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.

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.

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

Sodium Sulfate Na +1 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

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

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

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

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

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

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

Answers Calcium carbonate Aluminum sulfate Calcium chlorate Potassium phosphate Magnesium hydroxide

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

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