## Chemistry: Principles and Practice (3rd Edition)

(a) MgF$_2$ (b) Na$_2$O (c) Sc$_2$Se$_3$ (d) Ba$_3$N$_2$
(a) MgF$_2$: Magnesium (Mg), an alkaline earth metal, has a valence of 2+. Fluorine (F), a halogen, has a valence of 1-. The two elements combine in a 1:2 ratio, meaning we will need one magnesium ion for every two fluoride ions to balance the charges in this compound. (b) Na$_2$O: Sodium (Na) is an alkali metal and has a valence of 1+, and oxygen (O) has a valence of 2-. The two elements combine in a 2:1 ratio, meaning we will need two sodium ions for every oxide ion to balance the charges in this compound. (c) Sc$_2$Se$_3$: Scandium (Sc) is a transition metal and has a valence of 3+, and selenium (Se) has a valence of 2-. The two elements combine in a 2:3 ratio, meaning we will need two scandium ions for every three selenide ions to balance the charges in this compound. (d) Ba$_3$N$_2$: Barium (Ba), an alkaline metal, has a valence of 2+. Nitrogen (N) has a valence of 3-. We will need three barium ions for every two nitride ions to balance the charges in this compound.