Hybridization, Sigma/Pi Bonds, Delocalization & Resonance

Hybridization is a model that mixes atomic orbitals on a single atom to form new, equivalent hybrid orbitals suited to the observed bonding geometry. A carbon atom in its ground state has the configuration $1s^2 2s^2 2p^2$, which does not obviously explain four equivalent bonds in methane. The hybridization model resolves this: one $2s$ and three $2p$ orbitals combine to form four equivalent $sp^3$ hybrid orbitals directed toward the corners of a tetrahedron at $109.5°$. Each accepts one electron and overlaps with a hydrogen $1s$ orbital, giving four identical C-H bonds. The number of hybrid orbitals always equals the number of atomic orbitals mixed.

The three common hybridization states correspond directly to electron-domain geometry. An atom with four electron domains is $sp^3$ hybridize