Which of the following is a covalent bond? 1.Al2O3 2.AlF3 3.AlCl3 4.Al2(SO4)3 The correct answer is AlCl3.But I have a confusion between option c and d.A/c Fajan's rule more the size of anion more is the covalent character. So,SO4^-2 have large size than Cl^-3 .So option d is correct but option c is the right answer.Please explain this question .
Fajan's rule is correct, but the sulfate ion is complex. We need to ask what is being polarized, a large sulfate ion as a whole, or an oxygen atom specifically. As it turns out, the aluminum acts on a nearest oxygen, so that even if this oxygen is more polarizable because it has a sulfur to draw on, a chlorine is still more polarizable.
Another thing is that aluminum chloride is dimeric in the gas phase, whereas aluminum sulfate is just an ionic solid - although you might still say there is some covalent character in the aluminum-sulfate bonds.