Most transition metals can form more than one simple positive ion. For example, iron forms both $\mathrm{Fe}^{2+}$ and $\mathrm{Fe}^{3+}$ ions, and tin forms both $\mathrm{Sn}^{2+}$ and $\mathrm{Sn}^{4+}$ ions. Which is the smaller ion of each pair, and why?