Krypton (Kr)

Between 1960 and 1983 the metre was defined as 1,650,763.73 wavelengths of a particular orange-red line in the spectrum of krypton-86, which made this obscure gas the international standard of length for 23 years until lasers made the definition obsolete. Krypton is genuinely inert in ordinary conditions but not absolutely so — krypton difluoride exists, though it has to be made at cryogenic temperatures and falls apart above about −30 °C. It makes up about one part per million of the atmosphere.

Properties

Atomic number

36

Symbol

Kr

Standard atomic weight

83.798 u

Category

Noble gas

Group

18

Period

4

Block

p-block

Electron configuration

[Ar] 4s² 3d¹⁰ 4p⁶

Electrons per shell

2, 8, 18, 8

Melting point

115.79 K — -157.4 °C — -251.2 °F

Boiling point

119.93 K — -153.2 °C — -243.8 °F

Density

0.003733 g/cm³

Electronegativity

3 (Pauling)

Oxidation states

0, +2

Discovered

1898 by William Ramsay and Morris Travers

Electron configuration

Full

1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶

Noble gas shorthand

[Ar] 4s² 3d¹⁰ 4p⁶

Shells

K: 2 · L: 8 · M: 18 · N: 8

What it is used for

• High-performance window glazing, where it insulates better than argon
• Krypton fluoride excimer lasers in semiconductor lithography
• Photographic flash tubes and some high-intensity lamps
• Krypton-85 as a tracer for leak detection and for detecting plutonium production

Hazards

• Asphyxiant in confined spaces; denser than air

Density is the gas at 0 °C and 101.325 kPa.

Neighbours in the table

Group 18

Elements in the same group share their outer electron count, which is why they behave similarly in reactions. Krypton sits in group 18 with helium, neon, argon, xenon, radon, oganesson.