Oxygen is a colorless, odorless, tasteless gas; it is the first member of Group 16 of the periodic table. It is denser than air and only slightly soluble in water. A poor conductor of heat and electricity, oxygen supports combustion but does not burn. Normal atmospheric oxygen is a diatomic gas (O2) with molecular weight 31.9988. Ozone is a highly reactive triatomic (O3) allotrope of oxygen (see allotropy). When cooled below its boiling point oxygen becomes a pale blue liquid; when cooled still further the liquid solidifies, retaining its color. Oxygen is paramagnetic in its solid, liquid, and gaseous forms. Although eight isotopes of oxygen are known, atmospheric oxygen is a mixture of the three isotopes with mass numbers 16, 17, and 18.
Oxygen is extremely active chemically, forming compounds with almost all of the elements except the inert gases. Oxygen unites directly with a number of other elements to form oxides. It is a constituent of many acids and of hydroxides, carbohydrates, proteins, fats and oils, alcohols, cellulose, and numerous other compounds such as the carbonates, chlorates, nitrates and nitrites, phosphates and phosphites, and sulphates and sulphites.
The common reaction in which it unites with another substance is called oxidation (see oxidation and reduction). The burning of substances in air is rapid oxidation or combustion. The respiration of animals and plants is a form of oxidation essential to the liberation of the energy stored in such food materials as carbohydrates and fats. The rusting of iron and the corrosion of many metals results from the action of the oxygen in the air.