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 themetal 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.