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?

  • $\begingroup$ 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


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Browse other questions tagged or ask your own question.