Kr - Krypton
(Gr. kryptos, hidden) Discovered in 1898 by Ramsay and Travers in the residue leftafter liquid air had nearly boiled away. In 1960 it was internationally agreed that thefundamental unit of length, the meter, should be defined in terms of the orange-redspectral line of 86Kr. This replaced the standard meter of Paris, which was defined interms of a bar made of a platinum-iridium alloy. In October 1983 the meter, whichoriginally was defined as being one ten millionth of a quadrant of the earth's polarcircumference, was again redefined by the International Bureau of Weights and Measures asbeing the length of a path traveled by light in a vacuum during a time interval of1/299,792,458 of a second.
Krypton is present in the air to the extent of about 1 ppm. The atmosphere of Mars hasbeen found to contain 0.3 ppm of krypton. Solid krypton is a white crystalline substancewith a face-centered cubic structure which is common to all the "rare gases."