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 |
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 |
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. |
