Energies of molecular orbitals arising from 2s and 2p atomic orbitals

While, as shown in the diagrams at the right, the relative energies of the molecular orbitals on a species depend on the atoms involved in bonding, there are some generalizations that can be made.
 

• A bonding orbital is of lower energy than its antibonding orbital.
  2sσ<2sσ* etc - this is because the electron density between the nuclei is higher in the case of a bonding orbital.
  
   
• 2p molecular orbitals are of higher energy than 2s molecular orbitals.
  This is because the 2p atomic orbitals are higher in energy than the 2s atomic orbitals. Even the antibonding orbital 2sσ* is lower in energy than any of the 2p molecular orbitals.
  
   
• The energy difference between 2pσ and 2pσ* orbitals is greater than between 2pπ and 2pπ* orbitals.
  This is because end-on overlap is more effective than side-on overlap.
  
   

The 2pσ orbital is of higher energy for elements of lower atomic number because due to the energies of the 2s and 2p atomic orbitals being more similar, mixing of the 2pσ orbital with the 2sσ orbitals leads to an increase in its energy.