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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?

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  • $\begingroup$ OH group is smaller than Cl and Al(OH)3 isn't molecular, unlike the chloride. $\endgroup$
    – Mithoron
    Jun 28, 2022 at 19:39
  • $\begingroup$ In the pure state, $\ce{|AlCl3}$ dimerizes as a molecule $\ce{Al2Cl6}$. The structure is made of two squares having one edge in common (like an open book). Each aluminum atom is in the center of each square, and the six $\ce{Cl}$ atoms are at the six apexes. Covalent bonds $\ce{Al-Cl}$ are along the diagonals. The two aluminum atoms are negatively charged. The two central chlorine atom are positively charged. The whole is not ionic. $\endgroup$
    – Maurice
    Jun 28, 2022 at 20:39

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