<P> In chemistry, an unpaired electron is an electron that occupies an orbital of an atom singly, rather than as part of an electron pair . Each atomic orbital of an atom (specified by the three quantum numbers n, l and m) has a capacity to contain two electrons with opposite spins, an electron pair . 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 . </P> <P> Radicals are uncommon in s - and p - block chemistry, since the unpaired electron occupies a valence p orbital or an sp, sp or sp 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 . </P>

Give the number of unpaired electrons in the ground state of an oxygen atom