Krypton

Krypton (pronounced /ˈkrɪptən/ or /ˈkrɪptɒn/) is a chemical element with the symbol Kr and atomic number 36. A colorless, odorless, tasteless noble gas, krypton occurs in trace amounts in the atmosphere, is isolated by fractionally distilling liquified air, and is often used with other rare gases in fluorescent lamps. Krypton is inert for most practical purposes, but it is known to form compounds with fluorine. Krypton can also form clathrates with water when atoms of it are trapped in a lattice of the water molecules.

From 1960 to 1983, the distance of the meter was defined in terms of the orange-red spectral line of krypton-86, an isotope of krypton. It, as well as all other noble gases, can be used in lighting and photography. Krypton has an important role in production and usage of the krypton fluoride laser.

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Physical properties

Krypton is characterized by a brilliant green and orange spectral signature. It is one of the products of uranium fission.^[3] Solidified krypton is white and crystalline with a face-centered cubic crystal structure, which is a common property of all noble gases. The original name of krypton is "Hidden One." The melting point of krypton is -157.2 degrees Celsius, and its boiling point is -152.9 degrees Celsius.

History

Krypton (Greek κρυπτόν, kryptos meaning "hidden") was discovered in Great Britain in 1898 by Sir William Ramsay and Morris Travers in residue left from evaporating nearly all components of liquid air^[4]. William Ramsay was awarded the 1904 Nobel Prize in Chemistry for discovery of a series of noble gases, including krypton.

Metric role

In 1960, an international agreement defined the meter in terms of wavelength of light emitted by the krypton-86 isotope. This agreement replaced the longstanding standard meter located in Paris, which was a metal bar made of a platinum-iridium alloy (the bar was originally estimated to be one ten-millionth of a quadrant of the earth's polar circumference), and was itself replaced by a definition based on the speed of light — a fundamental physical constant. In October 1983, the Bureau International des Poids et Mesures (International Bureau of Weights and Measures) defined the meter as the distance that light travels in a vacuum during 1/299,792,458 s.^[5]

Occurrence

The concentration of krypton in earth's atmosphere is about 1 ppm. It can be extracted from liquid air by fractional distillation.^[6] The amount of krypton in space is uncertain, as is the amount is derived from the meteoritic activity and that from solar winds. The first measurements suggest an overabundance of krypton in space.^[7]

Compounds

Like the other noble gases, krypton is chemically inert. However, following the first successful synthesis of xenon compounds in 1962, synthesis of krypton difluoride was reported in 1963.^[8] Other fluorides and a salt of a krypton oxoacid have also been found. ArKr+ and KrH+ molecule-ions have been investigated and there is evidence for KrXe or KrXe+.^[9]

At the University of Helsinki in Finland, HKrCN and HKrCCH (krypton hydride-cyanide and hydrokryptoacetylene) were synthesized and determined to be stable up to 40K (M. Räsänen et al.).^[8]

If the kryptonite found in Superman stories followed the naming conventions of chemical compounds, it would be an oxyanion of krypton. Krypton cannot form an oxyanion.

Isotopes

Main article: isotopes of krypton

There are 31 known isotopes of krypton.^[10] Naturally occurring krypton is made of five stable and one slightly radioactive isotope. Its spectral signature can be produced with some very sharp lines. ⁸¹Kr, the product of atmospheric reactions is produced with the other naturally occurring isotopes of krypton. Being radioactive it has a half-life of 250,000 years. Krypton is highly volatile when it is near surface waters but ⁸¹Kr has been used for dating old (50,000 - 800,000 year) groundwater.^[11]

⁸⁵Kr is an inert radioactive noble gas with a half-life of 10.76 years. It is produced by the fission of uranium and plutonium, such as in nuclear bomb testing and nuclear reactors. ⁸⁵Kr is released during the reprocessing of fuel rods from nuclear reactors. Concentrations at the North Pole are 30% higher than at the South Pole as most nuclear reactors are in the northern hemisphere.^[12]

Applications

Krypton's multiple emission lines make ionized krypton gas discharges appear white, which in turn makes krypton-based bulbs useful in photography as a brilliant white light source. Krypton is thus used in some types of photographic flashes used in high speed photography. Fluorescent light bulbs are filled with a mixture of krypton and argon gases. Krypton gas is also combined with other gases to make luminous signs that glow with a bright greenish-yellow light.^[13]

Krypton's white discharge is often used to good effect in colored gas discharge tubes, which are then simply painted or stained in other ways to allow the desired color (for example, "neon" type advertising signs where the letters appear in differing colors, are often entirely krypton-based). Krypton is also capable of much higher light power density than neon in the red spectral line region, and for this reason, red lasers for high power laser light shows are krypton lasers with mirrors which select out the red spectral line for laser amplification and emission, rather than the more familiar helium-neon variety, which could never practically achieve the multi-watt red laser light outputs needed for this application.^[14]

Krypton has an important role in production and usage of the krypton fluoride laser. The laser has been important in the nuclear fusion energy research community in confinement experiments. The laser has high beam uniformity, short wavelength, and the ability to modify the spot size to track an imploding pellet.^[15]

In experimental particle physics, liquid krypton is used to construct quasi-homogenious electromagnetic calorimeters. A notable example is the calorimeter of the NA48 experiment at CERN containing about 27 tons of liquid krypton. This usage is rare, since the cheaper liquid argon is typically used. The advantage of krypton over agron is a small Molière radius of 4.7cm, which allows for excellent spatial resolution and low degree of overlapping. The other parameters relevant for calorimetry application are: radiation length of X₀ = 4.7cm, density of 2.4g/cm³.

References

Further reading

• Los Alamos National Laboratory - Krypton
• "Chemical Elements: From Carbon to Krypton" By: David Newton & Lawrence W. Baker
• "Krypton 85: a Review of the Literature and an Analysis of Radiation Hazards" By: William P. Kirk