IMAT Chemistry · Inorganic Compounds and Nomenclature

IUPAC and Traditional Names of Inorganic Compounds

Names encode charges and oxidation numbers: Roman numerals or -ous and -ic for metals, -ite and -ate for oxoanions, prefixes for covalent compounds.

All Inorganic Compounds and Nomenclature notes

Why this matters

The 2025 ministry paper asked for the traditional name of an iron salt, with the -ous and -ic forms side by side as options. Translating between a name and a formula also underlies every formula question on the previous page.

Concept 1 of 3: Naming metal compounds and binary covalent compounds

A name has to say everything the formula says. For a metal with only one possible charge, the name of the metal is enough. For a metal that can have two charges, the name must say which one, either with a Roman numeral or with an -ous or -ic ending. Between two non-metals there are no charges to balance, so prefixes count the atoms instead.

Definition

Ionic compounds: cation name, then anion name. A simple anion ends in -ide: chloride, oxide, sulfide, nitride, hydride.
Stock (IUPAC) system: a metal with more than one charge gets a Roman numeral for its charge: iron(II) chloride is $\mathrm{FeCl_2}$ , iron(III) chloride $\mathrm{FeCl_3}$ . Group 1, Group 2, Al, Zn and Ag need no numeral.
Traditional system: the Latin root with -ous for the lower charge and -ic for the higher: ferrous ( $\mathrm{Fe^{2+}}$ ) and ferric ( $\mathrm{Fe^{3+}}$ ), cuprous ( $\mathrm{Cu^+}$ ) and cupric ( $\mathrm{Cu^{2+}}$ ), stannous and stannic (Sn 2+ and 4+), plumbous and plumbic (Pb 2+ and 4+), mercurous ( $\mathrm{Hg_2^{2+}}$ ) and mercuric ( $\mathrm{Hg^{2+}}$ ).
Binary covalent compounds (two non-metals): Greek prefixes count the atoms: mono-, di-, tri-, tetra-, penta-, hexa-, hepta-. Mono- is dropped on the first element: CO is carbon monoxide, $\mathrm{N_2O_5}$ dinitrogen pentoxide, $\mathrm{SF_6}$ sulfur hexafluoride, $\mathrm{PCl_3}$ phosphorus trichloride.

Formula	Stock (IUPAC) name	Traditional name
$\mathrm{FeCl_2}$	iron(II) chloride	ferrous chloride
$\mathrm{FeCl_3}$	iron(III) chloride	ferric chloride
$\mathrm{Cu_2O}$	copper(I) oxide	cuprous oxide
CuO	copper(II) oxide	cupric oxide
$\mathrm{SnCl_4}$	tin(IV) chloride	stannic chloride
PbO	lead(II) oxide	plumbous oxide
$\mathrm{Hg_2Cl_2}$	mercury(I) chloride	mercurous chloride

Practice this conceptself-check · 4 quick reps

-ous and -ic on a metal give its charge, not the anion

Ferrous means

\mathrm{Fe^{2+}}

and ferric means

\mathrm{Fe^{3+}}

, whatever the anion. Do not confuse these endings with -ite and -ate, which belong to the anion. Ferrous sulfate is

\mathrm{FeSO_4}

; iron(II) sulfite is

\mathrm{FeSO_3}

Concept 2 of 3: Oxoacids and oxoanions: hypo-, -ous, -ic, per- and -ite, -ate

Many non-metals form several oxoacids with different numbers of oxygen atoms. The names rank them by the oxidation number of the central atom: -ous for lower, -ic for higher, with hypo- below the lowest and per- above the highest. Each acid's anion follows the same pattern with -ite and -ate.

Definition

Acid -ic gives anion -ate; acid -ous gives anion -ite; binary acid hydro...ic gives anion -ide (hydrochloric acid, chloride).
With four oxoacids (chlorine, bromine, iodine): hypo...ous (lowest), ...ous, ...ic, per...ic (highest).
With two (sulfur, nitrogen, phosphorus): just -ous and -ic.
Memory aid: the -ate has more oxygen than the -ite of the same element.
Salts take the anion name: $\mathrm{NaClO}$ is sodium hypochlorite, $\mathrm{KNO_2}$ potassium nitrite.

