The Criss-Cross Method for Writing Chemical Formulas

The Criss-Cross method is an easy way to find the subscripts on elements when writing formulas for ionic compounds based on their name. There are a few things to watch out for, so be careful!

When we use the criss-cross method we first find the ionic charge on each element. We can do that based on the group the element appears in on the Periodic Table. This video can help if you’re having trouble finding ionic charge.

Finding Ionic Charge:    • Figuring out the Charge on Ions  

When we have polyatomic ions, we need to either remember their charge or look them up on a table of polyatomic ions (if your instructor lets you). The criss-cross method works well with polyatomic ions, but you do need to be careful and use parentheses when you have a subscript after a polyatomic ion. For example:

CaNO3 2 is incorrect. It should be Ca(NO3)2.


Helpful video:

Memorizing Polyatomic Ions:    • Memorizing and Using the List of Poly...