Haha did you really expect me to know more than "any undergrad"?
You probably already have a ballpark idea of how much I know of MO theory just given that I'm still an A-level student. I'm not doubting that it's a vast and complicated field. My post only meant to say that I will not be grappling with anything more advanced than my immediate necessities as far as MO theory is concerned.
Most important thing, though, is:
Could you outline these very general rules for me, or say where I might find them? I can understand that having a proper bonding understanding is more important but I'd at least like to start with a complete foundation. Clearly my understanding of Lewis representation is less than complete if there still exist principles I don't know and which I should be using (my attempts were more trial and error; I know the basics of how to draw Lewis structures but not how to predict how many bonds there will be between two atoms, etc., except for a few basics).
I'm guessing it's things like this: if X (halogen) has 1 bond, O 2 bonds, N 3 bonds or C 4 bonds these will be neutral; if X, O or N have 1 bond less than these, they will be negative in formal charge, whereas if C has 1 bond less it will be positive; if X, O or N have 1 bond more, they will be positive, whereas if C has 1 bond more it will still typically be positive. (Assuming there are no free radicals.) Do not expect to find a pi bond between Period 3 elements. Try to avoid atoms with the same formal charges being bonded. Try to delocalize the formal charges over as many atoms as possible. Try to ensure that, in a molecule with several different elements, a less electronegative one never bears a more negative formal charge than a more electronegative one. If there is a radical, then, unless superceded by the rules above, I can expect to place the lone electron on the less electronegative atom.
However, even with all these rules running through my head, drawing the structure of P3O
93- isn't easy ... I would probably need more guidelines to avoid drawing the wrong structure. Any more principles I can use?
As I say, I understand the superiority of MO theory. But drawing Lewis structures isn't something I can avoid or go past for the time being, so please help me get them completely straight. It's less looking for a challenge and more making sure I can handle cases like these that I want to deal with this now.