# An ionic compound forms when calcium reacts with iodine. If the sample contains 7.4 xx 10^21 calcium ions, how many iodide ions does it contain?

It must contain $14.8 \times {10}^{21}$ iodide ions.
Calcium metal has 2 valence electrons. It readily forms a $C {a}^{2 +}$ ion. Because iodine readily forms a uninegative ion, ${I}^{-}$, a neutral salt will form with composition $C a {I}_{2}$.
$C a \left(g\right) + {I}_{2} \left(g\right) \rightarrow C a {I}_{2} \left(s\right)$