Krypton is element number 36 on the periodic table and is classified as one of the noble gases. At normal temperatures, krypton and the other elements in Group 18 are extremely unreactive. The reason is that, with the exception of helium, these elements' outer electron shells are filled to capacity with 8 electrons. This satisfies the octet rule, in which a full electron shell means maximum stability.
At high temperatures, krypton behaves like neon but glows purple rather than red. When mixed with other gases, krypton emits a greenish yellow glow. The reason is that at high temperatures, energetic excitation is strong enough to strip electrons away from krypton and other noble gases. This ionized gas state of nuclei and free electrons is called plasma. It is rare on Earth but abundant in stars like the sun.
At extremely low temperatures, krypton may form compounds with fluorine atoms. The most stable molecule containing krypton is KrF2 or krypton difluoride.
Because krypton and the other Group 18 elements are unreactive gases at room temperature, they were among the last elements on the periodic table to be discovered. Krypton was discovered in 1898 by Sir William Ramsey, who was studying liquefied air. Compared to other atmospheric gases, krypton is present in trace amounts, making up only 0.0001% of Earth's atmosphere.