We now ought to spell out one thing that we have never stated but rather assumed. Most atoms in stable organic molecules have a full complement of electrons (two in the case of hydrogen, eight in the cases of carbon, nitrogen, and oxygen) and so, if you make a new bond to one of those elements, you must also break an existing bond. Suppose you just ‘added’ Ph3P to MeI in this last example without breaking the C–I bond: what would happen?

This structure must be wrong because carbon cannot have five bonds—if it did it would have ten electrons in the 2s and the three 2p orbitals. Four orbitals can contain only eight electrons.

●B, C, N, and O never have more than four bonds. If you make a new bond to uncharged H, C, N, or O you must also break one of the existing bonds in the same step.