Science / Periodic Table of Elements / Krypton (Kr): Atomic number: 36, Atomic mass: 83.80 g.mol -1, Density: 3.73 10-3 g.cm-3 at 20°C, Melting point: - 157 °C, Boiling point: - 153° C, Vanderwaals radius: 0.197 nm, Isotopes: 15, Electronic shell: [ Ar ] 3d10 4s2 4p6, Energy of first ionisation: 1351 kJ.mol -1, Energy of second ionisation: 2350.4 kJ.mol -1, Energy of third ionisation: 3565 kJ.mol -1, Discovered by: Sir Ramsay in 1898. Krypton is present in the air at about 1 ppm. The atmosphere of Mars contains a little (about 0.3 ppm) of krypton. It is characterised by its brilliant green and orange spectral lines. The spectral lines of krypton are easily produced and some are very sharp. In 1960 it was internationally agreed that the fundamental unit of length, the metre, should be defined as 1 m = 1,650,763.73 wavelengths (in vacuo) of the orange-red line of Kr-33. Under normal conditions krypton is colourless, odourless, fairly expensive gas. Solid krypton is a white crystalline substance with a face-centered cubic structure which is common to all the 'rare gases'. Krypton difluoride, KrF2, has been prepared in gram quantities and can be made by several methods. Other compounds are unstable, unless isolated in a matrix at very low temperatures. Applications: Krypton is used to fill electric lamp bulbs which are filled with a mixture of krypton and argon, and for various electronic devices. Krypton is also used in photographic projection lamps, in very high-powered electric arc lights used at airports and in some strobo-lamps, because it has an extremely fast respons to an electric current. A mixture of stable and unstable isotopes of krypton is produced by slow neutron fission of uranium in nuclear reactors as Kripron-85, its most stable isotope. It is used to detect leaks in sealed containers, to excite phosphors in light sources with no external source of energy, and in medicine to detect abnormal heart openings. Krypton in the environment: Krypton might be one of the rarest gases in the atmosphere, but in total there are more than 15 billion tonnes of this metal circulating in the planet, of which only about 8 tonnes a year are extracted, via liquid air.