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

(a) LiI (b) Cs$_2$O (c) YCl$_3$
(a) LiI: Lithium, an alkali metal, has a valence of 1+. Iodine, a halogen, has a valence of 1-. The two elements combine in a 1:1 ratio, meaning we will need one lithium ion for every iodide ion to balance the charges in this compound. (b) Cs$_2$O: Cesium is an alkali metal and has a valence of 1+, and oxygen has a valence of 2-. The two elements combine in a 2:1 ratio, meaning we will need two cesium ions for every oxide ion to balance the charges in this compound. (c) YCl$_3$: Yttrium is a transition metal with a valence of 3+. Chlorine is a halogen and has a valence of 1-. We will need one yttrium ion for every three chlorine ions to balance the charges in this compound.