In chemistry, a dangling bond is an unsatisfied valence on an immobilized atom.
In order to gain enough electrons to fill their valence shells many atoms will form covalent bonds with other atoms. In the simplest case, that of a single bond, two atoms each contribute one unpaired electron, and the resulting pair of electrons is shared between both atoms. Atoms which possess too few bonding partners to satisfy their valences and which possess unpaired electrons are termed free radicals; so, often, are molecules containing such atoms. When a free radical exists in an immobilized environment, for example, a solid, it is referred to as an "immobilized free radical" or a "dangling bond".