Energy Levels of Different Molecular Orbitals

The energy of a molecular orbital depends on (i) the type of atomic orbitals from which it is formed and (ii) the extent of overlap of the atomic orbitals forming the molecular orbital. 

The number of molecular orbitals formed is always equal to the number of atomic orbitals combining. 

The more stable a bonding molecular orbital, the less stable the corresponding antibonding molecular orbital. 

The molecules are built up by adding electrons to available molecular orbitals, just in the same way as the filling up of electrons in atomic orbitals according to the principles of aufbau, Pauli and Hund s. These may be summed up as - 

The molecular orbital with the lowest energy is filled first. 

---