Learning Objectives

• Write the correct formula for an ionic compound.
• Recognize polyatomic ions in chemical formulas.

Introduction

Ionic compounds exist in a solid state as crystal lattices containing many ions of both cations and anions. The formula for an ionic compound, such as NaCl or Na₂S, represents the ratio of ions in the compound. In this section, we will learn how to find the correct ratio of ions and write a correct formula for an ionic compound.

Writing Formulas for Binary Ionic Compounds

To write the formula for a binary ionic compound, you need to know the names and charges of the ions involved. The formula should indicate the ratio of ions needed to balance the positive and negative charges.

For example, let’s write the formulas for aluminum nitride and lithium oxide.

1. Write the symbol and charge of the cation (metal) first and the anion (nonmetal) second.
2. Aluminum nitride: Al³⁺, N^3-

3. Lithium oxide: Li⁺, O^2-

4. Use a multiplier to make the total charge of the cations and anions equal to each other.

5. Total charge of cations = Total charge of anions
6. For aluminum nitride: 1(3+) = 1(3-) + 3

7. For lithium oxide: 2(1+) = 1(2-) + 2

8. Use the multipliers as subscripts for each ion.

9. Aluminum nitride: Al₁N₁

10. Lithium oxide: Li₂O₁

11. Write the final formula, leaving out all charges and subscripts that are 1.

12. Aluminum nitride: AlN
13. Lithium oxide: Li₂O

The Crisscross Method

An alternative way to write a correct formula for an ionic compound is to use the crisscross method. In this method, the numerical value of each ion’s charge is crossed over to become the subscript of the other ion, and the signs of the charges are dropped.

Let’s use the crisscross method to write the formula for lead (IV) oxide.

1. Write the symbol and charge of the cation (metal) first and the anion (nonmetal) second.

2. Lead (IV) oxide: Pb⁴⁺, O^2-

3. Transpose only the number of the positive charge to become the subscript of the anion and the number of the negative charge to become the subscript of the cation.

4. Lead (IV) oxide: Pb₂O₄

5. Reduce to the lowest ratio.

6. Lead (IV) oxide: PbO₂

Writing Formulas for Ionic Compounds Containing Polyatomic Ions

Writing a formula for ionic compounds containing polyatomic ions follows the same steps as for binary ionic compounds. The positive and negative charges must balance.

For example, let’s write the formula for calcium nitrate.

1. Write the symbol and charge of the cation (metal) first and the anion (polyatomic ion) second.

2. Calcium nitrate: Ca²⁺, NO₃^–

3. Transpose only the number of the positive charge to become the subscript of the anion and the number of the negative charge to become the subscript of the cation.

4. Calcium nitrate: Ca(NO₃)₂

5. Reduce to the lowest ratio.

6. Calcium nitrate: Ca(NO₃)₂

Recognizing Ionic Compounds

There are two ways to recognize ionic compounds.

Method 1: Metal and Nonmetal Elements

Compounds between metal and nonmetal elements are usually ionic. For example, CaBr₂ contains a metal element (calcium) and a nonmetal element (bromine), making it an ionic compound. On the other hand, NO₂ contains two nonmetal elements (nitrogen and oxygen) and is not an ionic compound.

Method 2: Polyatomic Ions

If you recognize the formula of a polyatomic ion in a compound, the compound is ionic. For example, Ba(NO₃)₂ contains the nitrate ion (NO₃^–), indicating that Ba is the Ba²⁺ ion. Therefore, this compound is ionic.

Conclusion

By following the steps outlined above, you can write correct formulas for ionic compounds. Remember to balance the charges of the cations and anions to ensure a neutral compound. Recognizing the presence of metal and nonmetal elements, as well as polyatomic ions, can help you identify whether a compound is ionic or not.