\▐ra-di-kәl\ n [ME, fr. LL radicalis, fr. L radic-, radix root] (1641) A group of atoms, normally part of a molecule that may replace a single atom (frequently H in organic compounds) and remain unchanged during reactions of the compound. Some examples are the ethyl radical, −C2H−, the acetate radical, CH3COO−, and the phenyl radical, −C6H5. Many chemical-reaction mechanisms postulate the transitory existence of unattached (“free”) radicals as intermediates, which, because of their charge, are extremely reactive. Free Radicals play important roles in addition polymerizations. A few free radicals are known that are sufficiently stable to permit their identification ad quantitative determination as chemical entities (Morrison RT, Boyd RN (1992) Organic chemistry, 6th edn. Prentice-Hall, Englewood Cliffs, NJ; Smith MB, March J (2001) Advanced organic chemistry, 5th edn. Wiley, New York; Odian GC (2004) Principles of polymerization. Wiley, New York).