3.4.2—Sigma and pi bonds

A covalent bond forms when atomic orbitals containing unpaired electrons overlap to form a molecular orbital containing the bonding pair. Greater effective overlap gives a stronger bond. The overlap model describes the shared electron pair without changing the covalent-bond definition.

A sigma (σ) bond forms by direct end-on overlap along the line joining the two nuclei, with electron density centred between them. A pi (π) bond forms by sideways overlap of adjacent p orbitals, placing electron density above and below the plane of the σ bond.

A double bond contains one σ and one π bond; a triple bond contains one σ and two mutually perpendicular π bonds. Use the supported cases: C₂H₄ is sp² at each carbon with one C–C σ and one π bond; C₂H₂/HCN/N₂ use sp hybridisation with one σ and two π components in the triple-bond region.

Hybridisation mixes orbitals from the same shell: sp³ gives four hybrid orbitals, sp² gives three, and sp gives two. Apply the source-supported mapping to tetrahedral, trigonal-planar and linear arrangements, while keeping detailed molecular-shape treatment for the neighbouring topic.