Krypton

Krypton
Atomic number	36
Symbol	Kr
Group	18 (noble gases)
Boiling point	−153.4 °C
Electron configuration	[Ar] 3d10 4s2 4p6
Period	4
Main isotopes	84Kr, 86Kr, 82Kr
Discovery	1898 (Ramsay, Travers)
Block	p
Phase STP	Gas
Wikidata	Q888

Krypton is a rare noble gas (atomic number 36) in Group 18 of the periodic table In its elemental form it is colorless, odorless, and exists as single (monatomic) atoms at standard temperature and pressure. Like other noble gases, krypton has a filled valence shell Ar] 3d¹⁰ 4s² 4p⁶) which makes it chemically inert under ordinary conditions. It is about three times heavier than air, with a gas density of roughly 3.7 g/L at 0 °C and 1 atm Krypton’s most common oxidation state is 0; it can be forced into a +2 state in a few exotic compounds In air it is very scarce (about 1.14 parts per million by volume) The element was discovered in 1898 by William Ramsay and Morris Travers, and its name comes from the Greek kryptos (“hidden”) because it was “hidden” unsuspected in the atmosphere

Atomic Structure and Electron Configuration

The krypton atom has 36 electrons arranged as [Ar] 3d¹⁰ 4s² 4p⁶ Its outer (4th) shell is completely filled with two 4s and six 4p electrons, giving it a stable noble-gas configuration. The full valence shell explains krypton’s lack of chemical reactivity – with no tendency to gain or lose electrons, krypton normally stays in the 0 oxidation state. Krypton’s atomic radii reflect its place in Period 4. Its van der Waals (nonbonded) radius is about 202 picometers and its covalent radius about 116 pm larger than argon and smaller than xenon as expected down the group. Its first ionization energy is high (about 1350.8 kJ/mol) meaning a lot of energy is needed to remove an electron – further evidence of its inertness. Krypton has essentially no stable negative ions (its electron affinity is not defined), and its electronegativity is typically not assigned on conventional scales (noble gases do not attract electrons in bonds). In periodic trends, krypton lies between argon (lighter, still inert) and xenon (heavier, more reactive); argon has no stable compounds, while xenon forms many, so krypton is intermediate in reactivity among the noble gases.

Isotopes and Nuclear Properties

Naturally occurring krypton is a mix of six stable isotopes: ^{78}Kr, ^{80}Kr, ^{82}Kr, ^{83}Kr, ^{84}Kr, and ^{86}Kr Their natural abundances are about 0.35% (^78Kr), 2.28% (^80Kr), 11.6% (^82Kr), 11.5% (^83Kr), 57.0% (^84Kr), and 17.3% (^86Kr) Most of these nuclei have zero nuclear spin (even-even nuclides), except ^{83}Kr which has spin 9/2. The isotopic atomic weight of naturally sourced krypton (weighted average) is 83.798. Krypton has no long-lived radioisotopes in nature, but many radioactive isotopes can be made artificially. In fact, krypton has isotopes of every mass from 69 to 101, mainly produced by nuclear fission Of these, the only relatively long-lived ones are ^{81}Kr and ^{85}Kr. Krypton-81 (half-life 2.29×10^5 years) is produced by cosmic rays in the atmosphere and then absorbed by groundwater; it is useful in archeological dating of old water and ice samples Krypton-85 (half-life 10.7 years) is generated by uranium and plutonium fission in reactors or bombs It is released in small amounts from nuclear fuel reprocessing, so atmospheric ^85Kr serves as a tracer of nuclear activities. Radioactive ^{85}Kr has been used in industry as a leak detector (escaping krypton atoms are detected by their beta radiation) After about a week or so, fission-derived krypton decays to leave essentially only ^{85}Kr (other short-lived isotopes vanish)

Allotropes and Typical Compounds

Allotropes: Krypton is monatomic under all ordinary conditions, so it has no distinct allotropes (structural forms) in the way carbon has diamond/graphite. As a pure element, its only condensed phases are liquid and solid, which form simple van der Waals structures. Solid krypton (below 115.8 K) crystallizes in a face-centered cubic lattice like other noble gases. Krypton can be trapped in cage-like “clathrate” structures (e.g. water ice cages) as a guest atom, but this is a physical inclusion rather than a chemical allotrope There are no diatomic Kr₂ molecules in the solid or gas; krypton remains monatomic.

