A molecule that is composed of two oxygen atoms, O₂, and has no color, odor, or taste. It is present in both the atmosphere and dissolved in the oceans and freshwater sources exposed to the atmosphere. Solar radiation with wavelengths less then 242 nm can break it back into oxygen atoms,

O₂ ---> O + O

One of these oxygen radicals in turn can combine with O₂ to form ozone,

O₂ + O ---> O₃

This last reactions requires quick collision with a third body to remove the excess energy of the ozone product.