Krypton was discovered in 1898 by W. Ramsay and M.W. Travers in the residue of liquid air. Its name comes from the Greek kryptos = 'the hidden one'. Kr has the atomic number 36. It has 6 stable isotopes 78Kr, 80Kr, 82Kr, 83Kr, 84Kr and 86Kr, with respective atomic masses of 77.920368, 79.913680, 81.913482, 82.914135, 83.911507 and 85.910616. Its atomic weight is 83.80 g/mol; melting point: 116.55 K; boiling point: 119.80 K. The atomic diameter is 3.58 Å. Gas density is 3.733 kg/m3 at 1 atm and 237.15 K. Krypton has the electron configuration 1s22s2p6 3s2p6d10 4s2p6 and a first ionization potential of 13.999 eV. Its valence is usually 0, but 2, 4, 6 and 8 are observed. It forms the hydride ion KrH+, krypton difluoride KrF2 and a hydroquinone clathrate [C6H4(OH)2]3. 0.74Kr, which forms stable crystals (Cook, 1961).
Dry air contains 1.14 × 10−4 vol.% krypton with 78Kr/84Kr, 80Kr/84Kr, 82Kr/84Kr, 83Kr/84Kr and 86Kr/84Kr ratios of 0.006087, 0.03960, 0.20217, 0.20136 and 0.30524 respectively.