Name simple ionic compounds (including transition metals and polyatomic ions), binary molecular compounds with prefixes, and short-chain organic molecules.
Why this matters
A name in chemistry is a precise instruction. On a reagent bottle, a safety data sheet or a lab request, "iron(II) chloride" and "iron(III) chloride" are different substances with different hazards and amounts. Chemists, lab technicians and engineers must be able to go from a name to a formula and back without guessing. The rules are systematic (set by IUPAC, the International Union of Pure and Applied Chemistry), so a handful of patterns cover most of what you will see.
Step 1: decide what kind of compound it is
- Metal + nonmetal (or a polyatomic ion such as NH₄⁺): ionic naming.
- Two nonmetals: molecular naming with prefixes.
- Carbon chains with hydrogen: organic naming.
Ionic compounds
Name the cation (positive ion) first, then the anion (negative ion). A single-element anion ends in -ide.
NaCl sodium chloride
MgO magnesium oxide
CaCl2 calcium chloride
Al2O3 aluminium oxide
K2S potassium sulfide
Ionic names do not use prefixes: CaCl₂ is calcium chloride, not calcium dichloride. The charges decide the ratio: Ca²⁺ needs two Cl⁻ to balance.
Metals with more than one charge
Many transition metals form more than one ion. Put the charge in Roman numerals after the metal (the Stock system):
FeCl2 iron(II) chloride Fe2+ with two Cl-
FeCl3 iron(III) chloride Fe3+ with three Cl-
CuO copper(II) oxide
Cu2O copper(I) oxide
To find the numeral from a formula, work out what the metal's charge must be to cancel the anions: in Fe₂O₃, three O²⁻ give −6, so each of the two irons is +3: iron(III) oxide.
Polyatomic ions
Some ions are groups of atoms that stay together. Learn the common ones:
| ion | name | ion | name |
|---|---|---|---|
| NH₄⁺ | ammonium | OH⁻ | hydroxide |
| NO₃⁻ | nitrate | SO₄²⁻ | sulfate |
| CO₃²⁻ | carbonate | HCO₃⁻ | hydrogen carbonate (bicarbonate) |
| PO₄³⁻ | phosphate | CH₃COO⁻ | acetate (ethanoate) |
NaOH sodium hydroxide
CaCO3 calcium carbonate
NaHCO3 sodium hydrogen carbonate (baking soda)
NH4NO3 ammonium nitrate
Na2SO4 sodium sulfate
Ca3(PO4)2 calcium phosphate
Use brackets when you need more than one polyatomic ion: Ca₃(PO₄)₂ has two phosphate groups.
Molecular (covalent) compounds of two nonmetals
Use Greek prefixes to give the number of each atom, and end the second element in -ide. Drop "mono-" on the first element.
| prefix | number | prefix | number |
|---|---|---|---|
| mono- | 1 | tetra- | 4 |
| di- | 2 | penta- | 5 |
| tri- | 3 | hexa- | 6 |
CO carbon monoxide
CO2 carbon dioxide
N2O4 dinitrogen tetroxide
PCl5 phosphorus pentachloride
SF6 sulfur hexafluoride
Some molecules keep common names you are expected to know: H₂O is water, NH₃ is ammonia, CH₄ is methane.
Simple organic names
Organic names are built from parts: prefix (branches) + root (chain length) + ending (functional group).
The root: the longest carbon chain
| carbons | root | alkane |
|---|---|---|
| 1 | meth- | methane CH₄ |
| 2 | eth- | ethane C₂H₆ |
| 3 | prop- | propane C₃H₈ |
| 4 | but- | butane C₄H₁₀ |
| 5 | pent- | pentane C₅H₁₂ |
| 6 | hex- | hexane C₆H₁₄ |
Alkanes follow CₙH₂ₙ₊₂, which is a quick way to check a formula.
The ending: the functional group
-ane alkane propane CH3CH2CH3
-ene alkene (C=C) propene CH2=CHCH3
-ol alcohol propan-1-ol CH3CH2CH2OH
-al aldehyde propanal CH3CH2CHO
-one ketone propanone CH3COCH3 (acetone)
-oic acid propanoic acid CH3CH2COOH
Numbers and branches
Number the main chain from the end that gives the functional group (or the first branch) the lowest number. Branches are named as alkyl groups (methyl, ethyl) with their position:
CH3CH(OH)CH3 propan-2-ol (isopropyl alcohol)
CH3CH(CH3)CH3 2-methylpropane
CH3CH(CH3)CH2CH3 2-methylbutane (not 3-methylbutane)
Common mistakes
- Prefixes on ionic compounds. MgCl₂ is magnesium chloride, not magnesium dichloride.
- Leaving out the Roman numeral for iron, copper and other variable-charge metals.
- Confusing sulfide (S²⁻), sulfite (SO₃²⁻) and sulfate (SO₄²⁻).
- Numbering from the wrong end of a chain; pick the end that gives the lowest numbers.
- Counting branch carbons in the root. The root is the longest continuous chain only.
Quick recap
- Ionic: cation + anion (-ide), Roman numerals for variable-charge metals, no prefixes.
- Molecular: prefixes for atom counts, second element ends in -ide.
- Organic: chain length root + functional-group ending + numbered branches.
Try it
Open the Do tab and take the quiz: you will name compounds and turn names back into formulas.