Alkenes: MO Model
In molecular orbital theory (MO theory), atomic orbitals (AOs) with identical symmetry are being combined to form molecular orbitals (MOs). These are then filled with available electrons according to the sequence of energy levels. In a reversal of the valence-bond method, MO theory first combines the orbitals and then fills them. In order to simplify the discussion, only the C=C double bond in ethene is being considered, i.e. C-H single bonds are not included.
Combination of two -atomic hybrid orbitals leads to one bonding σ-orbital and one antibonding σ*-molecular orbital. Interaction between two p-atomic orbitals oriented parallel to each other generates one π- and one π*-molecular orbital. Since in each case only two electrons are available for bonding, only the bonding σ MO and the π MO are filled. The antibonding σ*- und π*-molecular orbitals remain unoccupied. The distribution of resulting electron density is identical to the one observed with the valence bond theory.