Matter & Energy

Matter is composed of atoms or groups of atoms called molecules. The arrangement of particles in a material depends on the physical state of the substance. In a solid, particles form a compact structure that resists flow. Particles in a liquid have more energy than those in a solid. They can flow past one another, but they remain close. Particles in a gas have the most energy. They move rapidly and are separated from one another by relatively large distances.


Oxygen, symbol O, colorless, odorless, tasteless, slightly magnetic gaseous element. On earth, oxygen is more abundant than any other element. 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.


Gaseous oxygen can be condensed to a pale blue liquid that is strongly magnetic. Pale blue solid oxygen is produced by compressing the liquid. The atomic weight of oxygen is 15.9994; at atmospheric pressure, the element boils at -182.96° C (-297.33° F), melts at -218.4° C (-361.1° F), and has a density of 1.429 g/liter at 0° C (32° F).

Oxygen composes 21 percent by volume or 23.15 percent by weight of the atmosphere; 85.8 percent by weight of the oceans (88.8 percent of pure water is oxygen); and, as a constituent of most rocks and minerals, 46.7 percent by weight 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 structural forms of oxygen are known: ordinary oxygen, containing two atoms per molecule, formula O2; ozone, containing three atoms per molecule, formula O3; and a pale blue, nonmagnetic form, O4, containing four atoms per molecule, which readily breaks down 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).

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 many organic and inorganic compounds. It forms compounds called oxides with almost all the elements, including some of the noble gases. A chemical reaction in which an oxide forms is called oxidation. The rate of the reaction 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 burst into flame by spontaneous combustion; 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.


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 people 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 of the oxygen produced in the United States is used to make a mixture of carbon monoxide and hydrogen called synthesis gas, used for the synthesis of methanol and ammonia. High-purity oxygen is used also in the metal-fabrication industries; in liquid form it is of great importance as a propellant for guided missiles and rockets..