Answer
a) ($Al_{2}O_{3})$
b) ($BeI_{2})$
c) ($CaS)$
d) ($CaI_{2})$
Work Step by Step
An ionic compound is a special type of compound in chemistry where ions are held together by electrostatic forces (i.e. ionic bonds). Ionic compounds consists of a cation and an anion.
a) Aluminum Oxide ($Al_{2}O_{3})$ is an ionic compound made of aluminum and oxygen. Since Al forms $Al^{3+}$ and O forms $O^{2-}$, combing the two and crossing down the charges gives us ($Al_{2}O_{3})$
b) Beryllium Iodide ($BeI_{2})$ is an ionic compound made of beryllium and iodine. Since Be forms $Be^{2+}$ and I forms $I^{-}$, combining the two and crossing down the charges gives us ($BeI_{2})$
c) Calcium Sulfide ($CaS)$ is an ionic compound made of calcium and sulfur. Since Ca forms $Ca^{2+}$ and S forms $S^{2-}$, combining the two and crossing down the charges gives us ($CaS)$ because they both have the same magnitude of charge (i.e. 2) so it cancels out.
d) Calcium Iodide ($CaI_{2})$ is an ionic compound made of aluminum and sulfur. Since Ca forms $Ca^{2+}$ and I forms $I^{-}$, combining the two and crossing down the charges gives us ($CaI_{2})$
