The former is called ionic compound because oxygen atoms are strong enough to attract electrons, so aluminum is almost naked. For the latter, although chlorine is more electronegative than oxygen, its molecular structure (indeed, molecules! ! ! ) It's not AlCl3, but Al2Cl6. But also exists independently as a molecule, so it is a * * * valence compound.
In fact, this was discovered by humans because of the abnormal melting point.