Ozone (systematically named
1λ1,3λ1-trioxidane and
catena-trioxygen), or
trioxygen, is an inorganic molecule with the
chemical formula . It is a pale blue gas with a distinctively pungent smell. It is an
allotrope of
oxygen that is much less stable than the
diatomic allotrope , breaking down in the lower atmosphere to normal
dioxygen. Ozone is formed from dioxygen by the action of
ultraviolet light and also atmospheric electrical discharges, and is present in low concentrations throughout the
Earth's atmosphere (stratosphere). In total, ozone makes up only of the atmosphere.