Free radicals are molecules that, through photolysis or chemical reaction, have an unpaired electron in their outer valance shell. These radicals are very reactive and thus have a short life. When a free radical reacts with a more stable molecule, the radical often pulls an atom from it and becomes a stable molecule itself. The original molecule then becomes a free radical and will react with other species of atoms and molecules in a long series (or chain) of reactions until the process reaches the termination phase. In this phase two free radicals combine, sharing the pair of electrons and breaking the chain. [Journal of Organic Chemistry; v56; 5743-5; 1991.] [Journal of Organic Chemistry; v58; 3953-9; 1993.]