1,3-Butadiene is a conjugated system with four sp2 hybridized carbons, each with an unhybridized p orbital containing one electron. A linear combination of the four unhybridized atomic p orbitals gives rise to four π molecular orbitals, two bonding and two antibonding. The four π electrons occupy the two lowest molecular orbitals, ψ1 and ψ2. ψ2 is the highest occupied molecular orbital or HOMO, and ψ3 is the lowest unoccupied molecular orbital or LUMO. Notice that ψ1 is formed by an in-phase overlap of all four p orbitals leading to bonding interactions between all adjacent carbons. This explains the partial double-bond character of the C2–C3 bond and the stabilization gained from electron delocalization. In ψ2, the out-of-phase overlap between C2 and C3 gives rise to a node, a region of zero electron density with no bonding interaction. As the energy of the molecular orbitals increases from ψ1 to ψ4, the number of nodes increases incrementally, and the bonding interaction decreases.