The compound that is the triatomic form of oxygen and has the formula O₃ is ozone.
Ozone contains three oxygen atoms per molecule, unlike ordinary atmospheric oxygen, O₂, which contains two. It is a pale blue gas with a sharp odor and is chemically much more reactive than O₂.
Most atmospheric ozone is concentrated in the stratosphere, where the ozone layer absorbs much of the Sun’s harmful ultraviolet radiation. Near Earth’s surface, however, ozone is an air pollutant formed through reactions involving sunlight and nitrogen oxides.
Ozone and oxygen are allotropes of the same element, not separate elements. Ozone is also used in some water-treatment processes because its strong oxidizing action can destroy microorganisms and break down certain contaminants.