Question

Explain the purpose of the cris-cross method. How does it affect the overall charge of a ionic compound?

Answer 1

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

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. Crisscross the absolute values to give Al₂O₃ To find the formula for magnesium oxide:

The oxidation number of Mg is +2 and oxygen is -2. Crisscross the absolute values to give Mg₂O₂

In this example there is a common factor of 2 so divide by 2 to give MgO.