Oxygen, a nonmetallic chemical element that is contained in air and water, in the earth's crust, and in all forms of life. At ordinary temperatures, free (uncombined) oxygen is a gas with no odor, taste, or color. All animals and most plants require free oxygen to live—without it, they would suffocate.

Oxygen is the most abundant element on earth. Free oxygen is present in the atmosphere, and oxygen is combined with other elements to form water and various types of rocks. By weight, oxygen constitutes about 23 per cent of the earth's atmosphere, 89 per cent of the earth's water, and 47 per cent of the earth's crust.

A molecule of ordinary oxygen contains two atoms of oxygen, and the symbol for the molecule is written O2. Under certain conditions oxygen can exist as O3, or ozone. When oxygen is cooled to a liquid or solid state, it becomes pale blue.

Respiration and Combustion

Oxygen is the key element in the processes of respiration (which includes breathing) and combustion, In respiration, free oxygen is taken in by animals and plants and used to release energy from food. Plants, in turn, release free oxygen in the process of photosynthesis.

Ordinary burning, a type of combustion that takes place in campfires, furnaces, and automobile engines, is a chemical reaction between free oxygen and fuel. Oxygen itself is nonflammable, but it must be present before ordinary burning can take place. The chemical reaction that occurs in combustion is called oxidation.

An oxidation reaction also takes place when iron rusts. Free oxygen in the air combines with the iron to form iron oxide. Rusting is actually a form of combustion that take place at a very slow rate.

Production and Uses of Oxygen

Oxygen is produced commercially by the fractional distillation of liquid air. This process depends on the fact that the various gases that make up the air, including oxygen, have different boiling points.

Oxygen is used in the chemical industry in the manufacture of hydrogen, acetylene, and other chemicals. Pure oxygen is used in hospitals to help patients who have difficulty breathing. It is also carried on airplanes, spacecraft, and submarines for use when the oxygen content of the air becomes too low for normal breathing.

Combustion produces higher temperatures in pure oxygen than in air. Pure oxygen is therefore useful in industrial processes to raise the temperature of furnaces and other heating devices.

In one method of making steel, a jet of oxygen is used to burn impurities from iron. Liquid oxygen, called LOX, is used as an oxidizer in certain types of rocket engines These engines can operate in space, where there is no free oxygen to support combustion.


Oxygen was discovered independently by the Swedish chemist Karl Wilhelm Scheele and somewhat later by the English chemist Joseph Priestley. Priestley, who produced oxygen gas from mercuric oxide in 1774, is sometimes given sole credit for the discovery because Scheele's findings were not published until 1777.

The name oxygen is derived from Greek and means acid former. It was suggested in 1787 by the French chemist Antoine-Laurent Lavoisier, who thought (erroneously) that oxygen was an essential constituent of all acids.

Until 1961, oxygen was used as the reference standard for atomic weights. In that year, the atomic-weight scale was revised and a new standard, carbon 12, was adopted.

Symbol: O. Atomic number: 8. Atomic weight: 15.9994. Specific gravity: gas, 1.11 (air = 1); liquid, 1.14 (water = 1). Melting point. —361.8 F. (—218.8 C.). Boiling point: —297.4 F. (—183.0 C.). Oxygen has three stable isotopes: O-16, O-17, and O-18. It belongs to Group VI-A of the Periodic Table and has a valence of —2.