1. IV. Polyatomic Ions

2. Recall
Use the criss-cross method to write the formula for the ionic compound between nitrogen and aluminum. Write the name for this compound as well.
Write the name for the ionic compound Cu2S

3. A. What is a Polyatomic Ion?
Polyatomic Ion – ion composed of 2 or more different atoms that act like a single unit.
Usually end in –ate or –ide
Acetate: C2H3O2– Nitrate: NO3–
Carbonate: CO32– Nitrite: NO2–
Sulfate: SO42– Phosphate: PO43–
Sulfite: SO32– Phosphite: PO33–

4. More Common Poly Atomic Ions
Three polyatomic ions ending in –ide
Hydroxide: OH–
Cyanide: CN–
Peroxide: O22–
Ammonium: NH4+
One of the few positive polyatomic ions

5. B. Naming Polyatomic Ionic Compounds
Name cation first, then anion
Polyatomic cation = name of the ion
NH4+ = ammonium
Polyatomic anion = name of the ion
SO42– = sulfate
(NH4)2(SO4) = ammonium sulfate

6. Practice by writing the name for polyatomic ions Circle the cation and anion in each compound
MgSO4
NH4Cl
AlPO4
Magnesium sulfate
Ammonium chloride
Aluminum phosphate

7. C. Writing Ionic Formulas With Polyatomic Ions
Write symbol and charge of both cation and anion.
Use “criss-cross” method to balance formula
If more than one polyatomic ion is needed, parenthesis must be used.

8. Polyatomic Ion Example
Write the formula for the compound aluminum sulfate
aluminum sulfate
Al3+ SO42–
Al2(SO4)3

9. Polyatomic Ion Example
Write the formula for the compound magnesium hydroxide
magnesium hydoxide
Mg2+ OH–
Mg(OH)2

10. Write the Name
LiBrO3
Mg(ClO4)2
Al(CN)3
Ba(ClO)2
KC2H3O2

11. Write the Formula potassium hydroxide iron (III) chlorate
ammonium oxide
hydrogen peroxide
calcium nitrite