Typical Compounds: Krypton’s chemistry is extremely limited. For most of its history, it was thought to be totally inert. In fact, only one neutral molecular compound has been isolated in bulk: krypton difluoride (KrF₂). This colorless solid is prepared by combining krypton and fluorine at cryogenic temperature (around –183°C) in an electrical discharge or under UV irradiation KrF₂ slowly decomposes at room temperature, so it must be kept cold. In standard KrF₂, krypton is in the +2 oxidation state (Krypton difluoride is often written Kr^(+2)F₂^) From KrF₂, a few exotic ionic species have been obtained. For example, salts of the cation KrF^+ (with very strong acceptors like SbF₅) and even a Kr₂F₃^+ cation have been characterized These are only stable in superacidic media at low temperatures. No other simple krypton halides (chloride, bromide, iodide) are known. A few additional compounds have been reported under extreme conditions: for instance, a krypton–oxygen compound [Kr(OTeF₅)₂] shows a krypton–oxygen bond and HCNKrF^+ is an example of krypton bonded to nitrogen Outside of these highly specialized fluorine/oxygen chemistry experiments, krypton will not bond. Its characteristic bonding is thus “nil”; it exists in nature and in most uses only in the elemental monatomic form. In summary, krypton’s allotropes are just the gas, liquid, and solid phases, and its only normal compound of note is KrF₂, from which various odd ions and complexes are derived

Physical Properties

At standard conditions, krypton is a transparent, colorless gas. It has very low viscosity and thermal conductivity, as is typical for noble gases. Its density at 0 °C and 1 atm is about 3.73 g/L (compare air ~1.29 g/L). Its melting point is 115.79 K (–157.36 °C) and its boiling point is 119.735 K (–153.415 °C) This boiling point is higher than that of nitrogen or oxygen, which is why krypton collects in the bottom fraction when liquid air is distilled Below 115.8 K krypton solidifies; solid krypton is colorless and soft, with a density around 2.15 g/cm³ (2155 kg/m³) Solid krypton has a face-centered cubic (fcc) crystal structure as in the solid phases of other noble gases and some metals.

When electrically excited, krypton emits a characteristic spectrum. A low-pressure gas discharge in krypton produces a pale blue-white glow Its spectral lines span from infrared to ultraviolet; notably a sharp orange-red line (about 605.78 nm) of ^{86}Kr stands out. This line is so monochromatic that, from 1960 to 1983, it defined the international meter standard (One meter was exactly 1,650,763.73 times the wavelength of that Kr-86 line.) Krypton gas exhibits many other lines used in spectroscopy and calibration.

In terms of transport properties, krypton gas has a low thermal conductivity (about 9.4×10^–3 W·m^–1·K^–1 at room temperature) – lower than argon – and is an electrical insulator (no free electrons). Its speed of sound at 25 °C is around 220 m/s (liquid: ~1120 m/s) Krypton, like other noble gases, is diamagnetic (no unpaired electrons) It refracts light only slightly more than air (refractive index ≈1.00043). In high-energy contexts (e.g. in arcs or plasmas) krypton can form excited states, but in the normal condensed phases its thermal expansion is typical of a van der Waals solid and it shows no unusual mechanical properties.

