xenon (zēˈnŏn) [key] [Gr., = strange], gaseous chemical element; symbol Xe; at. no. 54; at. wt. 131.293; m.p. - 111.9°C; b.p. - 107.1°C; density 5.86 grams per liter at STP; valence usually 0. Xenon is a rare, colorless, odorless, tasteless, chemically unreactive gas. It is one of the inert gas elements found in Group 18 of the periodic table. Xenon was long considered incapable of chemical reaction, but in 1962 Neil Bartlett, a Canadian chemist, reported synthesis of xenon hexafluoroplatinate, XePtF6, a true compound. Since that time a number of other xenon compounds have been reported. Xenon is present in the atmosphere in extremely low concentration (about one part in 20 million). It is obtained commercially from liquid air. Xenon is used in certain photographic flash lamps, in high-intensity arc lamps for motion picture projection, and in high-pressure arc lamps to produce ultraviolet light. It is used in numerous instruments for radiation detection, e.g., neutron and X-ray counters and bubble chambers. It has found some use in medicine, e.g., as an experimental anesthetic. Naturally occurring xenon is a mixture of 9 stable isotopes; 20 short-lived radioactive isotopes are also known. A mixture of stable and unstable isotopes of xenon is produced in nuclear reactors during neutron fission of uranium; one of these, xenon-135, is a very good neutron absorber and must be removed since it poisons the reaction. Xenon was discovered spectroscopically in 1898 by William Ramsay and M. W. Travers, who obtained it by fractional distillation of an impure sample of krypton.