Molecular+Formulas+-+Criss-cross+Rule

How do you write the formula of a compound given its name? Use what is known as the criss-cross rule.

1. Write the down the symbol for each element. 2. Use the valence table to find the valence of each element. (The valence of the first element is always positive and the valence of the second element is always negative.) 3. Criss-cross. Use the valence of the first element as the subscript of the second element. Use the valence of the second element as the subscript of the first element. The charges do not need to be written down.
 * The basic rule:**

e.g. magnesium phosphide, calcium nitride, aluminum oxide, boron sulphide

Valences of one do not need to be written. e.g. sodium sulphide, magnesium chloride, calcium bromide, potassium oxide
 * Exception Number One:**

If the valences are the same, nothing needs to be written. e.g. calcium oxide, magnesium sulfide, aluminum nitride, boron phosphide
 * Exception Number Two:**

If the valences can be reduced, i.e. they are multiples of a number, they should be. e.g. calcium carbide, magnesium carbide
 * Exception Number Three:**