Name: _______________________________________
Date:__________________
Formulas with Polyatomic Ions
Instructions: Using the criss-cross method, write the formula for each ionic
compound using the charges given. Write all polyatomic ions in a different color.
Do not write the parentheses or outside subscript in this color.
Example: Co₂( CO₃ )₃
Ask yourself:
“How many of each ion do I need for the total charge to equal zero?”
That is what the subscript will tell you when looking at the formula.
OH⁻
Na⁺
Mg ²⁺
NH₄ ⁺
Ca²⁺
K⁺
Al ³⁺
Pb ⁴⁺
NO₃⁻
CO₃²⁻
SO₄²⁻
PO₄³⁻
Name: _______________________________________
Date:__________________
Example of the criss-cross method:
After you find the charges of each ion:
*Remember: The subscript is telling you how many of each ion there
is. You do NOT need to bring the + or – down with the number.