Whats wrong with the simple view of PCl5?
You will remember that the dots-and-crosses picture of PCl5 looks awkward because the phosphorus doesn’t end up with a noble gas structure. This diagram also shows only the outer electrons. In this case, a more modern view makes things look better by abandoning any pretence of worrying about noble gas structures. If the phosphorus is going to form PCl5 it has first to generate 5 unpaired electrons. It does this by promoting one of the electrons in the 3s orbital to the next available higher energy orbital. Which higher energy orbital? It uses one of the 3d orbitals. You might have expected it to use the 4s orbital because this is the orbital that fills before the 3d when atoms are being built from scratch. Not so! Apart from when you are building the atoms in the first place, the 3d always counts as the lower energy orbital. This leaves the phosphorus with this arrangement of its electrons: The 3-level electrons now rearrange (hybridise) themselves to give 5 hybrid orbitals, all of equ
