Krypton

group 18 · period 4 · p-block · noble gas

Identity

Name and symbol: Krypton, Kr
Atomic number: 36 protons
Position: group 18 · period 4 · p-block · noble gas
CAS number: 7439-90-9

Atomic structure

Atomic mass: 83.798 u
Electron configuration: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶
[Ar] 4s²³d¹⁰⁴p⁶
Electrons per shell: 2, 8, 18, 8
Valence electrons: 8 outer shell

isotope	mass (u)	abundance
⁷⁸Kr	77.920 366(2)	0.3 %
⁸⁰Kr	79.916 378(5)	2.2 %
⁸²Kr	81.913 481 15(4)	11.5 %
⁸³Kr	82.914 126 52(6)	11.5 %
⁸⁴Kr	83.911 497 73(3)	56.9 %
⁸⁶Kr	85.910 610 63(3)	17.2 %

natural isotopic composition, mole fraction

Physical properties

State at room temperature: Gas
Melting point: 115.79 K (-157.36 °C)
Boiling point: 119.93 K (-153.22 °C)
Density: 0.0037 g/cm3 (gas at STP, so 3.733 g/L)
Appearance: colorless gas, exhibiting a whitish glow in an electric field
Thermal conductivity: 9.43×10-3 W/(m·K)
Electrical resistivity: not in sources
Electrical conductivity: not in sources
Crystal structure: face-centered cubic
Molar heat capacity: not in sources

Chemical properties

Oxidation states: 0
Electronegativity: 3 (Pauling Scale)
Ionisation energy: 14 eV
1st 1,350.8, 2nd 2,350.4, 3rd 3,565 kJ/mol
Electron affinity: 0 eV
Atomic radius: empirical 116, covalent 116, van der Waals 202 pm
Ionic radius: not in sources

Reactivity: A noble gas with a closed-shell configuration, chemically almost inert; it forms a few compounds only under forcing conditions with fluorine, and its +2 chemistry parallels bromine's +1.
with water: Does not react; it dissolves only slightly and can be trapped in clathrates.
with oxygen, air: Does not react with oxygen.
with acids: Does not react with acids.
with halogens: Reacts only with fluorine, under electrical discharge or irradiation at low temperature, to krypton difluoride: $\ce{Kr + F2 -> KrF2}$ .

Typical compounds

KrF₂ krypton difluoride the only well-established binary compound, first made 1963

Occurrence, production and use

Crustal abundance: 1×10-4 milligrams per kilogram
Oceanic abundance: 2.1×10-4 milligrams per liter

Occurrence and sources

Krypton is present in the air to the extent of about 1 ppm. The atmosphere of Mars has been found to contain 0.3 ppm of krypton. Solid krypton is a white crystalline substance with a face-centered cubic structure which is common to all the "rare gases."

krypton gas the atmosphere at about 1 part per million by volume; 0.3 parts per million in the atmosphere of Mars

Extraction, production: Cryogenic distillation of air
Air is liquefied and fractionally distilled; krypton and xenon concentrate in the residue after the more volatile components boil away. A physical separation, so no reaction; the high cost of recovery limits practical applications.

Uses

The high cost of obtaining krypton from the air has limited its practical applications. Krypton is used in some types of photographic flashes used in high speed photography. Some 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 greenish-yellow light. In 1960, the length of the meter was defined in terms of the orange-red spectral line of krypton-86, an isotope of krypton.

Once thought to be completely inert, krypton is known to form a few compounds. Krypton difluoride (KrF2) is the easiest krypton compound to make and gram amounts of it have been produced.

For those that are curious, pictures of krypton gas and krypton plasma can be found in the Questions and Answers section of this site.

Krypton clathrates are prepared using hydroquinone and phenol. 85Kr can be used for chemical analysis by imbedding the isotope in various solids. During this process, kryptonates are formed. Kryptonate activity is sensitive to chemical reactions at the solution surface. Estimates of the concentration of reactants are therefore made possible. Krypton is used in certain photographic flash lamps for high-speed photography.

Lighting and lasers: filling gas, with argon, for energy saving fluorescent lamps and greenish yellow luminous signs; flash lamps for high speed photography; krypton fluoride excimer lasers

Safety, toxicity

GHS classification, signal word Warning

H280 Contains gas under pressure; may explode if heated Gases under pressure
H281 Contains refrigerated gas; may cause cryogenic burns or injury Gases under pressure

Discovery and name

Discovered by: William Ramsay and Morris Travers
Discovered: 1898
First isolated: not in sources
Named by: not in sources
Origin of the name: from Greek κρυπτός, 'hidden'

Krypton is a "noble" gas. It is characterized by its brilliant green and orange spectral lines.

Conventions

Valence electrons are counted by the usual convention: the outer shell for s- and p-block elements, ns and (n-1)d for the d-block, ns, (n-1)d and (n-2)f for the f-block. Lanthanides and actinides are placed in the f-block with no group number. Electrical conductivity is the reciprocal of the printed resistivity. Ionic radii are Shannon effective radii, six-coordinate unless noted. Where a field reads “not in sources” the value was not found; it is a gap, not a zero. Regulatory limits are the published values and change often, so check the standard in force at your site and the numbers written into your own permit before you design to them.

Data

Element records, isotopes, radii and the descriptive text come from PubChem (NCBI), the Los Alamos National Laboratory periodic table, IUPAC CIAAW and the IAEA Atomic Mass Data Center. Appearance, thermal conductivity, electrical resistivity, crystal structure, discovery and the origin of each name come from Wikipedia and Periodic-Table-JSON, used under CC BY-SA 4.0. Hazard classifications come from the ECHA C&L inventory via PubChem. Ionic radii follow R. D. Shannon (1976). The water chapters cite their own sources at the foot of each entry, and are written to the level of Snoeyink and Jenkins, Stumm and Morgan, MWH's Water Treatment and Metcalf and Eddy.