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.
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