IV POLYATOMIC IONS Recall Use the crisscross method

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 Cu 2 S

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: C 2 H 3 O 2– Nitrate: NO 3– � Carbonate: CO 32– Nitrite: NO 2– � Sulfate: SO 42– Phosphate: PO 43– � Sulfite: SO 32– Phosphite: PO 33–

More Common Poly Atomic Ions � Three polyatomic ions ending in –ide � Hydroxide: OH– � Cyanide: CN– � � Peroxide: O 22– Ammonium: NH 4+ �One of the few positive polyatomic ions

B. Naming Polyatomic Ionic Compounds � Name cation first, then anion � Polyatomic cation = name of the ion �NH 4+ = ammonium � Polyatomic anion = name of the ion �SO 42– = sulfate � (NH 4)2(SO 4) = ammonium sulfate

Practice by writing the name for polyatomic ions Circle the cation and anion in each compound �Mg. SO 4 • Magnesium sulfate �NH 4 Cl • Ammonium chloride �Al. PO 4 • 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 3+ Al SO 4 Al 2(SO 4)3 2–

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

Write the Name 1. Li. Br. O 3 2. Mg(Cl. O 4)2 3. Al(CN)3 4. Ba(Cl. O)2 5. KC 2 H 3 O 2

Write the Formula 1. potassium hydroxide 2. iron (III) chlorate 3. ammonium oxide 4. hydrogen peroxide 5. calcium nitrite