krypton

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properties
General
Name , symbol , atomic number Krypton, Kr, 36
Element category Noble gases
Group , period , block 18 , 4 , p
Appearance colorless
CAS number 7439-90-9
EC number 231-098-5
ECHA InfoCard 100.028.271
ATC code

V09 EX01 ( 81m Kr)

Mass fraction of the earth's envelope 1.9 · 10 −5  ppm
Atomic
Atomic mass 83,798 (2) et al
Covalent radius 116 pm
Van der Waals radius 202 pm
Electron configuration [ Ar ] 3 d 10 4 s 2 4 p 6
1. Ionization energy 13.999 605 3 (20) eV ≈ 1 350.76 kJ / mol
2. Ionization energy 24.35984 (12) eV ≈ 2 350.37 kJ / mol
3. Ionization energy 35.838 (20) eV ≈ 3 457.8 kJ / mol
4. Ionization energy 50.85 (11) eV ≈ 4 906 kJ / mol
5. Ionization energy 64.69 (20) eV ≈ 6 242 kJ / mol
Physically
Physical state gaseous
Crystal structure Cubic area-centered
density 3.7491 kg m −3 at 273.15 K.
magnetism diamagnetic ( Χ m = −1.6 10 −8 )
Melting point 115.79 K (−157.36 ° C)
boiling point 121.2 K (−152 ° C)
Molar volume (solid) 27.99 10 −6 m 3 mol −1
Heat of evaporation 9.03 kJ / mol
Heat of fusion 1.64 kJ mol −1
Speed ​​of sound 1120 m · s −1
Thermal conductivity 0.00949 W m −1 K −1
Chemically
Electronegativity 3.00 ( Pauling scale )
Isotopes
isotope NH t 1/2 ZA ZE (M eV ) ZP
78 kr 0.35% 2.0 x 10 21 a ε ε 2,868 78 Se
79 kr {syn.} 35.04 h ε 1.626 79 Br
80 kr 2.25% Stable
81 kr in traces 229,000 a ε 0.281 81 Br
82 kr 11.6% Stable
83 kr 11.5% Stable
84 kr 57.0% Stable
85 kr in traces 10,756 a β - 0.687 85 Rb
86 kr 17.3% Stable
For other isotopes see list of isotopes
NMR properties
  Spin
quantum
number I 		γ in
rad · T −1 · s −1 		E r  ( 1 H) f L at
B = 4.7 T
in MHz
83 kr 9/2 −1.033 10 7 0.000219 3.848
safety instructions
GHS labeling of hazardous substances
04 - gas bottle

Caution

H and P phrases H: 280
P: 403
As far as possible and customary, SI units are used.
Unless otherwise noted, the data given apply to standard conditions .

Krypton ( ancient Greek κρυπτός kryptós "hidden") is a chemical element with the element symbol Kr and the atomic number 36. In the periodic table it is in the 8th main group, so the 18th  IUPAC group and is therefore one of the noble gases . Like the other noble gases, it is a colorless, extremely inert, monatomic gas . In many properties, such as melting and boiling points or density , it stands between the lighter argon and the heavier xenon .

Krypton is one of the rarest elements on earth and only occurs in small amounts in the atmosphere.

The noble gas was discovered in 1898 by William Ramsay and Morris William Travers through fractional distillation of liquid air. Due to its rarity, krypton is only used in small quantities, primarily as a filling gas for incandescent lamps . A small number of crypto compounds is known, of which krypton difluoride one of the strongest oxidizing agent is known.

history

William Ramsay

After 1894 argon as the first noble gas by John William Strutt, 3rd Baron Rayleigh and William Ramsay discovered and so far only from the solar spectrum known helium in 1895 by Ramsay from uranium ores was isolated, he realized from the laws of the periodic table , that there are still other such Elements. Therefore, from 1896 onwards, Ramsay first investigated various minerals and meteorites and the gases released by them when heated or dissolved. However, he and his colleague Morris William Travers were unsuccessful, only helium and, more rarely, argon were found. The investigation of hot gases from Cauterets in France and from Iceland also yielded no results.

Eventually they began to examine 15 liters of crude argon and separate them by liquefaction and fractional distillation . When they examined the residue that remained when the raw argon was almost completely evaporated, they found previously unknown yellow and green spectral lines, i.e. a new element. It was named Krypton after the ancient Greek κρυπτός kryptós , "hidden" . After purification by further distillation, Ramsay and Travers were also able to determine the molar mass of about 80 g / mol. After this discovery they were able to extract another element, neon , from another, lower-boiling fraction , and finally, by separating the raw krypton, xenon .

