or radical
electron .
Most molecules contain even numbers of electrons, and their covalent bond s normally consist of shared electron pairs. Cleavage of such bonds produces two separate free radicals, each with an unpaired electron (in addition to any paired electrons). They may be electrically charged or neutral and are highly reactive and usually short-lived. They combine with one another or with atoms that have unpaired electrons. In reactions with intact molecules, they abstract a part to complete their own electronic structure, generating new radicals that go on to react with other molecules. Such chain reactions are particularly important in decomposition of substances at high temperatures and in polymerization . In the human body, oxidized (see oxidation-reduction ) free radicals can damage tissues. Antioxidant nutrients (e.g., {{link=vitamin C">vitamin C , vitamin E , selenium ) may reduce these effects. Heat, ultraviolet radiation , and ionizing radiation (see radiation injury ) all generate free radicals. They are magnetic, so their properties can be studied with such techniques as magnetic susceptibility and electron paramagnetic resonance measurements.