unhybridized

Unhybridized describes orbitals that have not been mixed into hybrid orbitals. In valence bond theory, atoms can form hybrid orbitals such as sp, sp2, or sp3 to accommodate sigma bonds and define molecular geometry. Any valence orbitals that are not involved in hybridization remain unhybridized and are available for other interactions, most commonly pi bonding or lone pairs.

In carbon dioxide, carbon adopts sp hybridization for the two sigma bonds to oxygen, leaving two unhybridized p orbitals that form the two pi bonds. The pi bonds are formed by sideways overlap with the corresponding p orbitals on the oxygen atoms. In benzene, each carbon is sp2 hybridized, with one unhybridized p orbital on each carbon that participates in the delocalized pi system around the ring. In methane, carbon is sp3 hybridized, so there are no unhybridized p orbitals and no pi bonding.

Unhybridized orbitals can also influence reactivity and spectroscopy. Their presence enables conjugation, aromaticity, and certain types of backbonding in metal complexes. While the concept is central to valence bond theory, molecular orbital theory describes pi bonding using overlapping p orbitals without requiring either explicit unhybridized orbital labels.