Ozone /ˈoʊzoʊn/ (systematically named 1λ1,3λ1-trioxidane and catena-trioxygen), or trioxygen, is an inorganic molecule with the chemical formula O3. 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 O2, breaking down in the lower atmosphere to normal dioxygen.