Compounds with Multivalent Elements

Compounds with Multivalent Elements

1. Some metals have more than one stable ion. For example, iron has two stable ion: Fe and Fe . Elements with more than one stable ion are called Multivalent Elements. Ionic compounds containing multivalent elements must have Roman numerals in their names to indicate which ions is forming that compound. The roman numeral is written in brackets after the element to indicate the charge. For example, chromium is multivalent, so Chromium (III) sulfide indicates that the Cr ion forms that compound. Roman numerals are not used in formulas, because you can figure out the charge on the ion by looking at the formula.

   • You can find the Roman numeral to use in the name of a multivalent ion by using the subscripts in the formula. For example, in FeBr , the subscript 2 after the Br is a guide to the iron ions charge. Recall that the positive and negative charges in an ionic compound must be equal. According to this rule, only an Fe could pair up with two Br to give this formula unit. FeBr would be written out as iron(II) bromide. In FeBr , only an Fe could pair up with three Br to give this formula unit. FeBr would be written out as iron(III) bromide.
     • When writing the names of ionic ionic compounds, use the anions charge to find the cations charge when the cation is multivalent. Remember that the Roman numeral is needed only if the metal element is multivalent. You can use the periodic table to find out which elements are multivalent. Select the first one listed in the periodic table if you are not given any other information. This is the most common ion for each element.
       • Example Problem: Write the name of the compound that has the formula Cu N .
         • 1. Identify the ions that form the compound.
           • Cu Copper Ion
           • N Nitride Ion
           • 2. Use the charge of the nitride ion (3-) and the rule that the total positive and negative charges in the formula unit must be equal. Three copper ions are present in the formula unit so each must have a charge of 1+.
             • 3. Write the name of the compound.
               • The name of the compound is copper(I) nitride.