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?