In 1924 A. von Antropoff claimed to have synthesized the first krypton compound in the form of a red stable solid from krypton and chlorine . However, it later turned out that this compound did not contain krypton, but nitrogen monoxide and hydrogen chloride . Greater efforts in the synthesis of krypton compounds began after the discovery of the first xenon compounds in 1962. As the first set Aristid of Grosse an encrypted connection is that he initially for krypton tetrafluoride held but which has been identified as krypton difluoride for further experiments.

The wavelength of electromagnetic radiation emitted by the krypton isotope 86 Kr was chosen as the basis for the definition of the meter in 1960. So they solved the imprecise definition over the standard meter from a platinum - iridium - alloy from. One meter was defined as 1,650,763.73 times the wavelength of the radiation emitted by the nuclide 86 Kr during the transition from the 5 d 5 to the 2 p 10 state and propagating in a vacuum . In 1983 this definition was finally replaced by a definition based on the distance that light travels in a vacuum in a specific fraction of a second.

Occurrence

Krypton is one of the rarest elements on earth. Only xenon and radioactive elements are rarer, which either, like plutonium, have largely already decayed or only occur as a short-lived intermediate product of a series of decays . The proportion of krypton in the earth's envelope is 1.9 × 10 −5  ppm, with the largest part of the gas being in the atmosphere, which consists of 1.14 ppm of krypton.

In the rest of the universe , krypton occurs in higher proportions, comparable to that of lithium , gallium and scandium . The ratio of krypton and hydrogen is largely constant in the universe. From this it can be concluded that the interstellar matter is rich in krypton. Krypton could also be detected in a white dwarf . The amount measured was 450 times that of the sun, but the reason for this high krypton content is still unknown.

Extraction

The extraction of krypton takes place exclusively from the air using the Linde process . In the case of nitrogen-oxygen separation, due to its high density, it is enriched together with xenon in the liquid oxygen that is located in the bottom of the column . This mixture is transferred to a column in which it is enriched to about 0.3% krypton and xenon. In addition to oxygen, the liquid krypton-xenon concentrate also contains a large amount of hydrocarbons such as methane , fluorinated compounds such as sulfur hexafluoride or tetrafluoromethane as well as traces of carbon dioxide and nitrous oxide . Methane and nitrous oxide can be converted into carbon dioxide, water and nitrogen via combustion on platinum or palladium catalysts at 500 ° C, which can be removed by adsorption on molecular sieves . Fluorine compounds, on the other hand, cannot be removed from the mixture in this way. In order to break them down and remove them from the mixture, the gas can be irradiated with microwaves , whereby the element-fluorine bonds break and the fluorine atoms formed can be caught in soda lime or passed over a titanium dioxide - zirconium dioxide catalyst at 750 ° C . The fluorine compounds react to form carbon dioxide and hydrogen fluoride and other compounds that can be separated off.

Then krypton and xenon are separated in a further column, which is heated at the bottom and cooled at the top. While xenon collects at the bottom, a gas flow forms at the top in which oxygen escapes from the column, and after a while krypton also escapes. The latter is freed of any remaining traces of oxygen by oxidation and collected in gas bottles.

properties

Physical Properties

Cubic closest packing of spheres of solid krypton, a  = 572 pm
When ionized in a high-voltage, high-frequency field, krypton glows

Krypton is a monatomic, colorless and odorless under normal conditions of gas , which at 121.2 K (-152 ° C) condenses and solidifies at 115.79 K (-157.36 ° C). Like the other noble gases apart from helium, krypton crystallizes in a cubic close packing of spheres with the lattice parameter a  = 572  pm .

Like all noble gases, krypton only has closed shells ( noble gas configuration ). This explains why the gas is always monatomic and the reactivity is low.

With a density of 3.749 kg / m³ at 0 ° C and 1013 hPa, krypton is heavier than air, so it sinks. In the phase diagram , the triple point is 115.76 K and 0.7315 bar, the critical point is −63.75 ° C, 5.5 MPa and a critical density of 0.909 g / cm³.

Krypton is somewhat soluble in water, a maximum of 110 ml of krypton can dissolve in one liter of water at 0 ° C.

Chemical properties

Like all noble gases, krypton is very inert. It can only react with the most electronegative element, fluorine , under special conditions, thereby forming crypton difluoride . In contrast to xenon fluorides, krypton difluoride is thermodynamically unstable, so its formation is endothermic and must take place at low temperatures. The fluorine radicals required for a reaction can be represented by exposure to UV radiation , bombardment with protons or electrical discharges.

With various compounds, krypton forms clathrates , in which the gas is physically enclosed in a cavity and thus bound. For example, water and water- chloroform mixtures form a clathrate at −78 ° C; a clathrate with hydroquinone is so stable that krypton stays in it for a long time. An inclusion compound of krypton in the oligosaccharide α-cyclodextrin is also known.

