Question

AlCl₃ is covalent where as AlF₃ is ionic. Why ?

Answer 1

According to Fajan’s rule, with increasing size of the anion and with decreasing size of the cation, ionic character of the compound decreases while the covalent character increases.

Here, since the cation is same in both the compounds, the size of the anion decides the nature of the compound. Size of chlorine being greater than fluorine, AlCl₃ is covalent while AlF₃ is ionic.