Krypton was an element, number 36 on the periodic table.
In 2267, the Galileo crash landed on Taurus II, near the 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")