IV. Polyatomic Ions

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

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–

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

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

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

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.

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

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

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

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