CBSE Class 11-science Answered

Hi Pavitra,

Due to the increase in polarization of X^- anion by Al³⁺ cation from F^- to I^- or due to the increase of the size of the anions from F^- to I^-, the covalent character of the trihalides of Al is in the order:

AlF₃  <  AlCl₃  <  AlBr₃  <  AlI₃

Thus, the fluorine is more electronegative than chlorine and is able to pull away the electron from aluminium atom.

In other words the fluorine atom's attraction on the electron in aluminium is much greater than chlorine. So with chlorine, Al shares the electrons forming a covalent bond while fluorine attracts the electron toward itself forming F^- ion and Al³⁺ ion and thus it is an electrovalent/ionic bond.