Crisscross rule, ionic compounds

Another way of deriving the formula of the ionic compound is the crisscross rule. In this technique the cation and anion are written side by side. The numerical value of the superscript charge on the cation (without the sign) becomes the subscript on the nonmetal in the compound, and the superscript charge on the anion becomes the subscript on the metal in the compound. Figure 11.3 illustrates the crisscross rule for the reaction between aluminum and oxygen. 

The crisscross rule can help determine the formula of an ionic compound. 

A quick way to determine the formula of cui ionic compound is to use the crisscross rule Take the numerical value of the metal ion s superscript (forget about the charge symbol) and move it to the bottom right-hcuid side of the nonmetal s symbol — as a subscript. Then tcike the numerical value of the nonmetal s superscript and make it the subscript of the metal. (Note that if the numerical value is 1, it s just understood and not shown.)... 

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