1. Polyatomic Ions

2. Binary vs. Polyatomic
So far, we have only been talking about binary compounds (compounds made up of only 2 different elements).
Many compounds are polyatomic (consisting of more than 2 different elements).

3. Several common polyatomic ions exist, consisting of 2 or more different nonmetals joined together by covalent bonds.
The formulas, names and charges of the most common ones must be memorized.

4. Name
Chemical Formula
ammonium
NH4+
hydroxide
OH–
carbonate
CO32–
nitrate
NO3–
phosphate
PO43–
sulfate
SO42–
chlorate
ClO3-
acetate
C2H3O2-

5. Rules for Dealing with Polyatomics
Treat the polyatomic ion as one ion – don’t think about the different elements.
Most end with “-ate” instead of “-ide”.
When you need to indicate more than one polyatomic, place it in brackets first.

6. Examples Write the formula for potassium hydroxide. K+ OH¯
Use the cross-over method to get the formula.
KOH

7. Examples Write the formula for sodium sulfate. Na+ SO42-
Use the cross-over method to get the formula.
Na2SO4

8. Examples Write the formula for ammonium sulfide. NH4+ S2-
Use the cross-over method to get the formula.
(NH4)2S