From the CRC handbook of Chemistry and Physics, CRC Press, Cleveland, 55th ed., 1974-75 "The Elements" by C.R. Hammond. page B-6
KRYPTON (Gr. kryptos, hidden), Kr; at. wt. 83.80; at. no. 36; m.p. -156.6 C; b.p. -152.30+/-0.10 C; density 3.733 g/l (0 C); valence usually 0. Discovered in 1898 by Ramsay and Travers in the residue left after liquid air had nearly boiled away. Krypton is present in the air to the extent of about 1 part per million. It is one of the "noble" gases. It is characterized by its brilliant green and orange spectral lines. Naturally-occuring krypton contains six stable isotopes. Fifteen other unstable isotopes are now recognized. 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 meter, should be defined in terms of the orange-red spectral line of Kr(56), corresponding tot he transition 5p[O 1/2]1-6d[O1/2]1, as follows: 1 meter = 1,650,763.73 wavelengths (in vacuo) of the orange-red line of Kr(86). This replaces the standard meter of Paris, which was defined in terms of a bar made of platinum-iridium alloy. Solid krypton is a white crystalline substance with a face-centered cubic structure which is common to all the "rare gases". WHile krypton is generally thought of as a rare gas that normally does not combine with other elements to form compounds, it now appears that the existence of some krypton compounds is established. Krypton and a salt of an oxyacid of krypton have also been reported. Molecule ions of ArKr+ or KrXe+ have been identified and investigated, and evidence is provided for the formation of KrXe or KrXe+. Krypton clathrates have been prepared with hydroquinone and phenol. Kr(85) has found recent application in chemical analysis. By imbedding the isotope in various solids, kryptonates are formed. The activity of these kryptonates is sensitve to chemical reactions at the surface. Estimates of the concentration of reactants is therefore made possible. Krypton is used commercially with argon as a low-pressure filling gas for fluorescent lights. It is used in certain photographic flash lamps for high-speed photography. Uses thus far have been limited because of its high cost. Krypton gas presently costs about $20/liter.
