Ionic Compounds: Writing Formulas, Formula Units, and Naming

Writing Formulas for Ionic Compounds

Principles of Ionic Compound Formulas

Ionic compounds are composed of positively charged cations and negatively charged anions. The chemical formula for an ionic compound is always written to reflect a neutral overall charge, even though the compound consists of charged ions.

• Neutrality Rule: The total positive charge must balance the total negative charge in the formula.

• Example: For potassium fluoride (K+ and F-): $(+1) + (-1) = 0$ Formula: KF

• Example: For calcium iodide (Ca2+ and I-): $(+2) + 2 \times (-1) = 0$ Formula: CaI2

• Example: For magnesium phosphate (Mg2+ and PO43-): $3 \times (+2) + 2 \times (-3) = 0$ Formula: Mg3(PO4)2

• Example: For ammonium sulfate (NH4+ and SO42-): $2 \times (+1) + (-2) = 0$ Formula: (NH4)2SO4

Formula Units

The smallest representative particle of an ionic compound is called a formula unit. Unlike molecules, formula units are the simplest ratio of ions that results in a neutral compound.

• Definition: A formula unit is the lowest whole-number ratio of ions in an ionic compound.

• Key Point: There is no such thing as a single neutral particle of an ionic compound; the formula unit is used to describe the composition.

Practice: Counting Particles in Formula Units

Examples

• KNO3: 1 K+ ion, 1 NO3- ion → 2 particles per formula unit

• NH4Cl: 1 NH4+ ion, 1 Cl- ion → 2 particles per formula unit

• Ba(NO3)2: 1 Ba2+ ion, 2 NO3- ions → 3 particles per formula unit

• Li3PO4: 3 Li+ ions, 1 PO43- ion → 4 particles per formula unit

Naming Ionic Compounds

Rules for Naming

To name an ionic compound, first name the cation (positive ion), then the anion (negative ion). For transition metals, indicate the charge with Roman numerals in parentheses.

• SrBr2: Strontium bromide

• Cs2HPO4: Cesium hydrogen phosphate

• (NH4)2SO4: Ammonium sulfate

• Na2S: Sodium sulfide

• Cu2O: Copper(I) oxide

• Fe2O3: Iron(III) oxide

Writing Formulas from Names

• Copper(II) sulfate: CuSO4

• Silver nitrate: AgNO3

• Lithium oxide: Li2O

• Iron(II) sulfide: FeS

• Potassium dihydrogen phosphate: KH2PO4

Introduction to Acids and Bases

Acids in Water

When an acid dissolves in water, it reacts to form hydronium ions (H3O+).

• General Reaction: $\mathrm{HA} + \mathrm{H_2O} \rightarrow \mathrm{H_3O^+} + \mathrm{A^-}$

Bases in Water

Metal hydroxides dissolve in water to yield aqueous hydroxide ions (OH-). Other bases, such as ammonia (NH3), react with water to produce hydroxide ions.

• Metal Hydroxide Example: $\mathrm{NaOH} \rightarrow \mathrm{Na^+} + \mathrm{OH^-}$

• Ammonia Example: $\mathrm{NH_3} + \mathrm{H_2O} \rightarrow \mathrm{NH_4^+} + \mathrm{OH^-}$