Krypton

This article is about the chemical element. For other uses, see Krypton (disambiguation).

Krypton, ₃₆Kr

A krypton-filled discharge tube glowing white
Krypton
Pronunciation	/ˈkrɪptɒn/ ​(KRIP-ton)
Appearance	colorless gas, exhibiting a whitish glow in an electric field
Standard atomic weight Ar°(Kr)
	83.798±0.002 83.798±0.002 (abridged)
Krypton in the periodic table
Hydrogen Helium Lithium Beryllium Boron Carbon Nitrogen Oxygen Fluorine Neon Sodium Magnesium Aluminium Silicon Phosphorus Sulfur Chlorine Argon Potassium Calcium Scandium Titanium Vanadium Chromium Manganese Iron Cobalt Nickel Copper Zinc Gallium Germanium Arsenic Selenium Bromine Krypton Rubidium Strontium Yttrium Zirconium Niobium Molybdenum Technetium Ruthenium Rhodium Palladium Silver Cadmium Indium Tin Antimony Tellurium Iodine Xenon Caesium Barium Lanthanum Cerium Praseodymium Neodymium Promethium Samarium Europium Gadolinium Terbium Dysprosium Holmium Erbium Thulium Ytterbium Lutetium Hafnium Tantalum Tungsten Rhenium Osmium Iridium Platinum Gold Mercury (element) Thallium Lead Bismuth Polonium Astatine Radon Francium Radium Actinium Thorium Protactinium Uranium Neptunium Plutonium Americium Curium Berkelium Californium Einsteinium Fermium Mendelevium Nobelium Lawrencium Rutherfordium Dubnium Seaborgium Bohrium Hassium Meitnerium Darmstadtium Roentgenium Copernicium Nihonium Flerovium Moscovium Livermorium Tennessine Oganesson Ar ↑ Kr ↓ Xe bromine ← krypton → rubidium
Atomic number (Z)	36
Group	group 18 (noble gases)
Period	period 4
Block	p-block
Electron configuration	[Ar] 3d10 4s2 4p6
Electrons per shell	2, 8, 18, 8
Physical properties
Phase at STP	gas
Melting point	115.78 K ​(−157.37 °C, ​−251.27 °F)
Boiling point	119.93 K ​(−153.415 °C, ​−244.147 °F)
Density (at STP)	3.749 g/L
when liquid (at b.p.)	2.413 g/cm3
Triple point	115.775 K, ​73.53 kPa
Critical point	209.48 K, 5.525 MPa
Heat of fusion	1.64 kJ/mol
Heat of vaporization	9.08 kJ/mol
Molar heat capacity	20.95 J/(mol·K)
Vapor pressure P (Pa) 1 10 100 1 k 10 k 100 k at T (K) 59 65 74 84 99 120
Atomic properties
Oxidation states	0, +1, +2 (rarely more than 0; oxide is unknown)
Electronegativity	Pauling scale: 3.00
Ionization energies	1st: 1350.8 kJ/mol 2nd: 2350.4 kJ/mol 3rd: 3565 kJ/mol
Covalent radius	116±4 pm
Van der Waals radius	202 pm
Spectral lines of krypton
Other properties
Natural occurrence	primordial
Crystal structure	​face-centered cubic (fcc) (cF4)
Lattice constant	a = 583.57 pm (at triple point: 115.78 K)
Thermal conductivity	9.43×10−3  W/(m⋅K)
Magnetic ordering	diamagnetic
Molar magnetic susceptibility	−28.8×10−6 cm3/mol (298 K)
Speed of sound	(gas, 20 °C) 221 m·s−1 (liquid) 1120 m/s
CAS Number	7439-90-9
History
Discovery and first isolation	William Ramsay and Morris Travers (1898)
Isotopes of krypton
Main isotopes Decay abun­dance half-life (t1/2) mode pro­duct 78Kr 0.360% 9.2×1021 y εε 78Se 79Kr synth 35 h ε 79Br β+ 79Br γ – 80Kr 2.29% stable 81Kr trace 2.3×105 y ε 81Br 81mKr synth 13.10 s IT 81Kr ε 81Br 82Kr 11.6% stable 83Kr 11.5% stable 84Kr 57.0% stable 85Kr trace 11 y β− 85Rb 86Kr 17.3% stable

Krypton (from [κρυπτός kryptos] Error: {{Lang-xx}}: text has italic markup (help) "the hidden one") is a chemical element with symbol Kr and atomic number 36. It is a member of group 18 (noble gases) elements. 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.

Krypton, like the other noble gases, can be used in lighting and photography. Krypton light has a large number of spectral lines, and krypton's high light output in plasmas allows it to play an important role in many high-powered gas lasers (krypton ion and excimer lasers), which pick out one of the many spectral lines to amplify. There is also a specific krypton fluoride laser. The high power and relative ease of operation of krypton discharge tubes caused (from 1960 to 1983) the official length of a meter to be defined in terms of the 605 nm (red-orange) spectral line of krypton-86.

