A molecular orbital energy level diagram shows the MOs resulting from the combination of the valence atomic orbitals of all atoms of the molecule, arranged in order of increasing energy. These diagrams are used to describe bonding within a molecule in molecular orbital terms. To obtain the molecular orbital description of the bonding in a molecule or atom, follow these steps: 

Drawing Molecular Orbital Diagrams

1. Select the appropriate molecular orbital energy level diagram by looking at the atomic orbitals in each atom involved in the molecule.
2. Determine the number of electrons in the molecule. Note that in applying MO theory, we will account for all electrons. This includes both the inner-shell electrons and the valence electrons.
3. Fill the molecular orbitals from bottom to top until all the electrons are added (lowest energy level available (Aufbau Principle). Describe the electrons with arrows.
   1. A maximum of  two arrows in each molecular orbital, with the first arrow pointing up and the second pointing down (Pauli Exclusion Principle).
4. Orbitals of equal energy are half filled with parallel spin before they begin to pair up.

For the diatomic species, H₂, He₂, Li₂, Be₂,B₂, C₂, and N₂, the ?_2p bonding orbitals are lower in energy than the ?_2p bonding orbital and the ?_2p antibonding orbitals are lower in energy than the ?_2p anti bonding orbital. Molecular orbital calculations indicate, however, that for O₂, F₂, and hypothetical Ne₂ molecules, the ?_2p bonding orbital is lower in energy than the ?_2p bonding orbitals while the ?_2p antibonding orbitals are lower in energy than the ?_2p antibonding orbital.

Example 1. 

Draw the energy level diagram for the diatomic helium molecule, He₂. 

Worked Problem

Example 2.

Draw the energy level diagram for the diatomic oxygen molecule, O₂.

Worked Problem

Subpages (2): Example 1 Example 2