Naming Rules at a Glance

The whole naming system on one page. The reasoning behind it — why the two families are named differently in the first place — is in Naming and Formulas, and the practice is in Naming Practice.

Start here: which kind of compound is it?

What the formula looks like	It is	Name it with
A metal and a non-metal	binary ionic	metal name, then non-metal stem + -ide
A metal and a polyatomic ion	ionic	metal name, then the polyatomic ion’s own name
Two non-metals	molecular	Greek prefixes on both elements
$\mathrm{H}$ first, in aqueous solution	an acid	the acid rules below
Ends in a dot and some water	a hydrate	the parent compound + prefix + hydrate

Everything below is that table expanded.

Ionic compounds

The cation is written and named first, then the anion. The formula comes from the charges balancing to zero overall.

Monatomic anion names take the stem and add -ide:

Element	Ion	Name
Fluorine	$\mathrm{F}{}^{-}$	fluoride
Chlorine	$\mathrm{Cl}{}^{-}$	chloride
Bromine	$\mathrm{Br}{}^{-}$	bromide
Iodine	$\mathrm{I}{}^{-}$	iodide
Oxygen	$\mathrm{O}{}^{2-}$	oxide
Sulfur	$\mathrm{S}{}^{2-}$	sulfide
Nitrogen	$\mathrm{N}{}^{3-}$	nitride
Phosphorus	$\mathrm{P}{}^{3-}$	phosphide

Charges you can read off the periodic table:

Group	Charge	Examples
1	$1+$	$\mathrm{Li}{}^{+}$, $\mathrm{Na}{}^{+}$, $\mathrm{K}{}^{+}$
2	$2+$	$\mathrm{Mg}{}^{2+}$, $\mathrm{Ca}{}^{2+}$, $\mathrm{Ba}{}^{2+}$
13	$3+$	$\mathrm{Al}{}^{3+}$
15	$3-$	$\mathrm{N}{}^{3-}$, $\mathrm{P}{}^{3-}$
16	$2-$	$\mathrm{O}{}^{2-}$, $\mathrm{S}{}^{2-}$
17	$1-$	$\mathrm{F}{}^{-}$, $\mathrm{Cl}{}^{-}$, $\mathrm{Br}{}^{-}$

Three transition metals behave as if they had a group: zinc is always $\mathrm{Zn}{}^{2+}$, silver is always $\mathrm{Ag}{}^{+}$, and cadmium is always $\mathrm{Cd}{}^{2+}$. They take no Roman numeral.

Multivalent metals need a Roman numeral giving the charge on the metal ion — not the number of atoms of anything.

Metal	Common ions
Copper	$\mathrm{Cu}{}^{+}$, $\mathrm{Cu}{}^{2+}$
Iron	$\mathrm{Fe}{}^{2+}$, $\mathrm{Fe}{}^{3+}$
Lead	$\mathrm{Pb}{}^{2+}$, $\mathrm{Pb}{}^{4+}$
Tin	$\mathrm{Sn}{}^{2+}$, $\mathrm{Sn}{}^{4+}$
Chromium	$\mathrm{Cr}{}^{2+}$, $\mathrm{Cr}{}^{3+}$, $\mathrm{Cr}{}^{6+}$
Manganese	$\mathrm{Mn}{}^{2+}$, $\mathrm{Mn}{}^{4+}$, $\mathrm{Mn}{}^{7+}$
Cobalt	$\mathrm{Co}{}^{2+}$, $\mathrm{Co}{}^{3+}$
Nickel	$\mathrm{Ni}{}^{2+}$, $\mathrm{Ni}{}^{3+}$
Mercury	$\mathrm{Hg}{}_2{}^{2+}$, $\mathrm{Hg}{}^{2+}$
Gold	$\mathrm{Au}{}^{+}$, $\mathrm{Au}{}^{3+}$

To find the numeral from a formula, work backwards from the anion, whose charge you already know. $\mathrm{Cu}{}_2\mathrm{O}$ contains one $\mathrm{O}{}^{2-}$ shared between two copper ions, so each is $1+$ and the name is copper(I) oxide. $\mathrm{CuO}$ is copper(II) oxide.

Mercury(I) is the odd one: it exists as a joined pair, $\mathrm{Hg}{}_2{}^{2+}$, so mercury(I) chloride is $\mathrm{Hg}{}_2\mathrm{Cl}{}_2$.

Polyatomic ions keep their own names and go in brackets whenever you need more than one. The list is Polyatomic Ions.

