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

(a) CaCl$_2$ (b) Rb$_2$S (c) Li$_3$N (d) Y$_2$Se$_3$
(a) CaCl$_2$: Calcium (Ca), an alkaline earth metal, has a valence of 2+. Chlorine (Cl), a halogen, has a valence of 1-. The two elements combine in a 1:2 ratio, meaning we will need one calcium ion for every two chloride ions to balance the charges in this compound. (b) Rb$_2$S: Rubidium (Rb) is an alkali metal and has a valence of 1+, and sulfur (S) has a valence of 2-. The two elements combine in a 2:1 ratio, meaning we will need two rubidium ions for every sulfide ion to balance the charges in this compound. (c) Li$_3$N: Lithium (Li) is an alkali metal and has a valence of 1+, and nitrogen (N) has a valence of 3-. The two elements combine in a 3:1 ratio, meaning we will need three lithium ions for every nitride ion to balance the charges in this compound. (d) Y$_2$Se$_3$: Yttrium (Y), a transition metal, has a valence of 3+. Selenium (Se) has a valence of 2-. We will need two yttrium ions for every three selenide ions to balance the charges in this compound.