Krypton is a chemical element with the symbol Kr and atomic number 36. It has six stable isotopes and several radioactive isotopes. It is a colorless, odorless, tasteless noble gas that occurs in trace amounts in the atmosphere and is often used with other rare gases in fluorescent lamps. Its name comes from the Greek word kryptos meaning hidden, because it remained undiscovered for a long time. Krypton is not flammable or toxic.