I tried with fajans' rules.
First I got AlCl3 and Al2O3. There, because O2- has high negative charge I decided that Al2O3 is less ionic. But my teacher told us that we can get the same thing by comparing the electronegativity difference. But When I applied it to this pair, an error occurred. Al-O has higher electronegativity difference, but it is less ionic. Is that supposed to happen ?
When I take all those 3 compounds I have no idea how to compare AlCl3 and Al(OH)3. Their charges are the same. I think (OH)- ion is much larger than Cl3. But not sure. Therefore I thought AlCl3 is more ionic. But it turns out to be that Al(OH)3 is the answer. A lot of places suggest that AlCl3 and Al2O3 are covalent. Can't I use fajans' rules to more covalent compounds?