I'm being taught that the kind of bonds that exist between elements depends on the electronegativity difference between the elements. A difference less than 1.7 is covalent and a difference higher than 1.7 is ionic.

Separately, I've been taught that bonds between metals and non-metals are ionic. What happens when these two 'rules' disagree? When Magnesium and Phosporus bond for example.

I understand that there isn't always a definite border between covalent and ionic bonding, but what should my rule of thumb be? That bonds between metals and non-metals are always ionic, and that bonds between non-metals are covalent unless there's a difference of more than 1.7?

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    $\begingroup$ In principle, it should be possible to exhaustively list all (simple stoichiometric) binary compounds (about 20 thousand) and all the electronegativity differences between elements (about 10 thousand), then parse the results to at least qualitatively determine which rule of thumb is more accurate. Just for fun, I've set up a spreadsheet with Pauling electronegativity differences, and have coloured the combinations which result in combinations "on the fence" of the (somewhat arbitrary) value of 1.7. Some further processing might be enlightening. $\endgroup$ Commented Nov 12, 2020 at 12:09
  • $\begingroup$ Though I do think there is something to be learned from digging into the data, I must reiterate the fuzziness of what makes a compound "ionic" or "covalent", and that the rules of thumb are another layer of fuzziness on top of that. One rather quickly realises such simple descriptions are too superficial. $\endgroup$ Commented Nov 12, 2020 at 12:25
  • $\begingroup$ chemistry.stackexchange.com/questions/9222/… $\endgroup$
    – Mithoron
    Commented Nov 12, 2020 at 14:05