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Krypton difluoride, KrF2 is a chemical compound of krypton and fluorine. It was the first compound of krypton discovered. It is a volatile, colourless solid at room temperature. The structure of the KrF2 molecule is linear, with Kr−F distances of 188.9 pm. It reacts with strong Lewis acids to form salts of the KrF+ and Kr2F+3 cations. The atomization energy of KrF2 (KrF2(g) → Kr(g) + 2 F(g)) is 21.9 kcal/mol, giving an average Kr–F bond energy of only 11 kcal/mol, the weakest of any isolable fluoride. In comparison, the dissociation of difluorine to atomic fluorine requires cleaving a F–F bond with a bond dissociation energy of 36 kcal/mol. Consequently, KrF2 is a good source of the extremely reactive and oxidizing atomic fluorine. It is thermally unstable, with a decomposition rate of 10% per hour at room temperature. The formation of krypton difluoride is endothermic, with a heat of formation (gas) of 14.4 ± 0.8 kcal/mol measured at 93 °C.