1. Ionic Compounds: Formulas and Naming
SNC2D – Academic Science

2. Determining the Chemical Formula without using a drawing
The Criss-Cross Rule!

3. The Criss-Cross Rule Al Cl AlCl
Example: What is the formula for the ionic compound formed by aluminum and chlorine?
First determine the ionic charge for each element.
Record the charge on the top right hand corner of the element symbol.
Criss-cross the charges into subscripts (without the + or -)
Record the chemical formula. -1 Al +3 Cl
AlCl 3 1
The formula is AlCl3.

4. The Criss-Cross Rule
Example: What is the formula for the ionic compound formed by sodium and oxygen?

5. Summary: Steps to Writing Formulas for Ionic Compounds
Write the symbols of the element with the metal first and their ionic charges above each symbol (in superscript).
Determine how many ions of each type you need to balance the charges using the criss-cross rule. i.e. Criss-cross the ionic charges.
Write the formula using subscripts and if necessary, reduce to lowest terms.

6. Naming Ionic Compounds

7. How to Name Ionic Compounds
The name of metal is written first, followed by the name of the non- metal.
The ending of the non-metal changes from “ine” to “ide”.
Examples:
calcium + chlorine, CaCl2 = Calcium chloride
copper + iodine, CuI = Copper iodide
copper + chlorine, CuCl2 = Copper chloride