(O₃), triatomic allotrope of oxygen (a form of oxygen in which the molecule contains three atoms instead of two as in the common form) that accounts for the distinctive odour of the air after a thunderstorm or around electrical equipment. The odour of ozone around electrical machines was reported as early as 1785; ozone's chemical constitution was established in 1872. Ozone is an irritating, pale blue gas that is explosive and toxic, even at low concentrations. It occurs naturally in small amounts in the Earth's stratosphere, where it absorbs solar ultraviolet radiation, which otherwise could cause severe damage to living organisms on the Earth's surface. Under certain conditions, photochemical reactions between nitrogen oxides and hydrocarbons in the lower atmosphere can produce ozone in concentrations high enough to cause irritation of the eyes and mucous membranes.

Ozone usually is manufactured by passing an electric discharge through a current of oxygen or dry air. The resulting mixtures of ozone and original gases are suitable for most industrial purposes, although purer ozone may be obtained from them by various methods; for example, upon liquefaction, an oxygen-ozone mixture separates into two layers, of which the denser one contains about 75 percent ozone. The extreme instability and reactivity of concentrated ozone makes its preparation both difficult and hazardous.

Ozone is 1.5 times as dense as oxygen; (oxygen) at -112° C (-170° F) it condenses to a dark blue liquid, which freezes at -251.4° C (-420° F). The gas decomposes rapidly at temperatures above 100° C (212° F) or, in the presence of certain catalysts, at room temperatures. Although it resembles oxygen in many respects, ozone is much more reactive; hence, it is an extremely powerful oxidizing agent, particularly useful in converting olefins into aldehydes, ketones, or carboxylic acids. Because it can decolorize many substances, it is used commercially as a bleaching agent for organic compounds; as a strong germicide it is used to sterilize drinking water as well as to remove objectionable odours and flavours. See also ozonosphere.