A free radical is an atom or group of atoms with a single unpaired electron. It is usually produced by breaking a covalent bond.

The concept of the covalent (or electron-pair) bond was first formulated by the American chemist Gilbert Newton Lewis in 1916. Lewis showed that, by sharing electrons, atoms can achieve the stability of the noble gases. For example, when two hydrogen atoms, each containing one electron, combine to form an H₂ molecule, each atom shares the two electrons and the hydrogen molecule takes on the electronic configuration of the noble gas helium with atomic number 2.

The only electrons that take part in a covalent bond are the electrons in the outer orbitals, i.e., the valence electrons. The rule that atoms in covalently bonded species prefer noble gas electron configurations is sometimes referred to as the octet rule because nonmetals, with the exception of...