a. Write balanced equations for the reactions of calcium, chromium, and lithium with oxygen. Identify the charge on the cation and the charge on the anion for each product.
$\mathrm{Ca}(s)+\mathrm{O}_{2}(g) \longrightarrow \mathrm{CaO}(s)$
$\mathrm{Cr}(s)+\mathrm{O}_{2}(g) \longrightarrow \mathrm{Cr}_{2} \mathrm{O}_{3}(s)$
$\mathrm{Li}(s)+\mathrm{O}_{2}(g) \longrightarrow \mathrm{Li}_{2} \mathrm{O}(s)$
b. Circle the elements that are oxidized in the chemical equations in part a.