A bond is formed between two atoms by sharing two electrons. When this bond is broken by supplying sufficient energy, either two electrons can be taken up by one atom forming ions by the cleavage called heterolysis, or the two electrons are shared by two atoms, with one each creatingradicalsby the cleavagecalled homolysis.

For example, HCl in solution cleaves into H⁺ and Cl⁻ ions, where the chlorine atom takes both bonding electrons with it, leaving a naked proton. However, at about 200 °C in the gas phase, the electron pair forming the H–Cl bond is shared between the two atoms.

Some weak bonds undergo homolysis at around room temperature. In such cases, light is the best energy source for the homolysis of bonds. Peroxides and halogens are quite readily homolysed by heat and light. Dibenzoyl peroxide and azobisisobutyronitrile (AIBN) are often used as initiators of radical reactions because they can easily homolyse to form radicals.