring (krypton-85), and experimental lung imaging (hyperpolarized krypton-83).

Hazards and Safety Considerations

Krypton is chemically inert and non-toxic, and it presents no direct chemical hazard. Its principal dangers are physical: as a dense gas it can displace oxygen and act as an asphyxiant in enclosed, poorly ventilated spaces, and because it is heavier than air it tends to accumulate in low-lying areas. Liquid krypton is a cryogenic fluid and poses risks of severe frostbite and pressure build-up in sealed containers. Radiological concerns attach mainly to krypton-85, whose release during nuclear fuel reprocessing is monitored under international safeguards; as a noble gas, it disperses readily in the atmosphere and is not readily incorporated into biological tissue.

Influence and Cultural Significance

Beyond its technical applications, krypton holds a notable place in the history of science and in popular culture. Its discovery completed the noble gas family and helped force the periodic table into its modern form, while its role in the 1960 redefinition of the metre marked the beginning of the era of quantum-based measurement standards. In popular culture, the element lent its name to Krypton, the fictional home planet of the superhero Superman, created by Jerry Siegel and Joe Shuster in 1938; the planet's name was taken directly from the then-famous element, and the fictional mineral kryptonite, depicted as Superman's fatal weakness, entered common usage as a metaphor for an otherwise powerful entity's singular vulnerability.

See Also

• Neon
• Xenon
• Argon
• Noble gas
• Krypton difluoride
• History of the metre