Naming Compounds
Naming compounds on the MCAT is not about memorizing thousands of names. It is about learning three sets of rules - one for ionic compounds, one for covalent compounds, and one for acids - and applying them consistently.
Ionic Compounds - Metal + Nonmetal
Ionic compounds form when a metal transfers electrons to a nonmetal. The naming rules are straightforward:
- Name the cation (metal) first, then the anion (nonmetal) second
- The anion gets the suffix -ide
- If the metal can form more than one charge (transition metals), use Roman numerals in parentheses to indicate the charge
| Formula | Name |
|---|---|
| NaCl | Sodium chloride |
| MgO | Magnesium oxide |
| FeCl₂ | Iron(II) chloride |
| FeCl₃ | Iron(III) chloride |
| CuSO₄ | Copper(II) sulfate |
Polyatomic Ions - Know the Pattern
You need to recognize common polyatomic ions. Rather than memorizing each one independently, learn the patterns:
The -ate/-ite pattern: For oxyanions (polyatomic ions with oxygen), the -ate form has more oxygen atoms, and the -ite form has fewer:
| More O (-ate) | Fewer O (-ite) |
|---|---|
| NO₃⁻ (nitrate) | NO₂⁻ (nitrite) |
| SO₄²⁻ (sulfate) | SO₃²⁻ (sulfite) |
| ClO₃⁻ (chlorate) | ClO₂⁻ (chlorite) |
Extended pattern for halogens: When there are four possible oxyanions, add per- (most O) and hypo- (least O):
| Ion | Name | Oxygens |
|---|---|---|
| ClO₄⁻ | Perchlorate | 4 (most) |
| ClO₃⁻ | Chlorate | 3 |
| ClO₂⁻ | Chlorite | 2 |
| ClO⁻ | Hypochlorite | 1 (least) |
Other essential polyatomic ions:
| Ion | Name |
|---|---|
| OH⁻ | Hydroxide |
| NH₄⁺ | Ammonium |
| CN⁻ | Cyanide |
| MnO₄⁻ | Permanganate |
| Cr₂O₇²⁻ | Dichromate |
| CrO₄²⁻ | Chromate |
| C₂O₄²⁻ | Oxalate |
| HCO₃⁻ | Bicarbonate (hydrogen carbonate) |
Covalent Compounds - Nonmetal + Nonmetal
Covalent compounds are named using Greek prefixes to indicate the number of each atom:
| Prefix | Number |
|---|---|
| mono- | 1 |
| di- | 2 |
| tri- | 3 |
| tetra- | 4 |
| penta- | 5 |
| hexa- | 6 |
Rules:
- The first element gets a prefix only if there is more than one atom (never say “monocarbon”)
- The second element always gets a prefix AND the -ide suffix
| Formula | Name |
|---|---|
| CO | Carbon monoxide |
| CO₂ | Carbon dioxide |
| N₂O₄ | Dinitrogen tetroxide |
| PCl₅ | Phosphorus pentachloride |
| SF₆ | Sulfur hexafluoride |
Naming Acids
Acid naming follows a predictable pattern based on the anion:
Binary acids (no oxygen, H + nonmetal):
- Use the prefix hydro- and the suffix -ic acid
- HCl = hydrochloric acid, HBr = hydrobromic acid, HF = hydrofluoric acid
Oxyacids (H + polyatomic ion with oxygen):
- If the ion ends in -ate, the acid ends in -ic acid
- If the ion ends in -ite, the acid ends in -ous acid
| Ion | Acid Formula | Acid Name |
|---|---|---|
| SO₄²⁻ (sulfate) | H₂SO₄ | Sulfuric acid |
| SO₃²⁻ (sulfite) | H₂SO₃ | Sulfurous acid |
| NO₃⁻ (nitrate) | HNO₃ | Nitric acid |
| NO₂⁻ (nitrite) | HNO₂ | Nitrous acid |
| ClO₄⁻ (perchlorate) | HClO₄ | Perchloric acid |
| ClO⁻ (hypochlorite) | HClO | Hypochlorous acid |