Free radicals react with molecules, atoms, other radicals in long series of chain reactions. A radical is formed in the atmosphere through photolysis or chemical reaction. Radicals are very reactive and will pull atoms or electrons from other molecules to fill their outer valance shell. New radicals may formed from the original molecule which then react with another molecule, etc. This reaction propagates from one species to another until a termination reaction occurs. The termination reaction usually involves the reaction of two free radicals to form a stable product.