The melting point of `MgBr_(2)` is `700^(@)C` while that of `AlBr_(3)` is only `97^(@)C`. Give reason.

Explanation

According to Fajan's rule ionic potential `(phi)` of the catins increases with increse in cationic charge and decresein cationic radii. Consequently the covalent character increases and the melting point of the corresponding salts decreases.
In case of `MgBr(=2) and AlBr_(3)`,
(i) Charge of `Mg^(2+)` is less than the charge of `Al^(3+)`
(ii) Radius of `Mg^(2+)` is greater than the radius of `Al^(3+)`.
Thus, melting point of `AlBr_(3)` is less tha that of `MgBr_(2)`.