Chemical nomenclature is the language of chemistry. It's how we name and identify compounds, from simple table salt to complex molecules. Understanding this system helps you decode the composition and properties of substances.

Naming compounds follows specific rules based on their type: ionic or molecular. For ionic compounds, we name the cation first, then the anion. Molecular compounds use prefixes to indicate the number of atoms. Mastering these rules is key to communicating in chemistry.

Chemical Nomenclature

Systematic nomenclature for inorganic compounds

• Ionic compounds consist of a cation (positive ion) and an anion (negative ion)
  • Cation named first, followed by the anion (sodium chloride, potassium sulfate)
  • Cation retains the element name
    • If cation has multiple possible charges, charge indicated using Roman numerals in parentheses (iron(II) chloride, copper(I) oxide)
  • Anion named by taking the root of the element name and adding the suffix "-ide"
    • Monoatomic anions (chloride Cl⁻, oxide O²⁻, sulfide S²⁻)
    • Polyatomic anions (nitrate NO₃⁻, sulfate SO₄²⁻, carbonate CO₃²⁻)
• Molecular compounds consist of two or more nonmetals
  • Named using prefixes to indicate the number of each element present (prefix system)
    • Mono- (1), di- (2), tri- (3), tetra- (4), penta- (5), hexa- (6), hepta- (7), octa- (8), nona- (9), deca- (10)
  • Prefix "mono-" usually omitted for the first element (carbon monoxide, not monocarbon monoxide)
  • Second element's name ends in "-ide" (carbon dioxide CO₂, dinitrogen pentoxide N₂O₅, sulfur trioxide SO₃)
• Acids named based on the anion they contain when dissolved in water
  • Anions ending in "-ide" form acids named with the prefix "hydro-" and the suffix "-ic" (HCl hydrochloric acid, H₂S hydrosulfuric acid)
  • Anions ending in "-ate" form acids named with the suffix "-ic" (HNO₃ nitric acid, H₂SO₄ sulfuric acid)
  • Anions ending in "-ite" form acids named with the suffix "-ous" (HNO₂ nitrous acid, H₂SO₃ sulfurous acid)

Ionic vs molecular compound formulas

• Ionic compounds contain a metal and a nonmetal
  • Formula represents the simplest whole-number ratio of the ions (NaCl, CaCl₂, AlCl₃)
  • Cations and anions attract each other due to opposite charges, forming a crystalline lattice structure
• Molecular compounds contain only nonmetals
  • Formula represents the actual number of atoms of each element in the molecule (CO₂, NH₃, CH₄)
  • Atoms held together by covalent bonds, sharing electrons to achieve a stable electronic configuration
• Binary compounds consist of two elements, regardless of whether they are ionic or molecular

Chemical formulas from compound names

• Ionic compounds
  1. Write the symbol and charge of the cation, followed by the symbol and charge of the anion
  2. Balance the charges to achieve a neutral compound by adjusting the subscripts
  • Sodium chloride: Na⁺ Cl⁻ → NaCl
  • Calcium chloride: Ca²⁺ Cl⁻ → CaCl₂
  • Aluminum oxide: Al³⁺ O²⁻ → Al₂O₃
• Molecular compounds
  • Use the prefixes to determine the number of each element in the molecule
  • Write the symbol of each element with the appropriate subscript (carbon dioxide CO₂, dinitrogen pentoxide N₂O₅, sulfur trioxide SO₃)
  • Prefixes indicate the number of atoms of each element present in the molecule

Additional Nomenclature Concepts

• Chemical formula: A concise way to represent the composition of a compound using element symbols and numerical subscripts
• IUPAC (International Union of Pure and Applied Chemistry): The organization that establishes official naming conventions for chemical compounds
• Polyatomic ions: Charged species composed of two or more atoms bonded together
• Oxyanions: Polyatomic anions containing oxygen, often named with suffixes -ate or -ite (e.g., sulfate, nitrite)