How to Write Ionic Chemical Formulas With the Crisscross Method
The ability to construct chemical formulas from chemical names represents one of the more important and difficult skills for chemistry students to master. Chemical reactions cannot be written and balanced without accurate formulas for the chemicals involved, and the "crisscross" method represents an easy-to-remember shortcut for constructing formulas.
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Instructions
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Identify the positive cation and its charge. As an example, in the chemical name iron(III) oxide, iron(III) (chemical abbreviation Fe3+) is the cation. By convention, the cation will be listed first in a chemical name and is usually a metal (and thus located on the left side or middle of the periodic table).
If no charge is given in the name, the charge is equal to the element's group number in the periodic table. For example, the cation of magnesium oxide is magnesium ion, which is located in group 2A of the periodic table. It's symbol is therefore Mg2+.
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Identify the negatively charged anion and its charge. For single-element anions (such as chloride, fluoride, oxide, phosphide), its charge will usually be equal to its group number from the periodic table minus 8. Oxide, for example, is the anion of oxygen, which is located in group 6A on the periodic table. Thus, 6 -- 8 = -2, meaning that oxide ion exists as O2-. Anions will usually be nonmetals (located on the right side of the periodic table).
The charges on polyatomic anions (those containing more than one atom) must be looked up in a table (such as that provided by Arkansas State University, see Resources) or committed to memory.
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Construct the chemical formula by making the charge of each ion the subscript of the other ion and neglect the sign of the charge. For example, in iron(III) oxide, the cation is Fe3+ and the anion is O2-. The 3 thus becomes the subscript of the oxygen and the 2 becomes the subscript of the iron: Fe2O3.
To double-check the formula, remember that nature requires charge neutrality for chemical compounds. Therefore, all of the positive and all of the negative charges in a compound must sum to zero. In the case of Fe2O3, the total positive charge is +3(2) = +6 (a +3 charge multiplied by two atoms in the chemical formula). The total negative charge is -2(3) = -6, and +6 + (-6) = 0.
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References
Resources
- Photo Credit H2o image by Renato Francia from Fotolia.com
