Krypton (pronounce /ˈkrɪptən/ or /ˈkrɪptɒn/; from Greek: kryptos "hidden") is a chemical element with the symbol Kr and atomic number 36. It is a member of Group 18 and Period 4. A colorless, odorless, tasteless noble gas, krypton occurs in trace amounts in the atmosphere, is isolated by fractionally distilling liquified air, and is often used with other rare gases in fluorescent lamps. Krypton is inert for most practical purposes, but it is known to form compounds with fluorine. Krypton can also form clathrates with water when atoms of it are trapped in a lattice of the water molecules.