Writing an ionic formula

1. Write the cation with its charge, then the anion with its charge.
2. Find the smallest numbers of each that make the total charge zero. Crossing the charges over as subscripts is a shortcut for this.
3. Reduce to the lowest whole-number ratio. $\mathrm{Mg}{}^{2+}$ with $\mathrm{O}{}^{2-}$ crosses over to $\mathrm{Mg}{}_2\mathrm{O}{}_2$, which is wrong. It is $\mathrm{MgO}$.
4. Bracket any polyatomic ion that needs a subscript greater than one.
5. Drop any subscript of 1 — it is understood.

Aluminium sulfate: $\mathrm{Al}{}^{3+}$ and $\mathrm{SO}{}_4{}^{2-}$ need two aluminium ions and three sulfate ions to balance, so it is $\mathrm{Al}{}_2(\mathrm{SO}{}_4){}_3$.

Molecular compounds

Two non-metals, so no charges to balance and nothing to tell you the ratio — the prefixes have to state it.

Number	Prefix	Number	Prefix
1	mono	6	hexa
2	di	7	hepta
3	tri	8	octa
4	tetra	9	nona
5	penta	10	deca

mono- is dropped from the first element and kept on the second. $\mathrm{CO}$ is carbon monoxide; $\mathrm{CO}{}_2$ is carbon dioxide; $\mathrm{N}{}_2\mathrm{O}{}_4$ is dinitrogen tetroxide; $\mathrm{P}{}_4\mathrm{O}{}_{10}$ is tetraphosphorus decoxide.

A few molecular compounds keep older names that predate the system: water, ammonia ($\mathrm{NH}{}_3$), methane ($\mathrm{CH}{}_4$). These are not exceptions to a rule, just names that were already in use.

Acids

A hydrogen-first formula in aqueous solution. Which of the three rules applies depends on what follows the hydrogen.

The anion	Rule	Example
A simple -ide anion, no oxygen	hydro- + stem + -ic acid	$\mathrm{HCl}(\mathrm{aq})$ is hydrochloric acid
An -ate polyatomic ion	stem + -ic acid	$\mathrm{H}{}_2\mathrm{SO}{}_4$ is sulfuric acid
An -ite polyatomic ion	stem + -ous acid	$\mathrm{H}{}_2\mathrm{SO}{}_3$ is sulfurous acid

The ones you will meet:

Formula	Name	From
$\mathrm{HCl}(\mathrm{aq})$	hydrochloric acid	chloride
$\mathrm{HF}(\mathrm{aq})$	hydrofluoric acid	fluoride
$\mathrm{HNO}{}_3$	nitric acid	nitrate
$\mathrm{HNO}{}_2$	nitrous acid	nitrite
$\mathrm{H}{}_2\mathrm{SO}{}_4$	sulfuric acid	sulfate
$\mathrm{H}{}_2\mathrm{SO}{}_3$	sulfurous acid	sulfite
$\mathrm{H}{}_2\mathrm{CO}{}_3$	carbonic acid	carbonate
$\mathrm{H}{}_3\mathrm{PO}{}_4$	phosphoric acid	phosphate
$\mathrm{CH}{}_3\mathrm{COOH}$	ethanoic (acetic) acid	acetate

The per- and hypo- ladder carries through: $\mathrm{HClO}{}_4$ is perchloric acid, $\mathrm{HClO}{}_3$ is chloric acid, $\mathrm{HClO}{}_2$ is chlorous acid, and $\mathrm{HClO}$ is hypochlorous acid.

Hydrates

Water built into a crystal structure is written after a dot and counted with the same Greek prefixes:

Formula	Name
$\mathrm{CuSO}{}_4\cdot 5\mathrm{H}{}_2\mathrm{O}$	copper(II) sulfate pentahydrate
$\mathrm{CaCl}{}_2\cdot 2\mathrm{H}{}_2\mathrm{O}$	calcium chloride dihydrate
$\mathrm{MgSO}{}_4\cdot 7\mathrm{H}{}_2\mathrm{O}$	magnesium sulfate heptahydrate

The water counts towards the molar mass, which matters as soon as you start weighing things in Molar Mass and Composition. Driving it off by heating is the basis of the percentage composition work there.

Read the formula before you reach for a rule

Almost every naming error is a misidentification rather than a forgotten rule. Decide first whether you are holding a metal with a non-metal, two non-metals, or an acid — and only then pick the naming system. Applying the ionic rules to a molecular compound produces confident nonsense, and the fix is one question asked earlier.