free radical
Most molecules keep their electrons in pairs, neat and balanced. A free radical is the odd one out: an atom or molecule with a single unpaired electron, a lone electron with no partner. Like a person at a dance who has lost their partner and will grab anyone passing by, a radical is highly reactive and constantly looking to pair up its lonely electron.
Radicals are born when a bond breaks evenly, with each atom keeping one of the two shared electrons (this is homolysis, often triggered by heat or light). Because radicals carry no overall charge but an unpaired electron, chemists track them with single-barbed 'fishhook' arrows, each showing the movement of just one electron, rather than the double-barbed arrows used for electron pairs. Their stability follows the same order as carbocations (tertiary > secondary > primary > methyl) and for the same reasons, hyperconjugation and electron donation from neighbors, and allylic or benzylic radicals are extra-stabilized by resonance. Radicals love to react in chains: one radical makes a product plus a new radical, which keeps the cycle going.
Radical chemistry explains a lot of everyday processes: combustion, the halogenation of alkanes, the spoiling of fats and oils (autoxidation), the action of antioxidants, and the polymerization that makes plastics like polyethylene. Because a radical reaction propagates in a chain, a tiny spark of initiation can transform a huge amount of material, which is both useful and, in the case of fires, dangerous.
Shining light on a mixture of methane and chlorine splits Cl2 into two chlorine radicals (Cl-with-a-dot); each then yanks a hydrogen off methane, starting a chain that produces CH3Cl.
Light initiates; then a self-sustaining chain of radicals does most of the work.
Radicals move one electron at a time, so they are drawn with single-barbed fishhook arrows, not the regular double-barbed curved arrows that depict electron pairs; mixing the two arrow types up is a common error.