Krypton, like the other noble gases, is used in lighting andphotography. Krypton light has manyspectral lines, and kryptonplasma is useful in bright, high-powered gas lasers (kryptonion andexcimer lasers), each of which resonates and amplifies a single spectral line.Krypton fluoride also makes a usefullaser medium. From 1960 to 1983, theofficial definition of the metre was based on thewavelength of one spectral line of krypton-86, because of the high power and relative ease of operation of kryptondischarge tubes.
Krypton was discovered inBritain in 1898 byWilliam Ramsay, a Scottish chemist, andMorris Travers, an English chemist, in residue left from evaporating nearly all components ofliquid air.Neon was discovered by a similar procedure by the same workers just a few weeks later.[12] William Ramsay was awarded the 1904Nobel Prize in Chemistry for discovery of a series ofnoble gases, including krypton.[13]
In 1960, theInternational Bureau of Weights and Measures defined the meter as 1,650,763.73wavelengths of light emitted in the vacuum corresponding to the transition between the 2p10 and 5d5 levels in theisotope krypton-86.[14][15] This agreement replaced the 1889international prototype meter, which was a metal bar located inSèvres. This also made obsolete the 1927 definition of theångström based on the redcadmium spectral line,[16] replacing it with 1 Å = 10−10 m. The krypton-86 definition lasted until the October 1983 conference, which redefined the meter as the distance that light travels invacuum during 1/299,792,458 s.[17][18][19]
Characteristics
Krypton is characterized by several sharp emission lines (spectral signatures) the strongest being green and yellow.[20] Krypton is one of the products ofuraniumfission.[21] Solid krypton is white and has a face-centeredcubiccrystal structure, which is a common property of all noble gases (excepthelium, which has a hexagonal close-packed crystal structure).[22]
Naturally occurring krypton in Earth's atmosphere is composed of fivestableisotopes, plus one isotope (78Kr) with such a longhalf-life (9.2×1021 years) that it can be considered stable. (This isotope has the third-longest known half-life among all isotopes for which decay has been observed; it undergoesdouble electron capture to78Se).[11][23] In addition, about thirty unstable isotopes andisomers are known.[24] Traces of81Kr, acosmogenic nuclide produced by thecosmic ray irradiation of80Kr, also occur in nature: thisisotope isradioactive with a half-life of 230,000 years. Krypton is highly volatile and does not stay in solution in near-surface water, but81Kr has been used fordating old (50,000–800,000 years)groundwater.[25]
Like the other noble gases, krypton is chemically highly unreactive. The rather restricted chemistry of krypton in the +2 oxidation state parallels that of the neighboring elementbromine in the +1 oxidation state; due to thescandide contraction it is difficult to oxidize the 4p elements to their group oxidation states. Until the 1960s no noble gas compounds had been synthesized.[27]
Following the first successful synthesis ofxenon compounds in 1962, synthesis ofkrypton difluoride (KrF 2) was reported in 1963. In the same year,KrF 4 was reported by Grosse,et al.,[28] but was subsequently shown to be a mistaken identification.[29] Under extreme conditions, krypton reacts with fluorine to form KrF2 according to the following equation:
Krypton gas in akrypton fluoride laser absorbs energy from a source, causing the krypton to react with fluorine gas, producing the exciplex krypton fluoride, a temporarycomplex in an excited energy state:[30]
The complex can undergo spontaneous or stimulated emission, reducing its energy state to a metastable, but highlyrepulsive ground state. The ground state complex quickly dissociates into unbound atoms:
The result is anexciplex laser which radiates energy at 248 nm, near theultraviolet portion of thespectrum, corresponding with the energy difference between the ground state and the excited state of the complex.[31]
Kr(H2)4 and H2 solids formed in adiamond anvil cell[32]Structure of Kr(H2)4. Krypton octahedra (green) are surrounded by randomly oriented hydrogen molecules.[32]
Compounds with krypton bonded to atoms other thanfluorine have also been discovered. There are also unverified reports of abariumsalt of a kryptonoxoacid.[33]ArKr+ and KrH+polyatomic ions have been investigated and there is evidence for KrXe or KrXe+.[34]
The reaction ofKrF 2 withB(OTeF 5) 3 produces an unstable compound,Kr(OTeF 5) 2, that contains a krypton-oxygen bond. A krypton-nitrogen bond is found in thecation [HC≡N–Kr–F]+ , produced by the reaction ofKrF 2 with [HC≡NH]+ [AsF− 6] below −50 °C.[35][36] HKrCN and HKrC≡CH (krypton hydride-cyanide and hydrokryptoacetylene) were reported to be stable up to 40K.[27]
