Crisscross method

The Thorpe's Crisscross method is a method of finding out the chemical formula of a metal and non-metal that combine to form an ionic bond created by high school chemistry teacher Kim Thorpe.

To use this method, the absolute value of the oxidation number of the first ion is used as the subscript of the second, and vice versa. The subscripts are then reduced by the greatest common factor. For instance, to find the formula for aluminium oxide:

The oxidation number of Al is +3 and oxygen is -2. Criss cross the absolute values to give Al₂O₃

To find the formula for magnesium oxide:-

The oxidation number of Mg is +2 and oxygen is -2. Criss cross the absolute values to give Mg₂O₂ In this example there is a common factor of 2 so divide by 2 to give MgO.

It is a helpful method just cross them and put them down without changes.