Krypton

About Krypton

Krypton is what's left when you've already boiled off the oxygen, nitrogen, and argon from liquid air — about 1 ppm of the atmosphere, recovered as a side product of large cryogenic air-separation plants. That scarcity makes it expensive enough that you only reach for it when nothing else does the job. Two cases qualify. First, the orange-red 605.78 nm line of ⁸⁶Kr was so spectrally clean that from 1960 to 1983 it defined the meter (1,650,763.73 wavelengths in vacuum), a job it lost only when stabilized lasers became more reproducible. Second, KrF excimer lasers at 248 nm drove deep-UV photolithography for two semiconductor generations and still show up in eye surgery. Chemically, Kr is almost completely inert — krypton difluoride exists but decomposes above −30 °C, and it's mostly a curiosity that proves the noble gases aren't truly noble. Double-pane windows fill argon's role; Kr only gets used when you need the lower thermal conductivity in narrow gaps.