How to determine ionic or covalent bond when writing Lewis Structure formula

Question

A bonding between a metal and a nonmetal is an ionic bond.

However, another rule says if the EN value difference between the two elements is equal or great than 1.7, then that's ionic.

Which rule we should always go with first to determine?

As I can see there are other elements like N & Be, C & Ca, or the Nonmetals with the transition metals have an EN difference of less than 1.7.

Just wondering what we should look at first to determine?

Answer 1

This is an age-old problem in introductory or general chemistry. It isn't black and white, it has shades of gray. The bottom line is that there is a continuum of polarity from zero to > 3.3 and therefore there also is a continuum of ionic character. There is actually a formula to calculate the PERCENT IONIC CHARACTER, but I won't present it here. Suffice it to say, you should look at the difference in EN values and not just assume that a metal and a non metal is 100% ionic. If you have to take a test and give an answer, you should use the "rule" that the professor has given you, but recognize that this is a guide, not really a "rule".