Ionic Compounds with Polyatomic Ions

Well, first things first. If you need a refresher on naming Ionic Compounds click the link below to be whisked back in time to the summer of love, otherwise known as November of last year and a post I wrote about how to name molecular and ionic compounds…

The Name Game (November 20, 2012)

Now, naming an Ionic compound that has a polyatomic ion in it is just as simple as naming any other ionic compound. It’s simply…

The name of the metal (space) The name of the Polyatomic Ion


The trick is that you need to know the names of the polyatomic Ions. Here is the first group of them (figure on the right). Here are the rules for naming polyatomic ions:

  1. “-ate” has 1 more Oxygen atom than “-ite”
  2. If you add an Oxygen to an ion that ends in “-ate” add the prefix “per-” (ChlorATE = ClO3 // PERchlorate = CLO4)
  3. If you subtract an Oxygen from an ion that ends in “-ite” add the prefix “hypo-” (ChlorITE = ClO2// HYPOchlorIte = CLO)

The entire molecule, NO3 for example, has a charge of  -1. So lets say I was joining Nitrate (which is NO3) with Magnesium…
Well, Magnesium’s charge is 2+ so how many Nitrate Ion’s would I need? If each NO3 has a charge of -1 then I need 2! So The formula would be…


The parentheses go around the entire polyatomic ion because the entire ion has a charge and therefore I need 2 of the whole thing.

So, to name the formula, I just use the names of each part: Magnesium Nitrate!

All the same rules for Ionic compounds still apply (Charges must balance & Metals with multiple charges get roman numerals). Try the first set of polyatomic Ion problems (Practice #1) and then move on to the 2nd group of Polyatomic ions, shown at the bottom, and then try their practice problems (Practice #2).