You also saw that when a CFC molecule absorbs a photon of UV radiation, one of the bonds in the molecule dissociates and a chlorine free radical is released. This occurs because photons of UV radiation at the right wavelength have enough energy to break the bond between the chlorine atom and carbon atom in a CFC molecule.

The interaction between UV radiation and gases has important consequences for life on our planet. As we have already discussed, UV radiation, oxygen and ozone interact in the stratosphere in the reactions of the Chapman cycle. Bond dissociation caused by UV radiation is an important component of this cycle. The cycle begins when a photon of UV radiation breaks apart molecules of O2, forming two oxygen atoms. These oxygen atoms then react with other O2 molecules to form ozone, O3. Ozone, in turn, is also dissociated by photons of UV radiation, producing O2 molecules and oxygen atoms, which undergo further reactions. Although the cycle is a little complicated, no new products are formed by the overall reactions. The net effect of the Chapman cycle is the conversion of UV radiation to increased kinetic energy of the stratospheric gases.

There is another instance involving the ozone layer in which bond dissociation caused by UV radiation is very important. Over the past century, human activity has released CFCs into the atmosphere. When CFC molecules reach the lower stratosphere, some of them are struck by photons of UV radiation. This causes C-Cl bonds to dissociate in the CFC molecules, releasing Cl free radicals (as shown in the image). The Cl free radicals from CFCs then act as catalysts that facilitate the conversion of ozone molecules into oxygen molecules. A single free radical can catalyze thousands of these reactions during its time in the stratosphere. These reactions significantly decreased the concentration of ozone in the stratosphere, causing the Chapman cycle to be disrupted and allowing more UV radiation to reach the surface of the Earth. To address this challenge, the international community signed the Montreal Protocol in 1987 to regulate the use of CFCs and protect the stratospheric ozone that is vital in protecting life from the harmful effects of UV radiation.