Nomenclature of Inorganic Compounds: Ions, Acids, and Hydrates

Nomenclature of Inorganic Compounds

Introduction

Nomenclature is the systematic method of naming chemical compounds. In general chemistry, understanding how to name ions, acids, and hydrates is essential for clear communication and comprehension of chemical formulas and reactions.

Monatomic and Polyatomic Ions

Monatomic Cations and Anions

Monatomic ions are ions consisting of a single atom with a positive (cation) or negative (anion) charge.

• Cations are named by adding the word 'ion' after the element name (e.g., Na+ is sodium ion).

• Anions are named by replacing the ending of the element name with '-ide' (e.g., Cl- is chloride ion).

Examples:

• Na+: Sodium ion

• O2-: Oxide ion

• S2-: Sulfide ion

Variable Charge (Transition Metal) Cations

Transition metals often form more than one type of cation. Their charges are indicated using Roman numerals in parentheses (Stock system) or classical names with suffixes:

• Stock System: Iron(II) ion (Fe2+), Iron(III) ion (Fe3+)

• Classical System: Ferrous ion (Fe2+), Ferric ion (Fe3+)

For classical names, the lower charge uses '-ous' and the higher charge uses '-ic'. Latin names are sometimes used for elements (e.g., Cu+: cuprous, Cu2+: cupric).

Table: Classical Names of Some Common Cations

Element	Lower Charge	Higher Charge
Iron	Ferrous (Fe2+)	Ferric (Fe3+)
Copper	Cuprous (Cu+)	Cupric (Cu2+)
Tin	Stannous (Sn2+)	Stannic (Sn4+)
Lead	Plumbous (Pb2+)	Plumbic (Pb4+)
Additional info:	See Table 6.2 for more examples.

Polyatomic Ions

Polyatomic ions are composed of two or more atoms covalently bonded, carrying a net charge.

• Common polyatomic anions: Nitrate (NO3-), Sulfate (SO42-), Phosphate (PO43-), Acetate (CH3COO-).

• Common polyatomic cations: Ammonium (NH4+).

Table: Names of Special Anions

Formula	Name
OH-	Hydroxide
CN-	Cyanide
CO32-	Carbonate
NO3-	Nitrate
SO42-	Sulfate
PO43-	Phosphate
CH3COO-	Acetate

Naming Ionic Compounds

Binary Ionic Compounds

Binary ionic compounds consist of a metal cation and a nonmetal anion. The cation is named first, followed by the anion with the '-ide' ending.

• NaBr: Sodium bromide

• Cu2S: Copper(I) sulfide

• Al2O3: Aluminum oxide

If the metal can form more than one charge, indicate the charge with Roman numerals (e.g., Iron(III) chloride).

Compounds with Polyatomic Ions

When a compound contains a polyatomic ion, use the name of the ion directly.

• NaNO3: Sodium nitrate

• BaSO4: Barium sulfate

• Al(CH3COO)3: Aluminum acetate

Naming Anions

Monatomic Anions

Monatomic anions are named by changing the ending of the element to '-ide'.

• F-: Fluoride

• Cl-: Chloride

• O2-: Oxide

Oxoanions

Oxoanions are polyatomic ions containing oxygen. Their names depend on the number of oxygen atoms:

• -ate: More oxygen atoms (e.g., NO3- nitrate)

• -ite: Fewer oxygen atoms (e.g., NO2- nitrite)

• Prefixes: 'per-' (most oxygen), 'hypo-' (least oxygen)

Examples:

• ClO4-: Perchlorate

• ClO3-: Chlorate

• ClO2-: Chlorite

• ClO-: Hypochlorite

Naming Acids

Binary Acids

Binary acids consist of hydrogen and one other nonmetal. The pure compound is named as hydrogen + nonmetal (e.g., hydrogen chloride), but when dissolved in water, it is named as an acid (e.g., hydrochloric acid).

• HF: Hydrogen fluoride (pure), hydrofluoric acid (aqueous)

• HCl: Hydrogen chloride (pure), hydrochloric acid (aqueous)

• H2S: Hydrogen sulfide (pure), hydrosulfuric acid (aqueous)

Oxoacids

Oxoacids are acids containing hydrogen, oxygen, and another element. Their names are derived from the corresponding oxoanion:

• If the anion ends in -ate, the acid ends in -ic acid (e.g., NO3- nitrate → nitric acid, HNO3).

• If the anion ends in -ite, the acid ends in -ous acid (e.g., NO2- nitrite → nitrous acid, HNO2).

• Prefixes 'per-' and 'hypo-' are retained (e.g., HClO4: perchloric acid, HClO: hypochlorous acid).

Examples:

• H2SO4: Sulfuric acid (SO42- sulfate)

• H2SO3: Sulfurous acid (SO32- sulfite)

• HNO3: Nitric acid (NO3- nitrate)

• HNO2: Nitrous acid (NO2- nitrite)

Number of Ionizable Hydrogens in Acids

The number of ionizable hydrogens in an acid is important for its chemical behavior.

• HCl: 1 ionizable hydrogen

• H2SO4: 2 ionizable hydrogens

• H3PO4: 3 ionizable hydrogens

For organic acids, only the hydrogen attached to the carboxyl group (-COOH) is ionizable (e.g., propanoic acid HC2H5O2 has 1 ionizable hydrogen).

Naming Hydrates

Hydrates

Hydrates are ionic compounds that contain a specific number of water molecules associated with each formula unit.

• The number of water molecules is indicated by a Greek prefix (e.g., mono-, di-, tri-, tetra-, penta-, etc.) followed by 'hydrate'.

• CuSO4·5H2O: Copper(II) sulfate pentahydrate

• If no water is present, the compound is called 'anhydrous' (e.g., anhydrous copper(II) sulfate).

Summary Table: Prefixes for Number of Water Molecules in Hydrates

Number	Prefix
1	Mono-
2	Di-
3	Tri-
4	Tetra-
5	Penta-
6	Hexa-
7	Hepta-
8	Octa-
9	Nona-
10	Deca-

Key Formulas and Equations

• General formula for ionic compounds: $\text{Cation}^{n+} + \text{Anion}^{m-} \rightarrow \text{Cation}_x\text{Anion}_y$ (where $x$ and $y$ are chosen to balance charges)

• Hydrate formula: $\text{Salt} \cdot n\text{H}_2\text{O}$

Additional info:

• Classical nomenclature is largely historical and less commonly used in modern chemistry, but may appear in older literature or specific contexts.

• Practice problems in the notes reinforce the application of these naming conventions.