Polyatomic Compounds Lecture Notes

Polyatomic Compounds

• Polyatomic compounds are groups of atoms that can be found as parts of a molecule.
• These atoms bond in such a way that they have a net electric charge, usually negative.
• The resulting group of atoms is an ion; therefore, it can form an ionic bond

Naming Polyatomic Compounds

1. Write the name of the metal, including the Roman numeral if necessary.
2. Write the name of the polyatomic ion. Note: Do not change the ending of the polyatomic ion name to -ide.

Common Polyatomic Ions

• Acetate: $C<em>2H</em>3O_2$-
• Hydroxide: $OH$-
• Ammonium: $NH_4$+
• Hypochlorite: $ClO$-
• Carbonate: $CO_3$2-
• Nitrate: $NO_3$-
• Chlorate: $ClO_3$-
• Nitrite: $NO_2$-
• Chlorite: $ClO_2$-
• Oxalate: $C<em>2O</em>4$2-
• Chromate: $CrO_4$2-
• Perchlorate: $ClO_4$-
• Cyanide: $CN$-
• Permanganate: $MnO_4$-
• Dichromate: $Cr<em>2O</em>7$2-
• Bicarbonate: $HCO_3$-
• Bisulfate: $HSO_4$-
• Phosphate: $PO_4$3-
• Sulfate: $SO_4$2-
• Sulfite: $SO_3$2-
• Bisulfite: $HSO_3$-

Polyatomic Exceptions: Ammonium

• Ammonium is the only common positive polyatomic ion
• Ammonium will form ionic compounds with non-metals.

Naming Compounds with Ammonium

1. Write the name of the polyatomic ion (Ammonium).
2. Write the name of the nonmetal, changing the end to -ide.

Writing Formulas Involving Polyatomic Ions

• Apply the rules for writing ionic compound formulas to polyatomic compounds; treat the polyatomic ion as a single unit.

Steps for Writing Formulas

1. Write the symbols of each ion, beginning with the cation (metal).
2. Write the ionic charge above each ion.
3. Cross over the charges to become subscripts (put any original subscript in brackets).
4. Rewrite the formula. Make sure to put brackets around the polyatomic ion if it has gained a new subscript!

Example: Sodium Phosphate

1. Symbols: Na $PO_4$
2. Charges: Na+1 $PO_4$-3
3. Cross over charges: Na3 $PO_4$
4. Final Formula: $Na<em>3PO</em>4$