Textbook Problem

For each of the following pairs, indicate which species is larger. Explain your reasoning in terms of the electron structure of each species.

msp;  $\begin{array}{l}\text{a}.{\text{ Mg}}^{\text{2}+}\text{or Mg} \text{c}.{\text{ Rb}}^{+}{\text{or Br}}^{-}\\ \text{b}.{\text{ Ca}}^{\text{2}+}{\text{or K}}^{+} \text{d}.{\text{ Se}}^{\text{2}-}\text{or Se}\end{array}$