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Contents Home Bookshelves Introductory, Conceptual, and GOB Chemistry Map: Introductory Chemistry (Tro) 5: Molecules and Compounds Expand/collapse global location
5.5: Writing Formulas for Ionic Compounds
Last updatedFeb 9, 2021
5.4: A Molecular View of Elements and Compounds
5.6: Nomenclature- Naming Compounds
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Learning Objectives
Write the correct formula for an ionic compound.
Recognize polyatomic ions in chemical formulas.
Ionic compounds do not exist as molecules. In the solid state, ionic compounds are in crystal lattice containing many ions each of the cation and anion. An ionic formula, like NaCl , is an empirical formula. This formula merely indicates that sodium chloride is made of an equal number of sodium and chloride ions. Sodium sulfide, another ionic compound, has the formula Na2S . This formula indicates that this compound is made up of twice as many sodium ions as sulfide ions. This section will teach you how to find the correct ratio of ions, so that you can write a correct formula.
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.
Example 5.5.1 : Aluminum Nitride and Lithium Oxide
Write the formulas for aluminum nitride and lithium oxide.
Solution
1. Write the symbol and charge of the cation (metal) first and the anion (nonmetal) second.
Al3+N3−
Li+O2−
2. Use a multiplier to make the total charge of the cations and anions equal to each other.
total charge of cations = total charge of anions
1(3+) = 1(3-)
+3 = -3
total charge of cations = total charge of anions
2(1+) = 1(2-)
+2 = -2
3. Use the multipliers as subscript for each ion.
Al1N1
Li2O1
4. Write the final formula. Leave out all charges and all subscripts that are 1.
AlN
