High energy ultraviolet rays (UVC) from the sun strike oxygen (O2) molecules, with just enough energy to break each molecule into two separate oxygen atoms, thus absorbing the UVC photons.

 

The single oxygen atoms are very reactive and combine with other oxygen molecules, to form ozone. The third O atom is not as strongly bound as is the original the pair. Thus when a lower energy ultraviolet photon (UVB) strikes the ozone molecule, it splits away the “extra” third oxygen, and it becomes available to bind to another single oxygen to make O2 or to an O2 molecule to make another ozone. 

 

This cycle of formation and breakdown sustains a constant level of ozone in a layer in the stratosphere which acts as the Earth’s sunscreen.