A student compares lattice-energy trends for alkaline earth oxide and chloride solids. The ionic radii for the relevant ions in comparable coordination environments are shown below.

Ion	Ionic radius, \(r\) (\(\text{pm}\))
\(\text{Mg}^{2+}\)	\(72\)
\(\text{Ca}^{2+}\)	\(100\)
\(\text{O}^{2-}\)	\(140\)
\(\text{Cl}^{-}\)	\(181\)

The student models the magnitude of the dominant attraction between neighboring ions using the index

\[ I=\dfrac{|z_+z_-|}{r_++r_-} \]

where a larger value of \(I\) predicts a larger lattice-energy magnitude. Assume that other structural factors are comparable within each cation replacement. Which statement correctly predicts whether \(\text{CaO}\rightarrow\text{MgO}\) or \(\text{CaCl}_2\rightarrow\text{MgCl}_2\) produces the larger increase in the modeled lattice-energy magnitude and gives the correct justification?