Chemical Reactivity and Trends

Krypton is extremely unreactive chemically under normal conditions. Because its valence shell is complete, it has virtually no tendency to gain, lose, or share electrons. It does not react with acids, bases, or most other chemicals at room temperature. (For example, krypton will not corrode or oxidize materials; it does not participate in aqueous chemistry.) Its standard oxidation state is 0, and aside from KrF₂ no stable lower-oxidation compounds exist. In fact, experiments have shown that krypton only reacts under very forcing conditions: combining it with fluorine gas at cryogenic temperature and providing an electrical discharge is one way to make KrF₂ Otherwise, krypton simply remains inert.

Among the noble gases, krypton marks a boundary: it is less reactive than xenon but slightly more reactive than argon. Argon (above Kr) forms essentially no stable compound (with rare exceptions in extreme labs), whereas xenon (below Kr) forms a rich chemistry of fluorides and oxides. Krypton sits between them. Its ionization energies are very large (first IE ~1351 kJ/mol so it does not easily lose an electron. Its electronegativity is undefined in practical terms. Krypton does not form acids or bases and does not complex with typical ligands. It can, however, be physically adsorbed by strongly polar or porous materials (like zeolites) without chemical bonding. Krypton also forms clathrate “solutions” (e.g. water or hydrocarbons forming cages around a Kr atom) under high pressure.

In redox terms, krypton will not act as a reducing agent or oxidizing agent in normal chemistry at ambient conditions. Its only observed redox reaction is the formation of KrF₂ (Krypton is oxidized to +2 by fluorine). There is no common “krypton oxide or chloride” under standard conditions. Therefore, krypton does not fit into typical reactivity series of metals – it simply does not react.

Occurrence and Production

Krypton is extremely scarce in nature. It is a trace component of Earth’s atmosphere, roughly 1.1 ppm (parts per million) by volume This is its main terrestrial reservoir. In the crust, krypton is essentially absent (<0.2 ppb by weight because it is non-reactive and does not form minerals. In oceans the concentration is even lower. In contrast, cosmically krypton was produced by stellar nucleosynthesis; some is found in meteorites and was present in the primordial solar nebula, but compared to hydrogen it is on the order of 0.06% of atoms in the universe

Commercially, krypton is obtained from liquid air. Because krypton’s boiling point (–153 °C) is higher than liquid nitrogen (–196 °C) and oxygen (–183 °C), it remains in the liquid residue when air is fractionally distilled In practice, air is liquefied and slowly warm, causing nitrogen and oxygen to boil off. Krypton (with xenon) concentrates in the remaining liquid. The krypton–xenon mixture is then purified further: first by adsorption (e.g., on silica gel) and distillation, then by passing the gas over hot titanium to remove impurities Major industrial gas companies (Air Liquide, Linde, etc.) perform these steps. Roughly speaking, several tons of air must be processed to yield a few liters of krypton gas, which makes it expensive.

In addition to atmospheric extraction, pure ^{85}Kr can be captured from nuclear fuel reprocessing off-gases. Nuclear reactors generate various krypton isotopes by fission, and specialized processing can isolate ^{85}Kr. This is more of a specialized operation (for instance, to reduce radioactive releases) than a primary commercial source of krypton gas.

Applications and Technology

Krypton’s unique properties lead to specialized applications:

• Lighting and Displays: Krypton is used in certain gas-discharge lamps. For example, it is added to fluorescent and keystone bulbs to improve efficiency. Krypton-mercury lamps and argon–krypton mixtures (in fluorescent tubes) yield “cooler” light and longer bulb life. Neon signs and plasma globes sometimes use krypton to produce purple/violet hues. A common use is in photographic flash lamps: arcing a capacitor through krypton (or krypton–xenon mixes) produces an intense light flash. High-speed photography and strobe lights have traditionally used krypton-filled flash bulbs Tungsten-halogen incandescent bulbs can also be filled with krypton or a mix of krypton and xenon to reduce filament evaporation and improve lamp life.

