7.1. Ionic Compounds http://www.ck12.org
HgCl. Likewise, the peroxide ion, O 22 −, is also a unit that must stay together in its formulas. For example, the
formula for potassium peroxide is K 2 O 2.
Sample Problem 7.2: Writing Formulas for Ternary Ionic Compounds
Write the correct formulas for the following ionic compounds:
- potassium sulfate
- zinc phosphate
Step 1: Plan the problem.
In each case, write the metal cation followed by the nonmetal anion. Crisscross the ion charges in order to make the
ionic compound neutral. Use parentheses around the polyatomic ion if more than one is present in the final formula.
Reduce to the lowest ratio if necessary.
Step 2: Solutions.
1.
2.
Step 3: Think about your result.
The formula for potassium sulfate is K 2 SO 4. Two potassium cations with 1+ charges balance out the 2−charge of
each sulfate ion. The formula for zinc phosphate is Zn 3 (PO 4 ) 2. Three zinc cations with 2+ charges balance out two
phosphate anions with 3−charges.