Recall that 1,3-butadiene is a four-carbon conjugated system. In comparison, the allyl cation and anion are three-carbon conjugated systems. A linear combination of the three unhybridized atomic p orbitals in allylic systems gives three π molecular orbitals. The lowest energy molecular orbital, ψ1, is formed by an in-phase overlap of all p orbitals and is entirely bonding. ψ2 has one node passing through the central carbon. Since there is no overlap between adjacent carbons, this is a nonbonding orbital and energetically similar to the atomic orbitals. ψ3 is entirely antibonding with three out-of-phase p orbitals forming two nodes. The two π electrons of the allyl cation fill up ψ1, which becomes the HOMO, making ψ2 the LUMO. In comparison, the allyl anion has four π electrons distributed between ψ1 and ψ2, where ψ2 is the HOMO and ψ3 the LUMO. Interestingly, in both systems, the charge is concentrated on the end carbons, which agrees with the resonance picture.