Writing Chemical Formulas for
Ionic Compounds
The Crisscross Method
Essential Question

How do I correctly write
chemical formulas for
ionic compounds?
What information do I need?
 Binary
Compounds
– Charges (Oxidation Numbers)
 Compounds with more than two
elements
– Charge of the Polyatomic ion
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.
-1
+2
Ca
1
Cl
CaCl2
2
Let’s Practice!
Try writing the formula for a compound made
from aluminum and sulfur.
Al+3 S-2
2
3
Al2S3
Try writing the formula for a compound made
from titanium and oxygen.
Ti+4 O-2
2
4
Ti2O4
This last one is not quite right!
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)
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:

SO4-2
OH -1
Sulfate
Hydroxide
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
Let’s Try Another!

Write the formula for a compound of Al+3
and SO4-2.
Al+3 SO4-2
2
3
Al2SO43
The 3 and 4 together make this formula
confusing. There must be something more!
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.
Conclusion
• Writing ionic chemical formulas is
easy!
All you have to do is