Un electrón desparejado o electrón desapareado es aquel que no tiene su espín compensado por otro electrón de espín opuesto en el mismo átomo (o, en un modelo de orbitales moleculares, en la misma molécula). Visto de otro modo, son aquellos que se encuentran solos en un orbital, que se dice que está semiocupado. Esto, que como se ve en la figura es muy común en átomos aislados, es relativamente infrecuente cuando estos átomos se encuentran formando sustancias. En sistemas orgánicos, los electrones desparejados dan lugar a radicales, que normalmente son muy reactivos, mientras que en metales de transición, por ejemplo, es fácil encontrar sistemas con electrones desparejados y que sean estables, tanto en estado elemental como en óxidos o complejos. En cualquier caso, los electrones sin emparejar le dan propiedades magnéticas al sistema en el que estén, y generalmente alteran sus propiedades ópticas.

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In chemistry, an unpaired electron is an electron that occupies an orbital of an atom singly, rather than as part of an electron pair (paired electrons means all two spin states of the orbital specified by three quantum numbers n, l and m are fully occupied by two electrons). As the formation of electron pairs is often energetically favourable, either in the form of a chemical bond or as a lone pair, unpaired electrons are relatively uncommon in chemistry, because an entity that carries an unpaired electron is usually rather reactive. In organic chemistry they typically only occur briefly during a reaction on an entity called a radical; however, they play an important role in explaining reaction pathways. Radicals are uncommon in s- and p-block chemistry, since the unpaired electron occupies a valence p orbital or an sp, sp2 or sp3 hybrid orbital. These orbitals are strongly directional and therefore overlap to form strong covalent bonds, favouring dimerisation of radicals. Radicals can be stable if dimerisation would result in a weak bond or the unpaired electrons are stabilised by delocalisation. 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. Relatively more stable entities with unpaired electrons do exist, e.g. the nitric oxide molecule has one. According to Hund's rule, the spins of unpaired electrons are aligned parallel and this gives these molecules paramagnetic properties. The most stable examples of unpaired electrons are found on the atoms and ions of lanthanides. 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 ion with the largest number of unpaired electrons is Gd3+ with seven unpaired electrons.

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