In chemistry, the criss-cross method is a way to write the formulas of ionic compounds. The criss-cross method makes it easier to determine the subscripts for each element in an ionic compound.
Using this method, the number associated with the charge of the first ion is assigned as the subscript of the second ion. The number of the charge on the second ion is assigned as the subscript of the first ion.
Using Li1+ and O2- as an example, the 1 from the Li1+ ion would be assigned as the subscript of O, while the 2 from the O2- ion would be assigned as the subscript of Li. The resulting formula is Li2O. This formula balances out the positive and negative charges of the ions. The criss-cross method does not work if the charges are multiples of each other, or if the charges have already been offset, as is the case with ions that have charges of 2+ and 2-.
If the criss-cross method does not work, use the traditional method of writing a formula. First, determine the ions present in the compound. Magnesium nitride has magnesium and nitrogen ions. Then write the symbol for each ion with the charge above it. Positively charged ions should be written first. For magnesium nitride, the ions are Mg2+ and N3-. The next step is to determine the number of ions needed to balance out the charges. Three magnesium ions are needed to balance out two nitrogen ions in magnesium nitride. Finally, write the number of ions needed for each element as a subscript after the element's symbol. The correct ionic formula for magnesium nitride is Mg3N2 using this method.