For the character, please see Kryton.
Krypton is an element, number 36 on the periodic table.
There were trace amounts of krypton in the atmosphere of Prospect VII. (SNW: "Lift Us Where Suffering Cannot Reach")
In 2267, the Galileo crash landed on Taurus II, near Murasaki 312. The crew found the atmosphere breathable, an oxygen-nitrogen mixture, with traces of krypton, neon, and argon, not dissimilar to that of Earth. (TOS: "The Galileo Seven")
Later that year, the Galileo II was forced to crash-land on a planetoid in the Gamma Canaris region. The crew found the atmosphere to also be an oxygen-nitrogen compound similar to that of Earth, again containing trace amounts of krypton, argon, and neon. (TOS: "Metamorphosis")