• Lasers: Krypton plays a vital role in laser technology. The most famous is the krypton fluoride (KrF) excimer laser. This excimer (excited dimer) emits ultraviolet light at 248 nm (in the deep UV), and it is widely used in photolithography for manufacturing integrated circuits The very short wavelength enables fine patterning on chips. Similarly, krypton chloride (KrCl) excimer lamps emit 222 nm UV (far-UVC), which has germicidal properties and is used to disinfect air and surfaces Krypton gas is also used in visible laser applications. A krypton-ion discharge laser can produce bright lines (for instance at 647 nm, 568 nm, 530 nm, etc.) used in research, medicine, and even laser light shows. While xenon-ion lasers are more common, krypton lasers have been used when specific red and yellow lines are desired.

• Insulation: Krypton’s low thermal conductivity makes it valuable as an insulating gas. It is used between the panes of high-efficiency double- or triple-glazed windows (especially in cold climates) to reduce heat transfer Krypton is about 2–3 times better an insulator than argon (though more expensive), so it helps improve a window’s R-value and reduces condensation. Similarly, in specialty cryogenic or thermal insulation systems, krypton gas may be chosen for its poor conductivity.

• Radiography and Imaging: Krypton isotopes have niche uses in medical and scientific imaging. Radioactive ^{81m}Kr (short-lived) is used as an inhaled tracer gas for lung ventilation scans (it highlights airways in the lungs without chemical interaction). This is generated from a parent ^{81}Rb source. Stable ^{83}Kr can be hyperpolarized and used in MRI/NMR studies of the lungs (an emerging medical imaging technique). Stable krypton isotopes are also used in tracer studies of the pulmonary system or in calibration of scientific instruments

• Luminescent and Calibration Standards: Krypton discharge lamps provide many sharp spectral lines. The orange-red line of ^{86}Kr was used from 1960 to 1983 as the definition of the meter Even today, krypton spectral lamps are used to calibrate spectrometers and optical instruments. In lighting, krypton arcs were once used in early “neon lamps” to produce specific colors.

• Propellant: Krypton is being adopted as a propellant in spacecraft electric thrusters. Xenon is the most common ion engine propellant, but it is expensive. Krypton offers a cheaper, though slightly less efficient, alternative. Modern Hall-effect thrusters on satellites (e.g. SpaceX’s Starlink constellation) use krypton or argon instead of xenon The performance (specific impulse) of krypton is roughly 35% that of xenon in the same engines, but the reduced propellant cost makes it attractive for large satellite fleets

• Leak Detection and Safety: Krypton-85, being radioactive, is used to detect leaks in vacuum systems or containers. A tiny amount of ^{85}Kr is released inside a sealed device, and any escaping krypton can be detected with a radiation sensor. This allows highly sensitive leak testing without opening the system (After testing, the remaining krypton in the device rapidly diffuses away.)

• Other Uses: Krypton iodide (KrI) tags have been explored for fast neutron detectors (KrI scintillators), and krypton’s inertness makes it a useful inert cover gas in specialized welding or metallurgy of sensitive materials. It is also sometimes used in low-temperature physics (as a refrigerant in cascade refrigeration) since it liquefies at accessible liquid-nitrogen-range temperatures. However, most uses take advantage of its atmospheric presence or specific properties (in lamps and lasers) rather than bulk material applications.

Biology, Environment, and Safety

Krypton is chemically inert and has no known biological role It does not interact with biological molecules, enzymes, or cells. At normal concentrations (parts per million in air), krypton has no physiological effect. Because of its inertness, it is not considered toxic in the usual chemical sense. Indeed, ICSC safety guidance lists enamelation by asphyxiation as the only health hazard

The primary health concern with krypton is asphyxiation: like any non-oxygen gas, in high concentration it can displace oxygen. Krypton is about 2.9 times heavier than air so it tends to accumulate in low-lying areas in an enclosure. In an unventilated space, breathing an atmosphere rich in krypton would reduce oxygen intake – safety precautions control oxygen levels, not exposures to krypton per se. Liquid or cold krypton can cause severe frostbite on skin contact due to rapid heat absorption. Krypton gas is nonflammable and noncombustible

