Boron (B 2 ) : B 2 is a good example of the energy level shift caused by the mixing of s and p orbitals. In the absence of mixing, the σ g (2p) orbital is expected to be lower in energy than the π u (2p) orbitals and the resulting molecule would be diamagnetic. However, mixing of the σ g (2s) orbital with the σ g (2p) orbital lowers the energy of the σ g (2s) orbital and increases the energy of the σ g (2p) orbital to a higher level than the π orbitals, giving the order of energies shown below. As a result, the last two electrons are unpaired in the degenerate (having the same energy) π orbitals, and the molecule is paramagnetic.

( σ 1s) 2 ( σ *1s) 2 ( σ 2s) 2 ( σ *2s) 2 ( π 2p 1 x = π 2p 1 y ) ( σ p z ) 0 .