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Krypton — (Gr. kryptos, hidden), Kr; atomic weight 83.80; atomic number 36; melting point — 156.6°C; boiling point — 152.30 ± 0.10°C; density 3.733 g/1 (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 ppm. The atmosphere of Mars has been found to contain 0.3 ppm of krypton. It is one of the "noble" gases. It is characterized by its brilliant green and orange spectral lines. Naturally-occurring 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,86 corresponding to the transition 5p[O1/2], – 6d[O1/2], as follows: 1 m = 1,650,763.73 wavelengths (in vacuo) of the orange-red line of Kr. This replaces the standard meter of Paris, which was defined in terms of a bar made of a 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 difluoride has been prepared in gram quantities and can be made by several methods. A higher fluoride of krypton and a salt of an oxyacid of krypton also have been reported. Molecule ions of ArKr+ and KrH+ 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. Kr85 has found recent application in chemical analysis. By imbedding the isotope in various solids, kryptonates are formed. The activity of these kryptonates is sensitive to chemical reactions at the surface. Estimates of the concentration of reactants are 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.

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