<P> In chemistry, a radical (more precisely, a free radical) is an atom, molecule, or ion that has an unpaired valence electron . With some exceptions, these unpaired electrons make free radicals highly chemically reactive towards other substances, or even towards themselves: their molecules will often spontaneously dimerize or polymerize if they come in contact with each other . Most radicals are reasonably stable only at very low concentrations in inert media or in a vacuum . </P> <P> A notable example of a free radical is the hydroxyl radical (HO), a molecule that has one unpaired electron on the oxygen atom . Two other examples are triplet oxygen and triplet carbene (: CH 2) which have two unpaired electrons . In contrast, the hydroxyl anion (HO −) is not a radical, since the unpaired electron is resolved by the addition of an electron; singlet oxygen and singlet carbene are not radicals as the two electrons are paired . </P> <P> Free radicals may be created in a number of ways, including synthesis with very dilute or rarefied reagents, reactions at very low temperatures, or breakup of larger molecules . The latter can be affected by any process that puts enough energy into the parent molecule, such as ionizing radiation, heat, electrical discharges, electrolysis, and chemical reactions . Radicals are intermediate stages in many chemical reactions . </P>

What is an example of a radical in chemistry