Binary compounds contain only two different elements. Binary ionic compounds are composed of a metal and a nonmetal. We can name these binary ionic compounds systematically using two naming schemes, depending on the type of metal. For Type One metals, which only form one type of ion, we use the name of the cation (same as the metal) followed by the base name of the nonmetal with an -ide ending. For example, NaCl is sodium chloride. We change the -ine ending in chlorine to the -ide ending of chloride to make it an anion. Another example is Al2O3, where aluminum is the metal and we change the ending of oxygen to oxide. Thus, the name of our ionic compound is aluminum oxide. Type Two metals can form ions with more than one possible charge. To name compounds with Type Two metals, we need to indicate the charge on the metal using roman numerals within the compound name. For example, let's name CuCl2. Copper is a Type Two metal because it can form more than one possible ion. So we use the charge of our anion to determine the charge on copper. Since the formula unit is electrically neutral, the charge on one copper ion plus the charge on two chloride ions must equal zero. From the periodic table, chloride has a one minus charge, so the copper ion must have a two plus charge. When naming this ionic compound, we put the charge of two in roman numerals after the word "copper". Thus, the name for this compound is copper(II) chloride. The alkaline metals, alkaline earth metals, aluminum, gallium, zinc, cadmium, and silver are all Type One metals. Alkaline metals form a cation with a one plus charge, and alkaline earth metals form a cation with a two plus charge. If the metal is not on this list of Type One metals, we can assume it is a Type Two metal. There is one unique Type Two metal, mercury, that can either form a two plus or a one plus ion. However, mercury one shows up as Hg2 2+ instead of just Hg+. For example, if we write the formula for mercury one bromide, we know that mercury one is Hg2 2+ and bromide is Br minus. So we need two bromide ions to balance the charge of mercury. Therefore, the formula becomes Hg2Br2. This may seem like an exception to the rule of wanting the lowest whole-number ratio of ions, but we must remember that the formula for mercury one is actually Hg2 2+.