Crisscross rule

One way of predicting the values of the subscripts is to crisscross the valences. This is not a rule of nomenclature, but for practice purposes in this exercise it will be referred to as the crisscross rule. It works most of the time and therefore is worth considering. Example ... 

If the crisscross rule is applied, you should reduce the formula if possible. For example Mn4+02 crisscrosses to Mn2C>4, which reduces to Mn02 If a formula is given, the crisscross rule can be reversed to give the valences ... 

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

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. 

If magnesium reacts with oxygen, then automatic application of the crisscross rule would lead to the formula Mg202, which is incorrect because the subscripts are not in the lowest whole-number ratio. For the same reason, lead(IV) oxide would have the formula Pb02 and not Pb204. Make sure the formula has the lowest whole-number ratio of elements. 

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

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

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

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 ... 

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

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. )... 

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.)... 

After you use the crisscross rule, reduce all the subscripts by a common factor, if possible, to get the right formula. 

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