homolysis
/ hoh-MOL-uh-sis /
When two people who shared one thing have to part, they can split it fairly, one piece each, or one person can grab the whole thing. Bonds can break the same two ways. Homolysis is the fair split: a covalent bond, which is two electrons shared between two atoms, breaks so that each atom walks away with one of the electrons.
Because each fragment keeps a single, unpaired electron, homolysis produces two free radicals. Chemists draw this with single-barbed 'fishhook' arrows, one for each electron, peeling apart from the middle of the bond. Compare this with heterolysis, the unfair split, where one atom takes both electrons and you get a cation and an anion instead. Homolysis is favored when the bond is non-polar (the two atoms pull equally, so there is no reason for one to take both electrons) and when energy is supplied as heat or ultraviolet light. Weak bonds with a low bond-dissociation energy, like the O-O bond in a peroxide or the Cl-Cl bond in chlorine gas, snap homolytically most easily.
Homolysis is the spark that starts nearly all radical chemistry: combustion, the chlorination of methane, the spoiling of oils, and the polymerization that makes plastics. A molecule that breaks apart this way on gentle heating, generating radicals on demand, is called an initiator. The key idea is that homolysis sets the chain in motion; from there a self-sustaining cycle does most of the work.
Heating a benzoyl peroxide gently snaps its weak O-O bond homolytically into two oxygen-centered radicals, which are then used to kick off the radical polymerization of styrene into polystyrene.
A weak O-O bond breaks evenly under heat, giving two radicals that start a chain.
Homolysis makes two radicals; heterolysis makes an ion pair. Don't confuse them: the fishhook (single-barbed) arrows of homolysis each move one electron, while the regular curved arrows of heterolysis each move an electron pair.