Rules for Filling Molecular Orbitals

The following rules are followed for filling up molecular orbitals with electrons in order to write molecular configurations:

1. The molecular orbital having the lowest energy is filled first. This is in accordance with Aufbau principle as already discussed for atoms.

2. Each molecular orbital can accommodate maximum of two electrons having opposite spins. This is in accordance with Pauli's exclusion principle.

3. If there are two molecular orbitals of the same energy, the pairing of electrons will occur only after each orbital of same energy has one electron. This is in accordance with Hund's rule.