In the environment, krypton is stable and benign. It does not break down, react, or bioaccumulate. Being a noble gas, it diffuses into the atmosphere and eventually escapes to space over geological timescales. Krypton does not contribute to ozone depletion or global warming (it has virtually no absorption bands in the infrared relevant to greenhouse effect). The notable exception is radioactive krypton isotopes. The atmosphere contains trace ^{85}Kr from nuclear activities, but its radioactivity in ambient air is extremely low (on the order of disintegrations per minute per liter). Environmental monitoring tracks ^{85}Kr as an indicator of nuclear reprocessing, but even at maximum concentrations it poses negligible radiation risk to humans. Halo.

Safety guidelines for krypton focus on oxygen displacement. No occupational exposure limit is given for krypton itself (it is not toxic), but confined spaces should be monitored for oxygen. Cylinders of compressed krypton require standard handling procedures (non-reactive gas precautions). Krypton, like all noble gases, is classified as a 2.2 non-flammable gas under UN transport regulations. In short, krypton is not chemically hazardous, but it is an asphyxiant hazard in concentrated doses.

History and Etymology

Krypton was discovered in 1898 by Sir William Ramsay and Morris W. Travers at University College London After isolating argon from air in 1894, Ramsay suspected more “inert” gases remained hidden in the atmosphere. In May 1898 they collected argon-free liquid air and slowly evaporated off gases. In the residue they observed new spectral lines with a spectrometer, indicating an unknown element They named this new gas krypton (Greek kryptos, “hidden”) because it had eluded earlier detection Ramsay and Travers simultaneously discovered neon and soon afterwards (with others) found xenon; these four noble gases reshaped the periodic table’s Group 18. Ramsay later won the Nobel Prize (1904) for his work on the noble gases.

Krypton’s name fits the theme of the inert gases: neon (“new”), argon (“inactive”), xenon (“strange”), and krypton (“hidden”). Its chemical symbol “Kr” follows the IUPAC convention. The historical krypton sample was less than a cubic inch of gas, but its unique spectral signature made its identification unambiguous.

In the 20th century, krypton found roles in modern metrology and exploration. Notably, in 1960 the orange-red ^{86}Kr spectral line was adopted as the standard of length: one meter was defined as exactly 1,650,763.73 wavelengths of that line This definition, based on krypton, was used until 1983, when the meter was redefined in terms of the speed of light. (Krypton’s contribution ensured a very precise, reproducible standard.)

Although krypton itself has no lore outside science, its element name inspired Superman’s fictional “Kryptonite” (though the story’s “kryptonite” is unrelated chemically). In recent decades, interest in krypton has waned except in high-tech niches (laser photolithography, advanced lighting, aerospace). However, research into inert-gas anesthesia and novel applications occasionally brings noble gases like krypton back into discussion.

Data Table

Property	Value
Symbol	Kr
Atomic number (Z)	36
Atomic weight (standard)	83.798
Electron configuration	[Ar] 3d¹⁰ 4s² 4p⁶
Group, Period, Block	18, 4th period, p-block
Oxidation states	0 (principal), +2 (in KrF₂)
Phase at STP	Gas
Density (gas, 0 °C, 1 atm)	3.73 g/L
Melting point	115.79 K (−157.36 °C)
Boiling point	119.735 K (−153.415 °C)
Crystal structure (solid)	Face-centered cubic
Van der Waals radius	202 pm
Covalent radius	116 pm
First ionization energy	1350.8 kJ/mol
Electronegativity (Pauling)	— (not defined for noble gas)
Stable isotopes (with abundance)	^78Kr (0.35%), ^80Kr (2.28%), ^82Kr (11.6%), ^83Kr (11.5%), ^84Kr (57.0%), ^86Kr (17.3%)