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Ionic Compounds with Polyatomic Ions

Polyatomic ions are a group of two or more elements that have an overall ionic charge. Most times the polyatomic ion will function as an anion, but there are a few polyatomic ions that are cations. To see a pop-up of some common polyatomic ions, click on polyatomic ions in the tools menu or click here.

Please note: All of the polyatomic ions are written with parenthesis. You CANNOT change any of the information inside the parenthesis. Think of anything inside parenthesis as being protected. When you cross oxidation numbers you must write it outside the parenthesis.

Writing ionic compounds that have polyatomic ions is, for the most part, a lot like writing binary ionic compounds. The only difference is one extra step.

Part I: How to Write a Formula for an Ionic Compound that contains a Polyatomic Ion

For the first example I’ll use calcium and carbonate.

Description of Action
1. Write the symbol of the cation with its charge. 1. Ca2+
2. To the right of the cation, write the polyatomic anion and its charge. 2. Ca2+ (CO3)2-
3. Cross each element’s oxidation number to the lower right side of the other element’s symbol. 3. Ca2+ (CO3)2-
Result: Ca2- (CO3)2+
4. Remove all (+) signs, (-) signs and ones. 4. Ca2(CO3)2
5. Reduce, if necessary. Remember, do not touch anything inside the parenthesis. 5. Ca(CO3)
6. If there is no subscript outside the anion’s parenthesis, remove the parenthesis. 6. Answer: CaCO3

Please note: Many of the polyatomic ions have very similar names that differ only in one letter. For example, phosphate is (PO4)3- and phosphite is (PO3)3-.

Part II: Naming Ionic Compounds with Polyatomic Ions

For the first example we will use our answer from the first example, CaCO3

Description of Action
1. Write the name of the cation. 1. calcium
2. To the right of the cation name, write anion’s name. 2. calcium carbonate

That’s it! Just two steps. Don’t get confused by the lack of parenthesis or any number that may appear outside of it. All that matters is the formula inside the parenthesis or what is left over.

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