Oxygen, O, can be encountered as a gas, cryogenic liquid, and liquid or solid in compound eith other materials. Although nontoxic, it is reactive with hydrocarbon based materials. Oxygen ias a strong oxidizer. Oxygen is a nonmetallic gasous element. Oxygen make up approximately 21% of the atmosphere. The boiling point of oxygen is -297F. It is nonflammable, but supports combustion. Liquid oxygen can explode when exposed to heat or organic materials.