A: Whether an aqueous solution of a metal ion is acidic depends on two principal factors:

(i) The first is the amount of charge on the cation; and 

(ii) The size of the ion.

The greater the charge on the cation, the greater is the ability of the metal ion to draw electron density away from the O—H bond in a H₂O molecule in its hydration sphere, favouring the release of a proton (H⁺). The smaller the cation, the more highly concentrated is the positive charge. Hence, for a given positive charge, the smaller the cation, the more acidic is the solution.

The ratio of the charge on the cation to the volume of the cation is called the charge density.

charge density (ρ) = ionic charge /ionic volume

Greater the charge density of a metal cation, move will be the extent of pulling electron density from the O—H bond and the more acidic is the hydrated cation.

A highly concentrated positive charge on a small cation is better able to pull electron density from the O-H bond than is a less concentrated positive charge on a larger cation.

Thus the small, highly charged Al³⁺ ion produces acidic solutions, but the larger Na⁺ cation, with a charge of just unit positive, does not increase the concentration of H₃O⁺.

In fact, none of the group 1 cations produces appreciably acidic solutions, and only Be²⁺ of the group 2 elements is small enough to do so (pka = 5.4).