hybridisation

Hybridisation is a concept in chemistry that describes the mixing of atomic orbitals within an atom to form new, equivalent hybrid orbitals that participate in covalent bonding. The approach helps explain the shapes and bond angles of molecules by providing directional orbitals that can overlap to form sigma bonds. Hybridisation is a modeling tool within valence bond theory and is not the sole description of bonding; it is most useful for predicting geometry and qualitative bonding properties.

Common types include sp3, sp2, and sp, reflecting the number of p orbitals mixed with one s orbital. In methane, carbon is described as sp3 hybridised, producing four equivalent orbitals arranged tetrahedrally at about 109.5 degrees. In ethene, each carbon is sp2 hybridised, giving three sigma bonds in a plane and leaving one unhybridised p orbital to participate in a pi bond, yielding a planar molecule with ~120-degree angles. In acetylene, sp hybridisation leaves two unhybridised p orbitals, forming two pi bonds and a linear molecule with ~180-degree angles. In water, oxygen is commonly described as sp3 hybridised, with two sigma bonds and two lone pairs giving a bent shape.

In aromatic compounds such as benzene, carbons are treated as sp2 to enable a delocalised pi system over the ring. For transition metals, bonding may involve d orbitals, leading to descriptions such as sp3d or sp3d2 in expanded octets. Limitations include dependence on energy considerations and the chosen model; molecular orbital theory can offer a more complete description in some cases.

Overall, hybridisation remains a fundamental tool for explaining molecular geometry, bonding, and reactivity in many inorganic and organic contexts.