Question

why are BeF2, AlCl3 and BeCl2 covalent compounds and not ionic compounds?

Answer 1

The electronegativity difference between Be and Cl and between Al and Cl just isn't large enough to support ionic behavior. Actually, both compounds are polar covalent which means they do have some ionic character. In a real sense, both Be and Al are small ions and they hold onto their electrons much more than the larger elements do; hence, their electronegativity is higher (which makes the difference between them and Cl smaller). Both Be and Al are exceptions to the rule of eight.