Ozone Ozone (O3) is a molecule consisting of three oxygen atoms, similar to the oxygen we breathe (O2), however oxygen consists of only two oxygen atoms. In the stratosphere, a region high up in the upper atmosphere, light rays are responsible for the breaking down of oxygen (O2), breathable oxygen into its two separate oxygen atoms. Lone oxygen atoms are markedly reactive. When a lone oxygen atom comes into contact with a breathable oxygen molecule (O2) it combines to form ozone (O3). The ozone layer is a small residual amount of ozone concentrated in a band in the upper atmosphere. This band of concentrated o