Using the crisscross rule

There s a quick way to determine the formula of an ionic compound Use the crisscross rule. 

Compounds involving polyatomic ions work exactly the same way. For example, here s the compound made from the ammonium cation and the sulfide anion  

Notice that because two ammonium ions (two positive charges) are needed to neutralize the two negative charges of the sulfide ion, the ammonium ion is enclosed in parentheses and a subscript 2 is added. 

The crisscross rule works very well, but there s a situation where you have to be careful. Suppose that you want to write the compound formed when magnesium reacts with oxygen. Magnesium, an alkaline earth metal, forms a 2+ cation, and oxygen forms a 2- anion. So you might predict that the formula is 

But this formula is incorrect. After you use the crisscross rule, you need to reduce all the subscripts by a common factor, if possible. In this case, you divide each subscript by 2 and get the correct formula  

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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Be careful when using the crisscross rule to reduce the subscripts to their lowest whole-number ratio. 

When using the crisscross rule be sure the subscripts are reduced to the lowest whole-number ratio. 

Figure 5-1 shows this process for magnesium and bromine. (Forget about the crisscrossing lines for now. 1 explain them in the upcoming section Using the crisscross rule. )... 

And finally, if you have the name, you can derive the formula and the charge on the ions. For example, suppose that you re given the name cuprous oxide. You know that the cuprous ion is Cu and the oxide ion is 0 . Applying the crisscross rule (from the earlier section Using the crisscross rule ), you get the following formula ... 

Going back to the previous section, where you were looking at how to determine the Roman numerals written after some elements, you may see how this can be done using the crisscross method. To determine the Roman numeral, you use a reverse crisscross technique to determine the charge on the cation. This charge then becomes the value of the Roman numeral. While you can probably eyeball a solution to these, we ll list a set of rules that will work for all examples ... 

So what happens if you react aluminum and oxygen Figure 6-4 shows the crisscross rule used for this reaction. 

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