Formulas of Ionic Compounds with Polyatomic Ions

Formulas of Ionic Compounds with Polyatomic Ions 

Writing formulas for ionic compounds that include polyatomic ions is done in the same way formulas are written for binary compounds. The polyatomic ions are treated as single units that bear a charge. The goal is to get the right ratio of cation to anion so that an overall electrically neutral combination is obtained. The same rules used for binary compounds apply here, with one modification  

The symbol of the cation always comes before the symbol of the anion in the formula. 

The sum of the positive charges on the cations must equal the sum of the negative charges on the anions so the formula of the compound is electrically neutral. 

If two or more of a particular polyatomic ion are needed in a formula, the ion is enclosed in parentheses, followed by a subscript number to indicate the number of that ion in the formula unit. The smallest set of numbers must be used. 

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