= Solution
\b[<Carbon dioxide> is linear, with two equivalent C–O bonds of formal <bond order> two; its <Homo> is predominantly oxygen nonbonding and its <LUMO> is C–O $\pi^*$ antibonding.] The carbon uses two opposite $sp$ hybrids for its <sigma bonds> and two perpendicular p <atomic orbitals> for <pi bonding>. A localized $\mathrm{O=C=O}$ picture assigns each terminal oxygen an $sp^2$ sigma/lone-pair framework and one p orbital for its local pi bond.
In a symmetry-adapted description, each of the two perpendicular sets of oxygen and carbon p functions forms bonding, oxygen-centred nonbonding and antibonding combinations. The occupied nonbonding pair is labelled $1\pi_g$ and the lowest empty pi pair $2\pi_u^*$. The latter has appreciable amplitude at carbon, explaining its electrophilic reactivity. Hybridization of a terminal atom with only one bonded neighbour is a choice of localized basis, not an independently measurable bond angle; an alternative oxygen $sp$ plus p basis can represent the same delocalized system.
There are 16 <valence electrons>: four in the two sigma bonds, four in the two net pi bonds, and eight in the remaining oxygen lone-pair combinations. These local electron counts and the delocalized orbital description are alternative bookkeeping schemes, not additional electrons.
Back to article page
