Molecular Orbitals of Li₂

Generating molecular orbitals of molecules more complex than hydrogen using the LCAO method requires following a few additional guidelines:

–          The number of MOs generated is equal to the number of atomic orbitals combined.

–          Combined atomic orbitals should be of similar energy levels.

–          The effectiveness of atomic orbital combination depends on the amount of orbital overlap. Increased overlap lowers the energy of the bonding molecular orbital further, and raises the energy of the antibonding molecular orbital.

Let’s follow these guidelines and generate a molecular orbital electron configuration diagram for Li₂ (Figure 9.21 “Molecular orbital electron configuration energy diagram for dilithium”):

Figure 9.21. Molecular orbital electron configuration energy diagram for dilithium.

Notice that we have combined the 1s atomic orbitals, as before in the H₂ example, to generate bonding and antibonding molecular orbitals that are completely filled by both atoms’ 1s electrons. Similarly 2s atomic orbitals combine, giving a bonding orbital and an antibonding orbital, which are filled with the remaining valence electrons starting from the bottom up. The atomic orbitals that combine are of similar energy levels; a 1s orbital does not combine with one of the 2s orbitals.

The bond order can be determined for this molecule to be:

Bond order = 1/2 (4 – 2) = 1

Therefore Li₂ would have a single bond.