Naming compounds — Chemistry Fundamentals

What you'll learn

Name simple ionic and organic compounds
Use Roman numerals for metals that form more than one ion
Name binary molecular compounds with Greek prefixes
Turn a compound's name back into its formula

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.

Key terms (6)

IUPAC: The International Union of Pure and Applied Chemistry, which sets the systematic naming rules.
cation: A positive ion, named first in an ionic compound.
anion: A negative ion, named second in an ionic compound; a single-element anion ends in -ide.
Stock system: Showing a metal ion's charge with Roman numerals after its name, as in iron(III) chloride.
Polyatomic ions: Ions made of a group of atoms that stay together, such as nitrate, sulfate and ammonium.
alkyl: A branch on a carbon chain named from its length, such as methyl or ethyl.