Radicals can be formed by adding a radical to a spin-paired molecule. This is typically observed with unsaturated species, where the addition of a radical across the π bond leads to the production of a new radical by dissolving the π bond. For example, the addition of a Br radical to an alkene yields a carbon-centered radical.

Similar to charge conservation in chemical reactions, spin conservation is implicit for radical reactions. Accordingly, the product formed must possess an unpaired electron if the reaction begins with a reactant having an unpaired electron.

The most facile category of radical formation via the addition process is reduction, where a single electron is added. The Birch reduction of organic compounds is one such example. It employs the electron generated as a metal of group 1 in the periodic table dissolves in liquid ammonia to form the corresponding stable M⁺¹ ion.