How would we combine atomic orbitals to make molecular orbitals for diatomic molecules such as HF? Would the 2s hydrogen orbital interact with the 2s orbital of fluorine, and the 2P of hydrogen with the 2p of fluorine? No, because the energies of orbitals with corresponding quantum numbers in H and F are very different. The increased nuclear charge of +9 on F, compared to +1 on H, pulls on the electrons in all the fluorine quantum levels and makes them more stable (lower energy). The is orbital of fluorine is so much lower in energy than the is of hydrogen that they cannot possibly interact. The outer occupied orbital in F and the is H orbital are the ones that are of similar energy. The first ionization energy of each kind of atom is the energy needed to remove one electron from an outer orbital: the is of hydrogen, and the 2p of fluorine. The first ionization energy of hydrogen is 313 kcal mole, and that of fluorine is 402 kcal mole, so the 2p orbitals in F are approximately 89 kcal mole lower in energy than the is orbital in H. These are the orbitals that are similar enough in energy to combine, and their relative energies are shown at the left and right of the energy-level diagram in the margin. The 2s orbital of fluorine is off scale at the bottom of the diagram, and the is is lower still. Molecular orbitals for HF are obtained by combining the 1s of H with the 2p of F.