Isotopes

A total of 32 isotopes and 10 other core isomers of krypton are known. Five isotopes are stable: 80 Kr, 82 Kr, 83 Kr, 84 Kr and 86 Kr. They occur in nature together with the extremely long-lived 78 Kr ( half-life 2 · 10 21 years). The largest share of the natural isotope mixture has 84 Kr with 57%, followed by 86 Kr with 17.3%; 82 Kr occurs at 11.58% and 83 Kr at 11.49%. In contrast, the isotopes 80 Kr with 2.28% and 78 Kr with 0.35% are rare. The most long-lived of the unstable isotopes after 78 Kr is 81 Kr with a half-life of 229,000 years . It is formed in traces in the upper atmosphere through reactions of stable krypton isotopes with cosmic rays and thus also occurs naturally in the air. Due to its formation in the atmosphere and its longevity, 81 Kr is used for dating fossil groundwater .

The radioactive isotope 85 Kr with a half-life of 10.756 years is also found in traces in the atmosphere. It is formed together with other (short-lived) isotopes during the nuclear fission of uranium and plutonium . It is released into the ambient air through nuclear explosions or during the reprocessing of fuel elements and is more common in the northern hemisphere than in the southern hemisphere due to the different distribution of emission sources. After the pollution of the atmosphere with 85 Kr decreased after the end of the atmospheric nuclear weapon tests in the 1960s, it increased significantly in a measuring station in Ghent between 1979 and 1999 - caused by the reprocessing plant in La Hague .

The only stable krypton isotope is 83 Kr NMR-active . Hyperpolarized 83 Kr was used in animal experiments on rats in magnetic resonance imaging to examine the lungs .

use

Most of the krypton is used as a filling gas for incandescent lamps . By the gas, the evaporation rate of the actual filament of tungsten low, which allows higher annealing temperature. This in turn causes a higher light output from the lamp. Halogen and fluorescent lamps can also contain krypton. It is also used as a filling gas in Geiger counters , scintillation counters and electronic devices. Even in insulating glass panes, it is used as a filling gas instead of the normally used argon, despite the high price, if one wants to achieve significantly better insulation with the same pane thickness.

Together with fluorine , krypton is used in the krypton fluoride laser . This is one of the excimer lasers and has a wavelength of 248 nm in the ultraviolet spectral range. Also, noble gas ion laser with krypton, in which the active medium is mono- or multiply charged ions are krypton, are known.

Like xenon, krypton absorbs X-rays , albeit to a lesser extent . Therefore it is being investigated whether xenon-krypton mixtures can be used as contrast media in computed tomography . You could achieve a better contrast than pure xenon, because its proportion of the contrast agent is limited to a maximum of 35 percent due to the narcotic effect when used on humans.

Liquid krypton is used as a material for calorimeters in particle physics . It enables a particularly precise determination of location and energy. An example of a particle detector that uses a liquid krypton calorimeter is the NA48 experiment at CERN .

The beta-emitting 85 krypton is used for preionization in fluorescent lamp glow starters . Also Ionisation contained earlier this gas.

In space travel , krypton is used as a supporting mass in ion drives .

Krypton discharge tube.jpg
KrTube.jpg


Krypton gas discharge tubes of various designs

Biological importance

Like the other noble gases, krypton has no biological significance due to its inertia and is also non-toxic. In higher concentrations, it has a suffocating effect by displacing the oxygen. At a pressure of more than 3.9 bar it has a narcotic effect .

links

Structural formula of krypton difluoride

Only a small number of krypton compounds are known. The most important and most stable of these is krypton difluoride . It is one of the strongest known oxidizing and fluorinating agents and is able, for example, to oxidize xenon to xenon hexafluoride or iodine to iodine pentafluoride . If krypton difluoride reacts with fluoride acceptors such as antimony pentafluoride , the cations KrF + and Kr 2 F 3 + , which are the strongest known oxidizing agents, are formed.

Compounds with ligands other than fluorine are also known. These include krypton bis (pentafluororthotellurate) Kr (OTeF 5 ) 2 , the only known oxygen-krypton compound, RCNKrF + AsF 6 - (R = H, CF 3 , C 2 F 5 or nC 3 F 7 ) with a krypton -Nitrogen binding and HKrCCH, in which an ethyne ligand is attached to the krypton.

literature

Web links

Wiktionary: Krypton  - explanations of meanings, word origins, synonyms, translations
Commons : Krypton  album with pictures, videos and audio files

Individual evidence

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  3. ^ IUPAC, Standard Atomic Weights Revised 2013 .
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This article was added to the list of excellent articles on March 7, 2010 in this version .