1. Writing Chemical Formulas for Ionic Compounds
The Crisscross Method

2. Essential Question
How do I correctly write chemical formulas for ionic compounds?

3. What information do I need?
Binary Compounds
Charges (Oxidation Numbers)
Compounds with more than two elements
Charge of the Polyatomic ion

4. Process for Writing Binary Chemical Formulas for Ionic Compounds
The element or polyatomic ion with the positive oxidation number (cation) is written first.
The element with the negative oxidation number (anion) goes second.
Crisscross the absolute values of the oxidation numbers.
Ca+2
Cl-1 1 2
CaCl2

5. Let’s Practice! Try writing the formula for a compound made
from aluminum and sulfur.
Al+3
S-2
Al2S3 2 3
Try writing the formula for a compound made
from titanium and oxygen.
Ti+4
O-2
Ti2O4 2 4
This last one is not quite right!

6. Now What Did We Do Wrong!
For ionic compounds, when subscripts are not in their simplest form, divide them through by their common multiple to get the empirical formula.
Ti2O4
TiO2
Empirical formulas show the simplest ratio
of the elements in a compound.
Molecular formulas show the actual number
of atoms in a molecule. (Covalent compounds only)

7. Process for Writing Non-binary Ionic Formulas
Some of the most common non-binary compounds include a polyatomic ion.
Polyatomic ions are ions that are made up of more than one element.
Examples are:
SO Sulfate
OH Hydroxide

8. Writing Formulas with Polyatomic Ions
The method for writing formulas that include polyatomic ions is similar to the one for writing binary formulas.
Let’s write the formula for a compound made of Cu+2 and CO3-2.
Cu+2
CO3-2 2 2
Taking it to its simplest form CuCO3

9. Let’s Try Another! Write the formula for a compound of Al+3 and SO4-2.
Al2SO43
The 3 and 4 together make this formula
confusing. There must be something more!

10. The Answer is Parentheses!
When you have more than one polyatomic ion in a formula, you must place parentheses around it.
Al2SO43 ( )
Al2
SO4 3
This formula shows that the compound is
made up of 2 aluminum atoms and 3 sulfate
polyatomic ions.

11. Conclusion Writing ionic chemical formulas is easy!
All you have to do is