History

Sir William Ramsay, the discoverer of krypton

Krypton was discovered in Britain in 1898 by Sir William Ramsay, a Scottish chemist, and Morris Travers, an English chemist, in residue left from evaporating nearly all components of liquid air. Neon was discovered by a similar procedure by the same workers just a few weeks later. William Ramsay was awarded the 1904 Nobel Prize in Chemistry for discovery of a series of noble gases, including krypton.

In 1960, an international agreement defined the meter in terms of wavelength of light emitted by the krypton-86 isotope (wavelength of 605.78 nanometers). 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. However, in 1927, the International Conference on Weights and Measures had redefined the meter in terms of a red cadmium spectral line (1 m = 1,553,164.13 wavelengths). In October 1983, the same bureau defined the meter as the distance that light travels in a vacuum during 1/299,792,458 s.

Characteristics

Krypton is characterized by several sharp emission lines (spectral signatures) the strongest being green and yellow. It is one of the products of uranium fission. Solidified krypton is white and crystalline with a face-centered cubic crystal structure, which is a common property of all noble gases (except helium, with a hexagonal close-packed crystal structure).

Isotopes

Main article: Isotopes of krypton

Naturally occurring krypton is made of six stable isotopes. In addition, about thirty unstable isotopes and isomers are known. ⁸¹Kr, the product of atmospheric reactions, is produced with the other naturally occurring isotopes of krypton. Being radioactive, it has a half-life of 230,000 years. Krypton is highly volatile when it is near surface waters but ⁸¹Kr has been used for dating old (50,000–800,000 years) groundwater.

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 due to convective mixing.

Chemistry

Like the other noble gases, krypton is chemically unreactive. However, following the first successful synthesis of xenon compounds in 1962, synthesis of krypton difluoride (KrF
₂) was reported in 1963. In the same year, KrF
₄ was reported by Grosse, et al., but was subsequently shown to be a mistaken identification. There are also unverified reports of a barium salt of a krypton oxoacid. ArKr⁺ and KrH⁺ polyatomic ions have been investigated and there is evidence for KrXe or KrXe⁺.

Compounds with krypton bonded to atoms other than fluorine have also been discovered. The reaction of KrF
₂ with B(OTeF
₅)
₃ produces an unstable compound, Kr(OTeF
₅)
₂, that contains a krypton-oxygen bond. A krypton-nitrogen bond is found in the cation [HC≡N–Kr–F]⁺
, produced by the reaction of KrF
₂ with [HC≡NH]⁺
[AsF⁻
₆] below −50 °C. HKrCN and HKrC≡CH (krypton hydride-cyanide and hydrokryptoacetylene) were reported to be stable up to 40 K.

Natural occurrence

The Earth has retained all of the noble gases that were present at its formation except for helium. Krypton's concentration in the atmosphere is about 1 ppm. It can be extracted from liquid air by fractional distillation. The amount of krypton in space is uncertain, as the amount is derived from the meteoric activity and that from solar winds. The first measurements suggest an overabundance of krypton in space.

Applications

Krypton gas discharge tube

Krypton discharge (spectrum) 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 brilliant white light source. Krypton is thus used in some types of photographic flashes used in high speed photography. Krypton gas is also combined with other gases to make luminous signs that glow with a bright greenish-yellow light.

Krypton mixes with argon as the fill gas of energy saving fluorescent lamps. This reduces their power consumption. Unfortunately this also reduces their light output and raises their cost. 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 higher operating temperatures to be used for the filament. A brighter light results which contains more blue than conventional lamps.

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 often 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.

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.

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

The sealed spark gap assemblies contained in ignition exciters used in some older jet engines contain a very small amount of Krypton-85 to obtain consistent ionization levels and uniform operation.

Krypton-83 has application in magnetic resonance imaging (MRI) for imaging airways. In particular, it may be used to distinguish between hydrophobic and hydrophilic surfaces containing an airway.

Although xenon has potential for use in computed tomography (CT) to assess regional ventilation, its anesthetic properties limit its fraction in the breathing gas to 35%. The use of a breathing mixture containing 30% xenon and 30% krypton is comparable in effectiveness for CT to a 40% xenon fraction, while avoiding the unwanted effects of a high fraction xenon gas.

Precautions

Krypton is considered to be a non-toxic asphyxiant. Krypton has a narcotic potency seven times greater than air, so breathing a gas containing 50% krypton and 50% air would cause narcosis similar to breathing air at four times atmospheric pressure. This would be comparable to scuba diving at a depth of 30 m (100 ft) (see nitrogen narcosis) and potentially could affect anyone breathing it. Nevertheless, that mixture would contain only 10% oxygen and hypoxia would be a greater concern.

Further reading

• William P. Kirk "Krypton 85: a Review of the Literature and an Analysis of Radiation Hazards", Environmental Protection Agency, Office of Research and Monitoring, Washington (1972)

External links

• Krypton Fluoride Lasers, Plasma Physics Division Naval Research Laboratory