1. Polyatomic Ions Writing Formulas / Naming Compounds
A polyatomic ion is a covalent molecule that has an ionic charge. (As opposed to being a neutral molecule.)
Poly = many
Atomic = atoms
Ion = charged particle
A charged particle that consists of more than one atom.

2. Polyatomic Ions Examples: Sulfate = SO4-2 Nitrate = NO3-1
Phosphate = PO4-3
Chlorate = ClO3-1
Carbonate CO3-2
Notice the ending has changed to –ate.

3. Writing Formulas Sodium Sulfate
Write the formula for the metal. Add the oxidation number from the periodic table.
Na+1
Write the formula for the polyatomic ion from the ion chart. Add its oxidation number.
SO4-2

4. Sodium Sulfate Na SO4-2
Determine the common factor of the two oxidation numbers. In this case, 2.
Decide how many of each ion is needed to equal the common factor. In this case, 2 sodium ions and 1 sulfate ion.
Write these numbers as the subscript for each ion.
Na2SO4

5. Calcium Phosphate
Write the formula for the metal. Add the oxidation number from the periodic table.
Ca+2
Write the formula for the polyatomic ion from the ion chart. Add its oxidation number.
PO4-3

6. Calcium Phosphate Ca +2 PO4 -3
Determine the common factor of the two oxidation numbers. In this case, 6.
Decide how many of each ion is needed to equal the common factor. In this case, 3 calcium ions and 2 phosphate ions.
Write these numbers as the subscript for each ion.
Ca3(PO4)2
Notice that the polyatomic ion must be placed in parenthesis or, instead of 2 phosphate ions, you would have 42 Oxygen atoms and 1 Phosphorus atom. Ca3PO42

7. Naming Compounds w/ Polyatomic Ions
KClO3
Write the name of the metal.
Potassium
Write the name of the polyatomic ion from the ion chart.
Chlorate
Name the compound.
Potassium Chlorate

8. Mg3(SO4)2 Name the metal. Name the polyatomic ion from the ion chart.
Magnesium
Name the polyatomic ion from the ion chart.
Sulfate
Name the compound.
Magnesium Sulfate

9. Some Additional Examples
Calcium Hydroxide
Ca+2 OH-1
The least common factor is 2. Therefore, 1 calcium ion will bond with 2 hydroxide ions to form a neutral compound.
Ca(OH)2
If you omitted the parenthesis, you would not have 2 hydroxide ions. Instead you would have 2 hydrogen atoms and one oxygen atom. CaOH2

10. Try these Write names for the following compounds. CaCO3 Al2(SO4)3
Ca(ClO3)2
K3PO4
Mg(OH)2

11. Answers Calcium carbonate Aluminum sulfate Calcium chlorate
Potassium phosphate
Magnesium hydroxide

12. Now, these! Write formulas for the following compounds.
Lithium sulfate
Calcium acetate
Aluminum nitrate
Magnesium phosphate
Sodium carbonate

13. Answers
Li2SO4
Ca(C2H3O2)2
Al(NO3)3
Mg3(PO4)2
Na2CO3