Kr

Krypton (symbol Kr) is a chemical element in the noble gases group, with atomic number 36. It is a colorless, odorless, monatomic gas at room temperature and pressure, and it is chemically inert under most conditions. In the Earth's atmosphere, krypton is present only at trace levels, typically measured in parts per million or smaller.

Krypton was discovered in 1898 by chemists Sir William Ramsay and Morris Travers during the liquefaction and analysis of air. Its name derives from the Greek kryptos, meaning hidden or concealed, reflecting its rarity in ordinary materials and the fact that it was found as a trace constituent of air.

Key physical properties include a relatively high density for a gas, a boiling point of about −153 °C, and a melting point around −157 °C. Krypton is typically obtained commercially through the fractional distillation of liquefied air. As a noble gas, it is highly unreactive, though under extreme electrical, chemical, or photochemical conditions a small number of krypton compounds, such as krypton difluoride (KrF2), can be formed.

Applications of krypton include lighting and signage, where krypton–argon mixtures are used in certain fluorescent and high-intensity discharge lamps. Krypton gas is also employed to improve thermal insulation in some energy-efficient window units. In addition, certain krypton isotopes are used in scientific and industrial contexts: Kr-85 is a radioactive tracer produced by nuclear fission, and isotopes like Kr-81 have been used in environmental dating and tracing groundwater movement.

Safety considerations are modest; krypton is non-toxic and inert, but as with any non-oxygen gas, high concentrations can displace air and pose asphyxiation risks in confined spaces.