Chemical nomenclature: traditional, IUPAC

Saturday 19 September 2026. In Year 3 of Italian high school (liceo), nomenclature comes straight after the periodic table and chemical bonds, between October and November, and the test that covers it falls between November and December, so six to ten weeks from now. But this post is not only for Year 3. Nomenclature also appears in Years 1 and 2 of technical school, and it comes back in every chemistry test up to the maturità (Italy’s school-leaving exam), because without being able to read a formula you cannot write a reaction or do a stoichiometry calculation. It is also the topic where students lose the most marks through pure inattention (an -ite/-ate mix-up, indices crossed the wrong way) rather than through any real conceptual difficulty.

Why there are two nomenclatures

In Italy two ways of naming an inorganic compound live side by side, and the teacher may ask for one, the other, or both in the same test. Italian names appear in italics below, with the English equivalent next to them.

The traditional nomenclature is the older one. It uses suffixes and prefixes that describe the oxidation state in relative terms: when an element has two possible oxidation numbers, the lower one takes the suffix -oso (English -ous) and the higher one the suffix -ico (English -ic). If it has more than two, the prefixes ipo- (hypo-) and per- are added at the two extremes. This is the nomenclature you hear in everyday language (“sulfuric acid”, “caustic soda”), and it is the one people still use when they talk about chemistry outside school.

The IUPAC nomenclature (named after the international body that defines it) is more recent and leaves no room for ambiguity. It uses the multiplying prefixes mono-, di-, tri-, tetra- to count the atoms in the formula, or else writes the oxidation number out explicitly in brackets, in Roman numerals. It is the nomenclature of the international scientific literature, and in recent years it is the one that textbooks tend to favour, but the traditional one has by no means disappeared from use.

Oxidation numbers: the key to everything else

The oxidation number is the charge (real or conventional) that an atom would have if all the bonds in the molecule were ionic. Before naming any compound you need to be able to assign it, using these rules, in the order in which it is convenient to apply them:

An element in its free state (not bonded to other elements) has oxidation number 0: $O_{2}$ , $Fe$ , $N_{2}$ .
In a monatomic ion, the oxidation number equals the charge of the ion: $Na^{+}$ has oxidation number $+ 1$ , $Cl^{-}$ has $- 1$ .
Oxygen almost always has oxidation number $- 2$ , except in peroxides ( $H_{2} O_{2}$ ), where it is $- 1$ .
Hydrogen almost always has oxidation number $+ 1$ , except in metal hydrides ( $NaH$ ), where it is $- 1$ .
The metals of group 1 always have oxidation number $+ 1$ , those of group 2 always $+ 2$ .
The algebraic sum of the oxidation numbers of all the atoms is 0 in a neutral compound, and equals the total charge in a polyatomic ion.

The last rule is the one you use to calculate the “unknown” oxidation number (usually that of the metal or of the central non-metal) once those of oxygen and hydrogen are fixed.

The classes of compounds, one by one

Basic oxides and anhydrides (acidic oxides)

A basic oxide (ossido basico) forms from a metal plus oxygen: $CaO$ , $N a_{2} O$ , $F e_{2} O_{3}$ . Traditional name: “oxide of” plus the name of the metal, with the suffix -oso/-ico (-ous/-ic) if the metal has several oxidation states. IUPAC name: multiplying prefix plus “oxide of” plus the name of the metal, or else “oxide of” plus the metal plus a Roman numeral.

An anhydride (anidride, or acidic oxide) forms from a non-metal plus oxygen: $C O_{2}$ , $S O_{3}$ , $N_{2} O_{5}$ . Traditional name: “anidride” (anhydride) plus the root of the non-metal with -osa/-ica. IUPAC name: exactly the same pattern as for oxides, with multiplying prefixes on both elements when they are needed.

