|Name, Symbol, Number||krypton, Kr, 36|
|Chemical series||noble gases|
|Group, Period, Block||18, 4, p|
|Atomic mass||83.798(2) g/mol|
|Electron configuration||[Ar] 3d10 4s2 4p6|
|Electrons per shell||2, 8, 18, 8|
|Density||(0 °C, 101.325 kPa)
|Melting point||115.79 K
(-157.36 °C, -251.25 °F)
|Boiling point||119.93 K
(-153.22 °C, -243.8 °F)
|Critical point||209.41 K, 5.50 MPa|
|Heat of fusion||1.64 kJ/mol|
|Heat of vaporization||9.08 kJ/mol|
|Heat capacity||(25 °C) 20.786 J/(mol·K)|
|Crystal structure||cubic face centered|
|Electronegativity||3.00 (Pauling scale)|
|1st: 1350.8 kJ/mol|
|2nd: 2350.4 kJ/mol|
|3rd: 3565 kJ/mol|
|Atomic radius (calc.)||88 pm|
|Covalent radius||110 pm|
|Van der Waals radius||202 pm|
|Thermal conductivity||(300 K) 9.43 mW/(m·K)|
|Speed of sound||(gas, 23 °C) 220 m/s|
|Speed of sound||(liquid) 1120 m/s|
|CAS registry number||7439-90-9|
Krypton (chemical symbol Kr, atomic number 36) is a colorless, odorless, tasteless noble gas. It occurs in trace amounts in the atmosphere and is isolated by fractionating liquefied air. Krypton is inert for most practical purposes, but it is known to form compounds with fluorine. Krypton can also form "clathrates" (cage-like molecules) with water, when atoms of the element are trapped in a lattice of water molecules. It is often used with other rare gases in fluorescent lamps.
Krypton (Greek κρυπτός, meaning "hidden") was discovered in Great Britain in 1898 by Sir William Ramsay and Morris Travers. They found it in the residue that remained after evaporating nearly all components of liquid air.
In 1960, an international agreement defined the meter in terms of light emitted from a krypton isotope. This agreement replaced the longstanding standard meter located in Paris, which was a metal bar made of platinum-iridium alloy. (The bar was originally estimated to be one ten-millionth of a quadrant of the Earth's polar circumference.) Just 23 years later, the krypton-based standard was replaced by a measure of the speed of light—the most reliable constant in the universe. 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 seconds.
Krypton is a member of the noble gas series in the periodic table. As such, it is an extremely unreactive element. It is situated between argon and xenon in group 18 (former group 8A), and is placed after bromine in period four. Its melting point is 156.6°C, and its boiling point is 152.3°C.
This element is characterized by a brilliant green and orange spectral signature. It is one of the products of the nuclear fission of uranium. Solidified krypton is white and crystalline, with a face-centered cubic structure, which is a common property of all "rare gases."
Naturally occurring krypton is made up of five stable and one slightly radioactive isotope. Krypton's spectral signature is easily produced with some very sharp lines. Kr-81 is the product of atmospheric reactions with the other naturally occurring isotopes of krypton. It is radioactive, with a half-life of 250,000 years. Like xenon, krypton is highly volatile when it is near surface waters, and Kr-81 has therefore been used for dating old (50,000 - 800,000 year) groundwater.
Kr-85, a radioactive isotope with a half-life of 10.76 years, is produced by the nuclear fission of uranium and plutonium. It has been produced by nuclear bomb tests, nuclear reactors, and the release of Kr-85 during the reprocessing of fuel rods from nuclear reactors. A strong gradient exists between the northern and southern hemispheres, where concentrations at the North Pole are approximately 30 percent higher than at the South Pole, because most Kr-85 is produced in the northern hemisphere, and north-south atmospheric mixing is relatively slow.
Although krypton is extremely unreactive, a few compounds of the element have been prepared. Following the first successful synthesis of xenon compounds in 1962, synthesis of krypton difluoride was reported in 1963. Other fluorides and a salt of a krypton oxoacid have also been made. ArKr+ and KrH+ molecular ions have been investigated, and there is evidence for KrXe or KrXe+.
At the University of Helsinki in Finland, HKrCN and HKrCCH (krypton hydride-cyanide and hydrokryptoacetylene) were synthesized and determined to be stable up to a temperature of 40K.
One major use of krypton is the krypton fluoride laser. Certain amounts of energy are added to force krypton gas to react with fluorine gas to produce the KrF excited state complex. Once the energy supply stops, the compound decomposes. During the decomposition process, the excess energy stored in the excited state complex is emitted in the form of strong ultraviolet laser radiation.
All links retrieved April 25, 2018.
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