Textbook Problem

or each of the following pairs, indicate which is smaller.

msp;  $\begin{array}{l}\text{a}.{\text{ Rb}}^{+}0{\text{r Na}}^{+} \text{c}.{\text{ F}}^{-}0{\text{r I}}^{-}\\ \text{b}.{\text{ Mg}}^{\text{2}+}{\text{or Al}}^{\text{3}+} \text{d}.{\text{ Na}}^{+}{\text{or K}}^{+}\end{array}$