Unpaired electron

Radicals can be stable if dimerisation would result in a weak bond or the unpaired electrons are stabilised by delocalisation.

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.

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 and actinides.