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# Rank each set of ions in order of decreasing size: Ca$^{2+}$, Sr$^{2+}$, Mg$^{2+}$.

$\begin{array}{1 1} Sr^{2+} > Ca^{2+} > Mg^{2+}. \\ Ca^{2+} > Sr^{2+} > Mg^{2+}. \\ Mg^{2+} > Sr^{2+} > Ca^{2+}.\\ Ca^{2+} > Mg^{2+} > Sa^{2+}. \end{array}$

Answer: Sr$^{2+}$ > Ca$^{2+}$ > Mg$^{2+}$.
Compare positions in the periodic table, formation of positive and negative ions and changes in size due to gain or loss of electrons.
These are members of the same Group 2A(2), and decrease in size going up the group.