ozone Unstable, pale-blue, gaseous allotrope of oxygen, formula (O³). It has a characteristic pungent odour and decomposes into molecular oxygen. It is present in the atmosphere, mainly in the ozone layer. Prepared commercially by passing a high-voltage discharge through oxygen, ozone is used as an oxidizing agent in bleaching, air-conditioning and purifying water. See also allotropy

allotropy Property of some chemical elements that enables them to exist in two or more distinct physical forms. Each form (an allotrope) can have different chemical properties but can be changed into another allotrope – given suitable conditions. Examples of allotropes are molecular oxygen and ozone, white and yellow phosphorous, and graphite and diamond (carbon).