Kryptonhydride (Kr(H2)4) crystals can be grown at pressures above 5 GPa. They have a face-centered cubic structure where krypton octahedra are surrounded by randomly oriented hydrogen molecules.[32]
Natural occurrence
Earth has retained all of the noble gases that were present at its formation excepthelium. Krypton's concentration in theatmosphere is about 1 ppm. It can be extracted from liquid air byfractional distillation.[37] The amount of krypton in space is uncertain, because measurement is derived from meteoric activity and solar winds. The first measurements suggest an abundance of krypton in space.[38]
Applications
Krypton gas discharge tube
Krypton's multiple emission lines make ionized krypton gas discharges appear whitish, which in turn makes krypton-based bulbs useful in photography as a white light source. Krypton is used in some photographic flashes for high speedphotography. Krypton gas is also combined with mercury to make luminous signs that glow with a bright greenish-blue light.[39]
Krypton is mixed withargon in energy efficient fluorescent lamps, reducing the power consumption, but also reducing the light output and raising the cost.[40] Krypton costs about 100 times as much as argon. Krypton (along with xenon) is also used to fill incandescent lamps to reduce filament evaporation and allow higheroperating temperatures.[41]
Krypton's white discharge is sometimes used as an artistic effect in gas discharge "neon" tubes. Krypton produces much higher light power than neon in the red spectral line region, and for this reason, red lasers for high-power laser light-shows are often krypton lasers with mirrors that select the red spectral line for laser amplification and emission, rather than the more familiar helium-neon variety, which could not achieve the same multi-watt outputs.[42]
Thekrypton fluoride laser is important in nuclear fusion energy research in confinement experiments. Thelaser has high beam uniformity, shortwavelength, and the spot size can be varied to track an imploding pellet.[43]
In experimentalparticle physics, liquid krypton is used to construct quasi-homogeneous electromagneticcalorimeters. A notable example is the calorimeter of theNA48 experiment atCERN containing about 27tonnes of liquid krypton. This usage is rare, since liquidargon is less expensive. The advantage of krypton is a smallerMolière radius of 4.7 cm, which provides excellent spatial resolution with little overlapping. The other parameters relevant for calorimetry are:radiation length of X0=4.7 cm, and density of 2.4 g/cm3.
Krypton-83 has application inmagnetic resonance imaging (MRI) for imaging airways. In particular, it enables the radiologist to distinguish betweenhydrophobic and hydrophilic surfaces containing an airway.[44]
Although xenon has potential for use incomputed tomography (CT) to assess regional ventilation, its anesthetic properties limit its fraction in the breathing gas to 35%. A breathing mixture of 30% xenon and 30% krypton is comparable in effectiveness for CT to a 40% xenon fraction, while avoiding the unwanted effects of a high partial pressure of xenon gas.[45] Themetastable isotope krypton-81m is used innuclear medicine for lungventilation/perfusion scans, where it is inhaled and imaged with agamma camera.[46] Krypton-85 in the atmosphere has been used to detect clandestine nuclear fuel reprocessing facilities inNorth Korea[47] andPakistan.[48] Those facilities were detected in the early 2000s and were believed to be producing weapons-grade plutonium. Krypton-85 is a medium livedfission product and thus escapes fromspent fuel when the cladding is removed.[49]
Krypton compared to other anaesthetic gases (minimum alveolar concentration is an inverse indicator of potency)
Krypton is considered to be a non-toxicasphyxiant.[52] Beinglipophilic, krypton has a significant anaesthetic effect (although the mechanism of this phenomenon is stillnot fully clear,[53] there is good evidence that the two properties are mechanistically related), withnarcotic potency seven times greater than air, and breathing an atmosphere of 50% krypton and 50% natural air (as might happen in the locality of a leak) causesnarcosis in humans similar to breathing air at four times atmospheric pressure. This is comparable to scuba diving at a depth of 30 m (100 ft) and could affect anyone breathing it.
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^Chon, D; Beck, KC; Simon, BA; Shikata, H; et al. (2007). "Effect of low-xenon and krypton supplementation on signal/noise of regional CT-based ventilation measurements".Journal of Applied Physiology.102 (4):1535–44.doi:10.1152/japplphysiol.01235.2005.PMID17122371.
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