Predicting Formulas of Compounds with Polyatomic Ions

Example Problem

Ammonium Ion
This is the structure of an ammonium ion. Todd Helmenstine

Polyatomic ions are ions made up of more than one atomic element. This example problem demonstrates how to predict the molecular formulas of several compounds involving polyatomic ions.


Predict the formulas of these compounds, which contain polyatomic ions:

  1. barium hydroxide
  2. ammonium phosphate
  3. potassium sulfate


The formulas of compounds containing polyatomic ions are found in much the same way as formulas are found for monoatomic ions.

Make sure you are familiar with the most common polyatomic ions. Here is a list of polyatomic ions to help you. Look at the locations of the elements on the Periodic Table. Atoms in the same column as each other (Group) tend to exhibit similar characteristics, including the number of electrons the elements would need to gain or lose to resemble the nearest noble gas atom. To determine common ionic compounds formed by elements, keep the following in mind:

  • Group I ions (alkali metals) have +1 charges.
  • Group 2 ions (alkaline earth metals) have +2 charges.
  • Group 6 ions (nonmetals) have -2 charges.
  • Group 7 ions (halides) have -1 charges.
  • There is no simple way to predict the charges of the transition metals. Look on a table listing charges (valences) for possible values. For introductory and general chemistry courses, the +1, +2, and +3 charges are most often used.

When you write the formula for an ionic compound, remember that the positive ion is always listed first.

When there are two or more polyatomic ions in a formula, enclose the polyatomic ion in parentheses.

Write down the information you have for the charges of the component ions and balance them to answer the problem.

  1. Barium has a +2 charge and hydroxide has a -1 charge, therefore
    1 Ba2+ ion is required to balance 2 OH- ions
  1. Ammonium has a +1 charge and phosphate has a -3 charge, therefore
    3 NH4+ ions are required to balance 1 PO43- ion
  2. Potassium has a +1 charge and sulfate has a -2 charge, therefore
    2 K+ ions are required to balance 1 SO42- ion


  1. Ba(OH)2
  2. (NH4)3PO4
  3. K2SO4

The charges listed above for atoms within groups are the common charges, but you should be aware that the elements sometimes take on different charges. See the table of the valences of the elements for a list of the charges that the elements have been known to assume.