Krypton Kr

Discovery

In 1898 the British chemists William Ramsay and Morris Travers let liquid air evaporate slowly and examined the small residue that remained with a spectroscope. They saw brilliant yellow and green lines that belonged to no known element and announced the discovery of krypton.

In the same year the pair used similar methods to find neon and xenon, filling three places in the new column of noble gases within a few weeks of one another. Ramsay received the 1904 Nobel Prize in Chemistry for his discovery of the noble gases.

The noble gases were long thought incapable of forming compounds. After xenon compounds were made in 1962, krypton difluoride (KrF₂) was prepared around 1963, showing that krypton, too, could be coaxed into chemical bonds.

Origin of the name

The name comes from the Greek kryptos, "hidden", because the gas had lain concealed in air. The symbol is Kr. Superman's fictional home planet shares the name but has nothing to do with the element.

Main uses

• Insulating glass: krypton conducts heat less than air or argon and is used to fill the narrow gaps in high-performance triple glazing.
• Lighting: in incandescent lamps it slows filament evaporation, improving brightness and life; it has also been used in airport runway lights and photographic flashes.
• Lasers: krypton fluoride excimer lasers emit ultraviolet light at 248 nm and have been used in photolithography to pattern semiconductor chips.
• Space propulsion: some satellite Hall-effect thrusters use krypton as a cheaper alternative to xenon.

Isotopes

Natural krypton contains six isotopes, the most abundant being krypton-84 (about 57%); krypton-78 decays so slowly that it is effectively stable. Radioactive krypton-85, with a half-life of about 10.7 years, is produced by nuclear fission, and its concentration in air is monitored to detect the reprocessing of spent nuclear fuel. Krypton-81, formed by cosmic rays in the atmosphere, has a half-life of about 230,000 years and is used to date very old groundwater and ice.

In everyday life

Krypton is obtained as a by-product of the cryogenic distillation of air, when oxygen and nitrogen are separated. Because it is so scarce in air, it is relatively expensive.

It is chemically inert and non-toxic, but like any inert gas it can displace oxygen in a confined space and cause asphyxiation. From 1960 to 1983 the metre was defined as a fixed number of wavelengths of the orange-red light emitted by krypton-86; today the metre is defined through the speed of light.