pibonding

Pi bonding is a type of covalent interaction formed by the sideways overlap of parallel p atomic orbitals on adjacent atoms, creating electron density above and below the internuclear axis. It accompanies sigma bonding to produce multiple bonds and is central to the chemistry of alkenes, alkynes, carbonyl groups, and aromatic systems. Unlike sigma bonds, pi bonds do not rely on end-to-end overlap along the bond axis.

In valence-bond terms, pi bonds arise when a p orbital on each atom remains unhybridized after the sigma framework forms (for example, after sp2 or sp hybridization). The sideways overlap of these p orbitals forms a pi bond that lies above and below the bond plane. In a double bond, there is one pi bond together with one sigma bond; in a triple bond, two pi bonds accompany one sigma bond.

Pi bonds are generally weaker than sigma bonds because their lobes overlap less completely and there is a nodal plane along the bond axis. They still contribute significantly to bond strength and shorten bond length relative to single bonds. Pi bonds also restrict rotation about the bond, leading to defined geometry; in conjugated systems, pi electrons can be delocalized over several atoms, enhancing stability (as in benzene) or enabling resonance structures.

Pi bonds are typical sites of chemical reactivity, undergoing electrophilic attack and addition reactions that break the pi bond while leaving the sigma framework intact. The concept is described in both molecular orbital and valence-bond theories, with MO theory treating pi bonding as the interaction of bonding and antibonding p-orbital combinations spread over the participating atoms.