13.3.3—Arrangement of σ and π bonds in molecules

A sigma (σ) bond forms by end-on overlap along the internuclear axis. A pi (π) bond forms by sideways overlap of parallel p orbitals and exists in addition to a σ bond.

A single bond is one σ bond; a double bond is one σ plus one π; a triple bond is one σ plus two π bonds. Hybridisation describes which orbitals make the σ framework and which unhybridised p orbitals make π bonds.

Ethene has five σ bonds and one π bond. Ethyne has three σ bonds and two π bonds, so rotation around the C≡C axis is not a simple single-bond rotation.

A double bond is not “two π bonds”, and a π bond is not a second atom-to-atom connection independent of the σ bond.