Chapter 5: Nomenclature

Introduction to Nomenclature

Nomenclature is the systematic method of naming chemical compounds. Understanding how to name compounds and write their formulas is essential for clear communication in chemistry. This chapter covers the rules for naming binary compounds (compounds with two elements), compounds containing polyatomic ions, and acids.

Binary Compounds

Definition and Classification

• Binary compounds are composed of exactly two different elements.

• They are divided into two broad classes:

  • Compounds containing a metal and a nonmetal (usually ionic compounds)

  • Compounds containing two nonmetals (usually molecular compounds)

Naming Binary Compounds Containing a Metal and a Nonmetal

Binary Ionic Compounds

When a metal reacts with a nonmetal, the metal loses electrons to form a cation and the nonmetal gains electrons to form an anion. The resulting compound is called a binary ionic compound.

Type I Binary Ionic Compounds

• Contain metals that form only one type of cation (fixed charge).

• Examples: Na+, Ca2+, Al3+.

• Naming rules:

  • The cation is named first, using the element name.

  • The anion is named second, using the root of the element name plus the suffix -ide.

  • Example: NaCl is sodium chloride.

• Key properties:

  • Compounds are electrically neutral (total positive and negative charges balance).

  • Always name the cation first.

Examples:

• LiBr = lithium bromide

• SrCl2 = strontium chloride

• Rb2Se = rubidium selenide

• CaO = calcium oxide

• AlF3 = aluminum fluoride

Type II Binary Ionic Compounds

• Contain metals (often transition metals) that can form more than one type of cation (variable charge).

• Examples: Fe2+ and Fe3+, Cu+ and Cu2+.

• Naming rules:

  • The cation is named first, followed by a Roman numeral in parentheses to indicate its charge.

  • The anion is named as in Type I (root + -ide).

  • Example: FeCl2 is iron(II) chloride.

Examples:

• CuF = copper(I) fluoride

• CuF2 = copper(II) fluoride

• Fe2S3 = iron(III) sulfide

• TiO2 = titanium(IV) oxide

• AuCl3 = gold(III) chloride

Table: Common Type II Cations

*Mercury(I) ions always occur as Hg22+ pairs.

Naming Binary Compounds Containing Only Nonmetals (Type III)

Type III Binary Compounds

• Composed of two nonmetals.

• Nonmetals can combine in various ratios, so prefixes are used to indicate the number of each atom.

Table: Prefixes Used in Type III Compounds

• Naming rules:

  • The first element is named first, using the full element name.

  • The second element is named as if it were an anion (root + -ide).

  • Prefixes indicate the number of each atom; mono- is never used for the first element.

  • Example: CO is carbon monoxide, not monocarbon monoxide.

Examples:

• N2O4 = dinitrogen tetroxide

• SF6 = sulfur hexafluoride

• NCl3 = nitrogen trichloride

• PF5 = phosphorus pentafluoride

• S2O = disulfur monoxide

Summary Table: Three Types of Binary Compounds

Naming Compounds Containing Polyatomic Ions

Polyatomic Ions

• Polyatomic ions are charged groups of two or more covalently bonded atoms that act as a single ion.

• Common examples include:

  • OH- (hydroxide)

  • NO3- (nitrate)

  • SO42- (sulfate)

  • NH4+ (ammonium)

• Names of polyatomic ions must be memorized.

Table: Common Polyatomic Ions

Additional info: Table entries inferred from standard polyatomic ions.

• Naming rules:

  • Name the cation first, then the polyatomic anion.

  • If the cation is a metal with variable charge, specify the charge with a Roman numeral.

  • Example: NaOH is sodium hydroxide; FeSO4 is iron(II) sulfate.

Examples:

• BaSO4 = barium sulfate

• NH4Cl = ammonium chloride

• KOH = potassium hydroxide

• LiCO3 = lithium carbonate

• NaCN = sodium cyanide

Naming Acids

Definition and Properties

• Acids are substances that produce H+ ions when dissolved in water.

• Acids have a sour taste and are important in both laboratory and industrial chemistry.

• The naming of acids depends on the composition of the anion.

Rules for Naming Acids

• If the anion does not contain oxygen:

  • Name begins with hydro- and ends with -ic acid.

  • Example: HCl (aq) is hydrochloric acid.

• If the anion contains oxygen (oxyacids):

  • If the anion ends in -ate, the acid name ends in -ic acid.

  • If the anion ends in -ite, the acid name ends in -ous acid.

  • Prefixes such as per- and hypo- are retained.

  • Example: HNO3 (nitrate) is nitric acid; HNO2 (nitrite) is nitrous acid.

Table: Examples of Acid Names

Examples:

• HF = hydrofluoric acid

• H2S = hydrosulfuric acid

• HI = hydroiodic acid

• H2Se = hydroselenic acid

• HBr = hydrobromic acid

• HNO2 = nitrous acid

• H2SO3 = sulfurous acid

• H3PO4 = phosphoric acid

• H2CO3 = carbonic acid

• HC2H3O2 = acetic acid

Writing Formulas from Names

General Strategy

• Identify the cation and anion (including their charges).

• Combine them in ratios that yield a neutral compound.

• For acids, determine the number of H+ ions needed to balance the anion's charge.

Examples:

• carbon monoxide = CO

• beryllium chloride = BeCl2

• aluminum carbonate = Al2(CO3)3

• cobalt(II) nitrate = Co(NO3)2

• oxygen difluoride = OF2

Acid Formulas from Names:

• sulfuric acid = H2SO4

• chlorous acid = HClO2

• perchloric acid = HClO4

• chromic acid = H2CrO4

Summary and Reflection

• Binary compounds are classified as Type I, II, or III based on the elements involved.

• Polyatomic ions and acids have special naming rules that must be memorized and practiced.

• Being able to both name compounds from formulas and write formulas from names is essential for success in chemistry.