You already know how to draw molecular orbital diagrams for diatomic molecules (ie molecules that have only two atoms) by combining orbitals with compatible symmetry. Now, you are going to do the same for polyatomic molecules (ie molecules with more than two atoms). You will start with the examples [HF2]\(-\), H2O, CO2, NH3, and BH3, shown below.
The approach to drawing a molecular orbital diagram for a polyatomic molecule changes only slightly compared to what you’ve done before with diatomic molecules. The main difference now is that the orbitals on pendant atoms are treated as a set that is fixed in space with respect to the central atom. Each group of orbitals on the pendant atoms takes on its own symmetry identity. Molecular orbitals form when the symmetry of the set is compatible with the atomic orbitals on the central atom.