READ: Naming Ionic Compounds
READ: Naming Ionic Compounds
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 charge 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 to 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.
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).
To review the rules for naming and writing formulas for Ionic Compounds, please click through the following animated review activity.
Georgia Virtual, Bonding and Chemical Reactions, CC BY-NC-SA 3.0
After you have completed this part of the lesson, you can check the associated box on the main course page to mark it as complete 