Chapter 15, Problem 118AP

Both $\text{M}{\text{g}}^{\text{+}}$ ' and $\text{C}{\text{a}}^{\text{2+}}$ are important biological ions. One of their functions is to bind to the phosphate group of ATP molecules or amino acids of proteins. For Group 2A metals in general, the equilibrium constant for binding to the anions increases in the order $\text{B}{\text{a}}^{\text{2+}}\text{<S}{\text{r}}^{\text{2+}}\text{<C}{\text{a}}^{\text{2+}}\text{<M}{\text{g}}^{\text{2+}}$. What property of the Group 2A metal cations might account for this trend?