gaseous element, symbol O, in group 16 (or VIa) of the periodic table (see PERIODIC LAW,); at.no. 8, at.wt. 15.9994. Oxygen melts at –218.4° C (–361.1° F), boils at –182.96° C (–297.33° F), and has a density of 1.429 g/liter at 0° C (32° F).

Oxygen was discovered in 1774 by the British chemist Joseph Priestley and, independently, by the Swedish chemist Carl Wilhelm Scheele; it was shown to be an elemental gas by the French chemist Antoine Laurent Lavoisier in his classic experiments on combustion.

Properties and Occurrence.

Oxygen is a colorless, odorless, tasteless, slightly magnetic, nontoxic gas. It can be condensed to a pale blue liquid that is strongly magnetic. Pale blue solid oxygen is produced by compression of the liquid.

Oxygen is the most abundant of all the elements. It composes 21 percent by volume or 23.15 percent by weight of the atmosphere; 85.8 percent of the oceans (88.8 percent of pure water is oxygen); and, as a constituent of most rocks and minerals, 46.7 percent of the solid crust of the earth. Oxygen comprises 60 percent of the human body. It is a constituent of all living tissues; almost all plants and animals, including all humans, require oxygen, in the free or combined state, to maintain life. See RESPIRATION,.

Three allotropic forms of oxygen are known: ordinary oxygen, containing two atoms per molecule, formula O2; OZONE (q.v.), containing three atoms per molecule, formula O3; and a pale blue, nonmagnetic form, O4, containing four atoms per molecule, which readily breaks down again into ordinary oxygen. Three stable isotopes of oxygen are known; oxygen-16 (atomic mass 16) is the most abundant. It comprises 99.76 percent of ordinary oxygen and was used in determination of atomic weights until the 1960s (see Atom and Atomic Theory).

Oxygen is prepared in the laboratory from salts such as potassium chlorate, barium peroxide, and sodium peroxide. The most important industrial methods for the preparation of oxygen are the electrolysis of water and the fractional distillation of liquid air. In the latter method, air is liquefied and allowed to evaporate. The nitrogen in the liquid air is more volatile and boils off first, leaving the oxygen. Oxygen is stored and shipped in either liquid or gaseous form.

Oxygen is a component of a great number of organic and inorganic compounds, in which it has a valence of 2. It forms compounds called oxides with practically all the elements, including some of the NOBLE GASES, (q.v.). The rate of the reaction, called oxidation, varies with different elements. Ordinary combustion, or burning, is a very rapid form of oxidation. In spontaneous combustion, the heat evolved by the oxidation reaction is sufficiently great to raise the temperature of the substance to the point that flames result. For example, phosphorus combines so vigorously with oxygen that the heat liberated in the reaction causes the phosphorus to melt and burn. Certain very finely divided powders present so much surface area to the air that they spontaneously combust; they are called pyrophoric substances. Sulfur, hydrogen, sodium, and magnesium combine with oxygen less energetically and burn only after ignition. Some elements, such as copper and mercury, form oxides slowly, even when heated. Inactive metals, such as platinum, iridium, and gold, form oxides only through indirect methods. For discussion of oxides of elements see separate articles on each element.

Uses.

Large amounts of oxygen are used in high-temperature welding torches, in which a mixture of oxygen and another gas produces a flame of much higher temperature than is obtained by burning gases in air. Oxygen is administered to patients whose breathing is impaired and also to persons in aircraft flying at high altitudes, where the poor oxygen concentration cannot support normal respiration. Oxygen-enriched air is used in open-hearth furnaces for steel manufacture.

Most oxygen produced in the U.S. is used to make a carbon monoxide and hydrogen mixture called synthesis gas, used for methanol and ammonia synthesis. High-purity oxygen is used also in the metal-fabrication industries; in liquid form it is an important propellant for guided missiles and rockets (see ROCKET,: Liquid Propellants).        S.Z.L., SEYMOUR Z. LEWIN, M.S., Ph.D.