We have seen that 1s atomic orbitals on two atoms form two molecular orbitals designated as σ1s and σ*1s. In the same manner, the 2s and 2p atomic orbitals (eight atomic orbitals on two atoms) give rise to the following eight molecular orbitals:
Antibonding MOs σ *2s σ*2p z π *2p x π *2py
Bonding MOs σ2s σ2pz π 2p x π 2py
The energy levels of these molecular orbitals have been determined experimentally from spectroscopic data for homonuclear diatomic molecules of second row elements of the periodic table. The increasing order of energies of various molecular orbitals for O2 and F2 is given below:
σ1s < σ * 1s < σ 2 s < σ * 2 s < σ 2 p z < ( π 2 p x = π 2 p y ) < ( π * 2 p x = π * 2 p y ) < σ * 2 pz
However, this sequence of energy levels of molecular orbitals is not correct for the remaining molecules Li2, Be2, B2, C2, N2. For instance, it has been observed experimentally that for molecules such as B2, C2, N2, etc. the increasing order of energies of various molecular orbitals is
σ1s < σ * 1s < σ 2s < σ * 2s < ( π 2 p x = π 2 p y ) < σ 2 pz < ( π * 2 p x = π * 2 p y ) < σ * 2 p z
The important characteristic feature of this order is that the energy of σ 2 p z molecular orbital is higher than that of π 2px and π 2py molecular orbitals.