**Recall:**

$\overline{){\mathbf{Lattice}}{\mathbf{}}{\mathbf{Energy}}{\mathbf{=}}\frac{|{\mathrm{cat}}^{+}\mathrm{charge}\times {\mathrm{an}}^{-}\mathrm{charge}|}{\mathbf{cat}\mathbf{.}\mathbf{}\mathbf{period}\mathbf{}\mathbf{\#}\mathbf{+}\mathbf{an}\mathbf{.}\mathbf{}\mathbf{period}\mathbf{}\mathbf{\#}}}$

**(a) CaO or CaS**

CaO → Ca^{2+} + O^{2–}

CaS → Ca^{2+} + S^{2–}

Ca: Period 4

O: Period 2

S: Period 3

For each pair, choose the compound with the higher lattice energy, and explain your choice:

(a) CaO or CaS;

(b) BaO or SrO

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