Ionic and Molecular Compound Nomenclature, Polyatomic Ions, and Acid Naming

Ionic Compounds

Cations with a Fixed Charge

Ionic compounds are formed from the combination of cations (positively charged ions) and anions (negatively charged ions). The nomenclature for these compounds depends on the type of ions involved.

• Cations: Named as the element itself. Typically metals with a fixed positive charge.

• Anions: Named by adding the suffix -ide to the root of the nonmetal or other species with a negative charge.

• Example: NaCl is named Sodium Chloride.

Cations with Variable Charges (Transition Metals)

Some metals, especially transition metals, can form cations with different charges. Their charge is indicated using Roman numerals in parentheses after the element name.

• Cations: Element name followed by Roman numeral indicating charge (e.g., Iron(III)).

• Anions: Suffix -ide is used.

• Example: NiCl2 is named Nickel(II) Chloride.

• Determining Charge: The charge of the metal cation can be deduced from the formula by considering the charges of the anions and the overall neutrality of the compound.

  • For NiCl2: Each Cl has a -1 charge, so two Cl- ions give -2. Nickel must be +2 to balance.

  • General formula: $n(x) + z(1) = 0$ where n = number of cations, x = charge of cation, z = number of anions.

Molecular Compounds

Nonmetals and Nonmetals

Molecular compounds are formed between nonmetals. Their names use prefixes to indicate the number of atoms of each element present in the molecule.

• Prefixes:

  • 1 – mono-

  • 2 – di-

  • 3 – tri-

  • 4 – tetra-

  • 5 – penta-

  • 6 – hexa-

  • 7 – hepta-

  • 8 – octa-

  • 9 – nona-

  • 10 – deca-

• Examples:

  • N2O5: Dinitrogen Pentoxide (the "a" in penta is dropped before a vowel)

  • NO2: Dinitrogen Dioxide

  • CO2: Carbon Dioxide (if mono- is in the front, it is always dropped)

Polyatomic Ions

Common Polyatomic Ions

Polyatomic ions are ions composed of two or more atoms covalently bonded, carrying a net charge. Their names and formulas are essential for naming ionic compounds.

Name	Formula	Polyatomic Ion
Ammonium	NH4+	Ammonium ion
Nitrate	NO3-	Nitrate ion
Hydroxide	OH-	Hydroxide ion
Phosphate	PO43-	Phosphate ion
Sulfate	SO42-	Sulfate ion
Carbonate	CO32-	Carbonate ion
Chlorate	ClO3-	Chlorate ion
Perchlorate	ClO4-	Perchlorate ion
Hypochlorite	ClO-	Hypochlorite ion
Acetate	C2H3O2-	Acetate ion

Patterns in Polyatomic Ion Nomenclature

• Periods are horizontal rows; groups are vertical columns in the periodic table.

• -ate ions have more oxygens than -ite ions.

• Period Plus Rule (for groups 2 and 3):

  • Period number + 1 = maximum number of oxygens the nonmetal will have.

  • After determining the most possible oxygens, total charge is the sum of all oxygens and the group number.

  • Example: Sulfate (SO42-): Sulfur is in group 6, so 4 oxygens and a total charge of -2.

• Patterns in -ate and -ite Nomenclature:

  • The ion with the most oxygens uses the suffix -ate.

  • The ion with fewer oxygens uses -ite.

  • Examples:

    • NO3-: Nitrate

    • NO2-: Nitrite

    • SO42-: Sulfate

    • SO32-: Sulfite

• Patterns in Chlorine Oxyanion Nomenclature (Halogens):

  • ClO4-: Perchlorate

  • ClO3-: Chlorate

  • ClO2-: Chlorite

  • ClO-: Hypochlorite

Acid Nomenclature

Acids without Oxygen

Acids are named based on the presence or absence of oxygen in the anion. Binary acids (no oxygen) use the prefix hydro- and the suffix -ic.

• Example: HF is Hydrofluoric Acid

• Example: HCl is Hydrochloric Acid

Acids with Oxygen

Oxyacids contain oxygen and are named based on the polyatomic ion present.

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

  • Example: H2SO4 (SO42- is sulfate): Sulfuric Acid

  • Example: HNO3 (NO3- is nitrate): Nitric Acid

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

  • Example: HNO2 (NO2- is nitrite): Nitrous Acid

• If the acid has one oxygen less than the -ite form, add the prefix hypo-.

  • Example: HClO (one oxygen less than chlorite): Hypochlorous Acid

• If the acid has one oxygen more than the -ate form, add the prefix per-.

  • Example: HClO4: Perchloric Acid

Summary Table: Acid Nomenclature Patterns

Ion Name	Formula	Acid Name
Chloride	Cl-	Hydrochloric Acid
Sulfate	SO42-	Sulfuric Acid
Nitrate	NO3-	Nitric Acid
Nitrite	NO2-	Nitrous Acid
Phosphate	PO43-	Phosphoric Acid
Phosphite	PO33-	Phosphorous Acid
Hypochlorite	ClO-	Hypochlorous Acid

Additional info:

• For transition metals, the Roman numeral indicates the charge of the cation, which is essential for writing correct formulas and names.

• Polyatomic ions are often encountered in both ionic and acid compounds, so memorizing their names and formulas is crucial for success in General Chemistry.

• Acid nomenclature is based on the anion present; binary acids use "hydro-" and "-ic", while oxyacids use "-ic" or "-ous" depending on the polyatomic ion.