sp2

sp2 hybridization refers to the mixing of one s orbital and two p orbitals on a valence-shell atom to form three equivalent sp2 hybrid orbitals. This concept is used in valence bond theory to describe molecular geometry and bonding, especially for atoms like carbon, nitrogen, and boron in certain environments.

The three sp2 orbitals are arranged trigonal planar with about 120-degree separation. Each sp2 orbital can form a sigma bond or hold a lone pair. The remaining unhybridized p orbital on the atom extends perpendicular to the plane and can participate in pi bonding by overlapping with p orbitals on adjacent atoms, enabling pi systems.

In practice, sp2 centers tend to be planar. Ethene (ethylene) features two sp2-hybridized carbons; each carbon uses its sp2 set to form two C–H sigma bonds and one C–C sigma bond, while the C=C pi bond arises from the unhybridized p orbitals. In benzene and other arenes, all six carbons are sp2, producing a planar ring with a delocalized pi system across the ring.

sp2 is also relevant in other contexts, such as carbonyl-containing compounds where the carbon in C=O is sp2, and in graphite where each carbon is sp2-hybridized in a two-dimensional sheet with delocalized pi electrons. It is distinct from sp3 hybridization, which forms tetrahedral geometries with four sigma bonds, and from sp, which forms linear geometries with two sigma bonds and two leftover p orbitals.