Chemical Formulas and Nomenclature
Chemical nomenclature is the systematic naming of chemical compounds, and chemical formulas are shorthand representations of their composition. Together, they form the language of chemistry — a precise, internationally agreed-upon system that lets chemists around the world communicate without ambiguity.
Types of Chemical Formulas
Empirical formula: Gives the simplest whole-number ratio of atoms in a compound. - Example: H₂O₂ (hydrogen peroxide) has the empirical formula HO. - Useful for ionic compounds and when the actual molecular size is unknown.
Molecular formula: Gives the actual number of each type of atom in one molecule. - Example: H₂O₂ (hydrogen peroxide). Glucose: C₆H₁₂O₆.
Structural formula: Shows how atoms are connected (the bonding pattern), using lines to represent bonds. - Example: H–O–O–H for hydrogen peroxide.
Condensed structural formula: A compact version showing groups of atoms. - Example: CH₃CH₂OH for ethanol.
Lewis structure (electron dot structure): Shows bonding pairs and lone pairs of electrons. Used to predict molecular geometry and reactivity.
Naming Ionic Compounds (Binary)
Binary ionic compounds consist of a metal cation and a nonmetal anion.
Rule 1: Name the cation (positive ion, the metal) first, using the element's name unchanged. Rule 2: Name the anion (negative ion, the nonmetal) second, replacing the ending with -ide.
| Nonmetal | Anion Name |
|---|---|
| Chlorine (Cl) | Chloride (Cl⁻) |
| Bromine (Br) | Bromide (Br⁻) |
| Oxygen (O) | Oxide (O²⁻) |
| Sulfur (S) | Sulfide (S²⁻) |
| Nitrogen (N) | Nitride (N³⁻) |
Examples: - NaCl → Sodium chloride (table salt) - MgO → Magnesium oxide - Al₂O₃ → Aluminum oxide (alumina) - CaF₂ → Calcium fluoride
Transition Metals and Roman Numerals
Transition metals often form cations with more than one possible charge. To specify the charge, use Roman numerals in parentheses after the metal name (the Stock system).
- FeCl₂ → Iron(II) chloride (Fe²⁺)
- FeCl₃ → Iron(III) chloride (Fe³⁺)
- CuO → Copper(II) oxide (Cu²⁺)
- Cu₂O → Copper(I) oxide (Cu⁺)
An older system uses the suffixes -ous (lower charge) and -ic (higher charge): - Fe²⁺: ferrous; Fe³⁺: ferric - Cu⁺: cuprous; Cu²⁺: cupric
The Stock system with Roman numerals is preferred in modern chemistry.
Polyatomic Ions
Many ions are not single atoms but groups of atoms with an overall charge. These polyatomic ions must be memorized (or looked up):
| Ion | Formula | Charge |
|---|---|---|
| Hydroxide | OH⁻ | −1 |
| Nitrate | NO₃⁻ | −1 |
| Sulfate | SO₄²⁻ | −2 |
| Carbonate | CO₃²⁻ | −2 |
| Phosphate | PO₄³⁻ | −3 |
| Ammonium | NH₄⁺ | +1 |
| Acetate | CH₃COO⁻ | −1 |
| Permanganate | MnO₄⁻ | −1 |
When writing the formula of a compound containing a polyatomic ion, enclose the ion in parentheses if more than one is needed: - Ca(OH)₂ — calcium hydroxide (one Ca²⁺, two OH⁻) - Al₂(SO₄)₃ — aluminum sulfate (two Al³⁺, three SO₄²⁻)
Naming Covalent (Molecular) Compounds
Binary covalent compounds (two nonmetals) use Greek prefixes to indicate the number of each atom:
| Prefix | Number |
|---|---|
| mono- | 1 |
| di- | 2 |
| tri- | 3 |
| tetra- | 4 |
| penta- | 5 |
| hexa- | 6 |
| hepta- | 7 |
| octa- | 8 |
Rules: - Name the first element with a prefix (omit "mono" if only one atom of the first element). - Name the second element with a prefix and change the ending to -ide.
Examples: - CO → Carbon monoxide - CO₂ → Carbon dioxide - N₂O → Dinitrogen monoxide (laughing gas) - SO₃ → Sulfur trioxide - PCl₅ → Phosphorus pentachloride - N₂O₄ → Dinitrogen tetroxide
Naming Acids
Binary acids (H + nonmetal, in aqueous solution): - Add "hydro-" prefix and "-ic acid" suffix. - HCl(aq) → Hydrochloric acid - HBr(aq) → Hydrobromic acid - H₂S(aq) → Hydrosulfuric acid
Oxyacids (H + polyatomic ion): - If the anion ends in -ate, the acid ends in -ic acid: H₂SO₄ (sulfate → sulfuric acid), HNO₃ (nitrate → nitric acid). - If the anion ends in -ite, the acid ends in -ous acid: H₂SO₃ (sulfite → sulfurous acid), HNO₂ (nitrite → nitrous acid).
Writing Chemical Formulas from Names
To write the formula of an ionic compound, balance the charges so the overall compound is neutral. Use a "criss-cross" method if needed:
Example: Aluminum sulfate - Al³⁺ and SO₄²⁻ - To balance: 2 × Al³⁺ gives +6; 3 × SO₄²⁻ gives −6. - Formula: Al₂(SO₄)₃
Example: Calcium phosphate - Ca²⁺ and PO₄³⁻ - LCM of 2 and 3 is 6: 3 Ca²⁺ (+6) and 2 PO₄³⁻ (−6). - Formula: Ca₃(PO₄)₂
IUPAC Nomenclature for Organic Chemistry
Organic compounds have their own systematic naming rules governed by IUPAC (International Union of Pure and Applied Chemistry). Organic names encode the carbon skeleton length, type of bonds, and functional groups:
- Methane (CH₄), ethane (C₂H₆), propane (C₃H₈), butane (C₄H₁₀) — saturated hydrocarbons.
- Ethanol (C₂H₅OH) — a two-carbon chain (eth-) with an alcohol group (-ol).
- Acetic acid (CH₃COOH) — systematic name: ethanoic acid.
Mastering IUPAC nomenclature is a major component of organic chemistry and allows the structure of a molecule to be decoded directly from its name.