Oxygen

Oxygen, chemical element, symbol O; for physical constants see Periodic Table. Oxygen was discovered by Joseph Priestley in 1772. It is present in air to the extent of 21% and is commercially obtained from the air by liquefaction and fractional distillation. The element makes up 49.2%, by weight, of the earth's crust and is the most abundant element. Oxygen is a colorless, odorless, reactive gas, capable of combining with most other elements. Ozone, a poisonous allotrope of oxygen, is formed by the action of ultraviolet light, or an electrical discharge on oxygen. The greatest use of the gas is in the oxygen enrichment of steel blast furnaces. Large quantities of oxygen are used in the synthesis of ammonia, methanol, and ethylene oxide, as well as for oxy-acetylene welding, and as rocket propellent. Oxygen consumption in the United States is 20 million short tons per year.