Transcription: So far I've only numbered the first four groups. What about the next four? Because I'm relying on valency there's a problem. Some of the elements in these groups, like phosphorus, have more than one prominent valency. Which one should I choose to do the numbering? One last set of compounds will tell us. We've looked at the empirical formulae of the highest hydrides and the highest chlorides. Now let's turn to the highest oxides.