Hydroxides

A hydroxide (idrossido) is obtained, conceptually, from basic oxide + water: $CaO + H_{2} O \to Ca (OH)_{2}$ . Hydroxides always contain the group $OH^{-}$ (the hydroxyl group), which appears as many times as the oxidation number of the metal. Traditional name: “hydroxide of” plus the metal (with -oso/-ico if needed). IUPAC name: multiplying prefix plus “hydroxide of” plus the metal.

Oxyacids (oxygen-containing acids)

They are obtained, conceptually, from anhydride + water: $S O_{3} + H_{2} O \to H_{2} S O_{4}$ . Traditional name: “acid” plus the root of the non-metal with -oso/-ico, which directly inherits the suffix of the anhydride it comes from (anidride solforosa → acido solforoso, sulfurous acid; anidride solforica → acido solforico, sulfuric acid).

Phosphorus is a special case. From the same anhydride ( $P_{2} O_{5}$ ) it forms three different acids, depending on how many molecules of water are added, distinguished by the prefixes meta-, piro-, orto- (English meta-, pyro-, ortho-):

$P_{2} O_{5} + H_{2} O \to 2 HP O_{3}$ : metaphosphoric acid (one water);
$P_{2} O_{5} + 2 H_{2} O \to H_{4} P_{2} O_{7}$ : pyrophosphoric acid (two waters);
$P_{2} O_{5} + 3 H_{2} O \to 2 H_{3} P O_{4}$ : orthophosphoric acid, usually called simply phosphoric acid (three waters, the most common form).

Hydracids and their salts

Hydracids (idracidi) are acids without oxygen: hydrogen bonded directly to a non-metal, typically a halogen or sulfur: $HCl$ , $HF$ , $H_{2} S$ . The name is “acid” plus the root of the non-metal plus -idrico (English -hydric): acido cloridrico (hydrochloric acid), acido solfidrico (hydrosulfuric acid). It is the same in traditional and IUPAC nomenclature.

Binary and ternary salts

A salt is obtained from a neutralisation reaction, acid + hydroxide → salt + water. It is a binary salt if it comes from a hydracid ( $HCl + NaOH \to NaCl + H_{2} O$ ), and a ternary salt if it comes from an oxyacid ( $H_{2} S O_{4} + 2 NaOH \to N a_{2} S O_{4} + 2 H_{2} O$ ).

The name of the salt always starts from the suffix of the acid, changed according to a fixed pattern:

Suffix of the acid	Suffix of the salt
-oso	-ito
-ico	-ato
-idrico (hydracids)	-uro

So acido solforoso → solfiti (sulfites), acido solforico → solfati (sulfates), acido cloridrico → cloruri (chlorides).

Summary table

Class	General formula	Example	Traditional name	IUPAC name
Basic oxide	metal + O	$F e_{2} O_{3}$	ossido ferrico (ferric oxide)	triossido di diferro (diiron trioxide) / ossido di ferro(III) (iron(III) oxide)
Anhydride	non-metal + O	$S O_{3}$	anidride solforica (sulfuric anhydride)	triossido di zolfo (sulfur trioxide)
Hydroxide	metal + OH	$Ca (OH)_{2}$	idrossido di calcio (calcium hydroxide)	diidrossido di calcio (calcium dihydroxide)
Oxyacid	anhydride + H₂O	$H_{2} S O_{4}$	acido solforico (sulfuric acid)	acido solforico (common name)
Hydracid	H + non-metal	$HCl$	acido cloridrico (hydrochloric acid)	acido cloridrico (hydrochloric acid)
Ternary salt	oxyacid + hydroxide	$N a_{2} S O_{4}$	solfato di sodio (sodium sulfate)	solfato di sodio (sodium sulfate)
Binary salt	hydracid + hydroxide	$NaCl$	cloruro di sodio (sodium chloride)	cloruro di sodio (sodium chloride)

