Why do group 3 metals form more acidic aqueous solutions than group 2 metals?

The equation of a metal ion forming a metal hexaaqua ion in aqueous solution is as follows.

Mⁿ⁺ + 6H₂O > [M(H₂O)₆)]ⁿ⁺

These are acidic because the metal is positively charged, and so attracts electron density away from the oxygen and from the O-H bonds. The O-H bonds therefore become weaker, and are easier to break. When the O-H bond breaks, a proton is given off which means that the metal hexaaqua ion is more acidic than regular water.

Because group 3 metals form 3+ cations, and group 2 metals form 2+ cations, group 3 metals form more positive ions than group 2 metals. This means that group 3 metals attract more electron density from the O-H bonds than group 2 metals, and so the O-H bonds become even weaker. The protons are therefore released easier causing the pH to decrease further.