krypton

Colourless, odourless, gaseous, non-metallic element, atomic number 36, relative atomic mass 83.80. It is grouped with the noble gases (rare gases) in Group 0 of the periodic table, and was long believed not to enter into reactions, but it is now known to combine with fluorine under certain conditions; it remains inert to all other reagents. As with the other noble gases, krypton's lack of reactivity is due to its full outer shell of electrons. It is present in very small quantities in the air (about 114 parts per million). It is used chiefly in fluorescent lamps, lasers, and gas-filled electronic valves.

Krypton was discovered in 1898 in the residue from liquid air by British chemists William Ramsay and Morris Travers; the name refers to their difficulty in isolating it.