Inchemistry, anunpaired electron is anelectron that occupies anorbital of anatom singly, rather than as part of anelectron pair. Each atomic orbital of an atom (specified by the threequantum numbers n, l and m) has a capacity to contain two electrons (electron pair) with oppositespins. As the formation of electron pairs is often energetically favourable, either in the form of achemical bond or as alone pair, unpaired electrons are relatively uncommon in chemistry, because an entity that carries an unpaired electron is usually rather reactive. Inorganic chemistry they typically only occur briefly during a reaction on an entity called aradical; however, they play an important role in explaining reaction pathways.
Radicals are uncommon in s- and p-block chemistry, since the unpaired electron occupies avalence p orbital or an sp, sp2 or sp3hybrid orbital. These orbitals are strongly directional and therefore overlap to form strong covalent bonds, favouringdimerisation of radicals. Radicals can be stable if dimerisation would result in a weak bond or the unpaired electrons are stabilised bydelocalisation. In contrast, radicals in d- and f-block chemistry are very common. The less directional, more diffuse d and f orbitals, in which unpaired electrons reside, overlap less effectively, form weaker bonds and thus dimerisation is generally disfavoured. These d and f orbitals also have comparatively smaller radial extension, disfavouring overlap to form dimers.[1]
Relatively more stable entities with unpaired electrons do exist, e.g. thenitric oxide molecule has one. According toHund's rule, the spins of unpaired electrons are aligned parallel and this gives these moleculesparamagnetic properties.
The most stable examples of unpaired electrons are found on the atoms and ions oflanthanides andactinides. The incomplete f-shell of these entities does not interact very strongly with the environment they are in and this prevents them from being paired. The ions with the largest number of unpaired electrons areGd3+ andCm3+ with seven unpaired electrons.
An unpaired electron has amagnetic dipole moment, while an electron pair has no dipole moment because the two electrons have opposite spins so their magnetic dipole fields are in opposite directions and cancel. Thus an atom with unpaired electrons acts as amagnetic dipole and interacts with amagnetic field. Only elements with unpaired electrons exhibitparamagnetism,ferromagnetism, andantiferromagnetism.