Naming Ionic Compounds

Ionic compounds are made up of a metal cation and a nonmetal anion. The metal will always be the first element in the formula, while a nonmetal will always be the second. The metal cation is named first and remains unchanged. In other words, the metal is the same name as it appears on the periodic table. The metal cation is followed by the nonmetal anion. The ending of the nonmetal's name is changed to -ide. Subscripts in the formula do not affect the name.

Refer to the table below for some examples.

Chemical Formula	Name
KF	Potassium fluoride
Na₃N	Sodium nitride
Ca₃P₂	Calcium phosphide
MgCl₂	Magnesium chloride

Writing Formulas for Ionic Compounds

If you know the name of a binary ionic compound, you can write its chemical formula. Start by writing the metal ion with its charge, followed by the nonmetal ion with its charge. Because the overall compound must be electrically neutral, decide how many of each ion is needed in order for the positive and negative charges to cancel each other out. Consider the compound aluminum nitride. The ions are:

Al⁺³ N^-3

Since the ions have charges that are equal in magnitude, one of each will be the lowest ratio of ions in the formula. The formula of aluminum nitride is AlN.

The ions for the compound lithium oxide are:

Li⁺¹ O^-2

In this case, two lithium ions are required to balance out the charge of one oxide ion. The formula of lithium oxide is Li₂O.

An alternative way of writing a correct formula for an ionic compound is to use the crisscross method. In this method, when the charges do not equal zero the numerical value of each of the ion charges is crossed over to become the subscript of the other ion. Signs of the charges are dropped. Shown below is the crisscross method for aluminum oxide.

Aluminum Oxide with crisscross of the superscripts

The red arrows indicate that the 3 from the 3+ charge will cross over to become the subscript of the O.The 2 from the 2− charge will cross over to become the subscript of the Al. The formula for aluminum oxide is Al₂O₃. You can check that the resulting charge is 0 by the fact that now Al has a total of +6 charge (2 x +3) and O has a total of -6 charge (3 x -2).

This cross-over method is only used when the charges do not equal zero. Note that we did not use this method for Al⁺³ and N^-3 because +3 - 3 = 0. Since the overall charges equal zero, we need just one of each atom for the formula (AlN).

For a summary of the text above as well as a demonstration of practicing writing formulas, watch the video below. Please be patient while the video loads. It may take a few minutes.

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