The recurring mistakes

Swapping -ito and -ato. The suffix -ito always comes from the acid in -oso (lower oxidation number), -ato always from the acid in -ico (higher oxidation number): sulfite from sulfurous acid, sulfate from sulfuric acid. They are different compounds, not variants of the same name.
Forgetting to cross the indices. When you write a formula from a name, the index of the cation is the absolute value of the charge of the anion, and vice versa. This is the mechanism behind exercise 1 below. Skipping it produces formulas with unbalanced charges.
Confusing an anhydride with a basic oxide. It depends only on what bonds with the oxygen: metal → basic oxide, non-metal → anhydride. It does not depend on the number of atoms in the formula.
Getting the oxidation number of iron and copper wrong. Iron has $+ 2$ (ferroso, ferrous) and $+ 3$ (ferrico, ferric); copper has $+ 1$ (rameoso, cuprous) and $+ 2$ (rameico, cupric). They are the most common two-oxidation-state elements in tests, and you need to remember them as a pair, not one at a time.
Writing the IUPAC name without the brackets. “Iron III oxide” without brackets is ambiguous (it looks like “three iron oxides”); it must be written “iron(III) oxide”, with the Roman numeral enclosed.

Three worked exercises

1. From name to formula: aluminium sulfate

Aluminium has a fixed oxidation number of $+ 3$ . The sulfate ion, $S O_{4}^{2 -}$ , has charge $- 2$ . To write a neutral formula you cross the absolute values of the charges and use them as indices:

Al^{3 +} S O_{4}^{2 -} ⟶ A l_{2} (S O_{4})_{3}

Aluminium takes $2$ as its index (from the charge of the sulfate ion), and the sulfate ion takes $3$ as its index (from the charge of aluminium). Check: $2 \times (+ 3) + 3 \times (- 2) = 6 - 6 = 0$ . The compound is neutral, as it must be.

2. From formula to name, in both nomenclatures: Fe₂O₃

In the compound $F e_{2} O_{3}$ oxygen has oxidation number $- 2$ (rule 3), for a total of $3 \times (- 2) = - 6$ . For neutrality (rule 6), the two iron atoms must add up to $+ 6$ , so each has oxidation number $+ 6/2 = + 3$ .

With iron at $+ 3$ (the higher of its two possible oxidation numbers), the traditional name uses the suffix -ico: ossido ferrico (ferric oxide). In IUPAC you can use the atom count, triossido di diferro (diiron trioxide), or the explicit oxidation number, ossido di ferro(III) (iron(III) oxide). Both forms are accepted.

3. The whole chain: from anhydride to salt

Follow the same substance, sulfur, through three steps.

S O_{3} (sulfuric anhydride) + H_{2} O ⟶ H_{2} S O_{4} (sulfuric acid)

H_{2} S O_{4} + 2 NaOH ⟶ N a_{2} S O_{4} (sodium sulfate) + 2 H_{2} O

Checking the balance of the second reaction: on the left $2$ Na, $1$ S, $6$ O, $4$ H; on the right $2$ Na, $1$ S, $6$ O ( $4$ in the sulfate $+ 2$ in the water), $4$ H. It balances. Sulfur keeps the same oxidation number, $+ 6$ , from the anhydride all the way to the salt: the name changes at every step, but the chemical nature of the central atom does not.

How I work on this topic

With students who come to nomenclature for the first time, or who revise it before the maturità, I always start from oxidation numbers, not from names. Once assigning them is automatic, nomenclature, in both versions, becomes an exercise in applying a fixed pattern, not a list to learn by heart. I take for granted the atom, the periodic table and bonds, which are the topic immediately before: if you need them, I cover them in the post on atoms and bonds in Year 3. This post deliberately stops at names and formulas. For the next step, the mole, stoichiometry and calculations with balanced reactions, there is the guide on stoichiometry and its typical mistakes, and for balancing equations themselves the post on balancing chemical equations.

An honest note: the free practice tests currently cover maths and physics, not yet chemistry. The same logic would apply if chemistry arrives in future: the correct answer is free, and the guided solution unlocks with the first lesson. If you want to revise nomenclature before the November-December test, or you already have a test to go over together, you will find the full picture of how I work on the sciences page or directly on the contact page.

Chemical nomenclature: traditional vs IUPAC