Hybridization

Hybridization is a concept in valence bond theory used to explain the shapes of molecules. It describes the mixing of atomic orbitals on a central atom to form a set of new, equivalent orbitals, called hybrids, arranged in fixed geometries. The resulting hybrids participate in sigma bonding and host lone pairs, producing bond angles that closely match observations. Hybridization is a model, not a direct measurement of orbital energies.

Common hybrids include sp, sp2, and sp3. Sp hybrids yield linear geometries; sp2 yields trigonal planar; sp3 yields tetrahedral arrangements. For example, carbon uses sp3 hybrids in methane to form four equivalent C-H sigma bonds, carbon uses sp2 in ethene to make a planar framework with a pi bond, and carbon uses sp in acetylene to create two sigma bonds and two pi bonds. Heavier main-group elements can form expanded octets described by sp3d or sp3d2 hybrids, though such descriptions are debated.

Hybridization simplifies bonding in valence bond theory but is not the only description of bonding. Molecular orbital theory offers an alternative that can account for delocalization and aromaticity without invoking discrete hybrids. In practice, chemists use hybridization as a heuristic to rationalize molecular geometry and reactivity, while recognizing that the actual electronic structure is a mixture of configurations and that the role of d-orbitals is context-dependent.