Acid	Acid name	Anion	Anion name	Oxidation number
HClO	hypochlorous acid	$\mathrm{ClO^-}$	hypochlorite	Cl +1
$\mathrm{HClO_2}$	chlorous acid	$\mathrm{ClO_2^-}$	chlorite	Cl +3
$\mathrm{HClO_3}$	chloric acid	$\mathrm{ClO_3^-}$	chlorate	Cl +5
$\mathrm{HClO_4}$	perchloric acid	$\mathrm{ClO_4^-}$	perchlorate	Cl +7
$\mathrm{H_2SO_3}$	sulfurous acid	$\mathrm{SO_3^{2-}}$	sulfite	S +4
$\mathrm{H_2SO_4}$	sulfuric acid	$\mathrm{SO_4^{2-}}$	sulfate	S +6
$\mathrm{HNO_2}$	nitrous acid	$\mathrm{NO_2^-}$	nitrite	N +3
$\mathrm{HNO_3}$	nitric acid	$\mathrm{NO_3^-}$	nitrate	N +5
$\mathrm{H_3PO_4}$	phosphoric acid	$\mathrm{PO_4^{3-}}$	phosphate	P +5
$\mathrm{H_2CO_3}$	carbonic acid	$\mathrm{CO_3^{2-}}$	carbonate	C +4
HCl	hydrochloric acid	$\mathrm{Cl^-}$	chloride	Cl −1

Practice this conceptself-check · 4 quick reps

-ite is the lower oxidation state, not the higher

Sulfite (S

+4

) has one O fewer than sulfate (S

+6

); nitrite (N

+3

) one fewer than nitrate (N

+5

). Swapping the two endings gives a formula with the wrong number of oxygen atoms.

Concept 3 of 3: From name to formula and from formula to name

Going from a name to a formula is two lookups and one balance: identify each ion and its charge from the name, then balance the charges. Going back the other way, you work out the metal's charge from the anion first, and only then choose the ending.

Definition

Name to formula:

Read the cation and its charge (a Roman numeral, or -ous/-ic, or the group).
Read the anion and its charge (-ide, -ite, -ate, with hypo- or per-).
Balance the charges, bracketing polyatomic ions when needed.

Formula to name:

Identify the anion and its charge; from that, work out the metal's charge.
Name the metal (with its numeral or -ous/-ic if it has more than one charge), then the anion.

Charge on each metal ion

\text{charge on each cation} = \frac{\text{number of anions} \times |\text{anion charge}|}{\text{number of cations}}

Worked example

(a) Write the formula of calcium hypochlorite. (b) Name

\mathrm{CuSO_4}

in both systems.

Practice this conceptself-check · 4 quick reps

Work out the metal's charge from the anion first

\mathrm{Cu_2O}

the oxide is

-2

, shared by two copper ions, so each is

+1

: copper(I) oxide, cuprous oxide. The subscript 2 is not the charge. Always divide the anion charge by the number of metal atoms.

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Formulas (1)

From name to formula and from formula to name
Charge on each metal ion
$\text{charge on each cation} = \frac{\text{number of anions} \times |\text{anion charge}|}{\text{number of cations}}$

Reference tables (2)

Naming metal compounds and binary covalent compounds7 rows

Formula	Stock (IUPAC) name	Traditional name
$\mathrm{FeCl_2}$	iron(II) chloride	ferrous chloride
$\mathrm{FeCl_3}$	iron(III) chloride	ferric chloride
$\mathrm{Cu_2O}$	copper(I) oxide	cuprous oxide
CuO	copper(II) oxide	cupric oxide
$\mathrm{SnCl_4}$	tin(IV) chloride	stannic chloride
PbO	lead(II) oxide	plumbous oxide
$\mathrm{Hg_2Cl_2}$	mercury(I) chloride	mercurous chloride

Oxoacids and oxoanions: hypo-, -ous, -ic, per- and -ite, -ate11 rows

Acid	Acid name	Anion	Anion name	Oxidation number
HClO	hypochlorous acid	$\mathrm{ClO^-}$	hypochlorite	Cl +1
$\mathrm{HClO_2}$	chlorous acid	$\mathrm{ClO_2^-}$	chlorite	Cl +3
$\mathrm{HClO_3}$	chloric acid	$\mathrm{ClO_3^-}$	chlorate	Cl +5
$\mathrm{HClO_4}$	perchloric acid	$\mathrm{ClO_4^-}$	perchlorate	Cl +7
$\mathrm{H_2SO_3}$	sulfurous acid	$\mathrm{SO_3^{2-}}$	sulfite	S +4
$\mathrm{H_2SO_4}$	sulfuric acid	$\mathrm{SO_4^{2-}}$	sulfate	S +6
$\mathrm{HNO_2}$	nitrous acid	$\mathrm{NO_2^-}$	nitrite	N +3
$\mathrm{HNO_3}$	nitric acid	$\mathrm{NO_3^-}$	nitrate	N +5
$\mathrm{H_3PO_4}$	phosphoric acid	$\mathrm{PO_4^{3-}}$	phosphate	P +5
$\mathrm{H_2CO_3}$	carbonic acid	$\mathrm{CO_3^{2-}}$	carbonate	C +4
HCl	hydrochloric acid	$\mathrm{Cl^-}$	chloride	Cl −1

Watch out for (3)

-ous and -ic on a metal give its charge, not the anion
→ Naming metal compounds and binary covalent compounds
-ite is the lower oxidation state, not the higher
→ Oxoacids and oxoanions: hypo-, -ous, -ic, per- and -ite, -ate
Work out the metal's charge from the anion first
→ From name to formula and from formula to name

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