Naming ions and compounds

On the reagent shelf stands a jar of pale green crystals labelled “FeSO₄·7H₂O, iron(II) sulfate heptahydrate”. Older books call it ferrous sulfate; the alchemists called it green vitriol. Many names, one substance — and apart from old trade names such as green vitriol, each name is a code for the formula: if you can read it, you can write the formula from the name and the name from the formula.

This lesson teaches the code. There are only a few rules: the charges of the ions fix the formula, and the formula fixes the name. Chinese and English say the same thing in opposite order — the Chinese name of this salt starts with the anion, the English name with the cation.

The charges of main-group ions can be read straight from the periodic table: metals lose their valence electrons and non-metals gain electrons until each has a noble-gas configuration. Group 1 gives +1 (Na⁺, K⁺), group 2 gives +2 (Mg²⁺, Ca²⁺) and aluminium +3; group 17 gives −1 (Cl⁻), group 16 −2 (O²⁻, S²⁻) and nitrogen −3 (N³⁻). The heavier p-block metals are the exception: tin and lead can lose all four valence electrons or, often, only their two p electrons, so they form both +4 and +2 (the inert-pair effect).

Transition metals are different: iron forms Fe²⁺ and Fe³⁺, copper Cu⁺ and Cu²⁺, and mercury(I) is even two atoms bonded together, $\ce{Hg2^2+}$. That is exactly why their names must say which charge is meant.

Other ions are groups of atoms that carry a charge together — polyatomic ions. They often go in and out of reactions as a unit, and when a formula has more than one of them they take brackets: $\ce{Ca(OH)2}$, $\ce{(NH4)2SO4}$. The dozen most common are worth knowing by heart.

A Chinese name reads the formula from the right: the anion first, then the cation. A monatomic anion gives one ending, an oxoanion another, and of a metal’s two common charges the lower one is marked with a special character meaning “sub-”, so iron(II) and iron(III) sulfate get different names; binary compounds often state the atom counts instead. The widgets show these names beside the English ones.

An English name puts the cation first and the anion second. A monatomic anion takes the ending -ide (sodium chloride); a polyatomic ion keeps its own name (the sulfate of sodium sulfate). A metal with more than one common charge carries that charge as a Roman numeral in brackets: iron(II) sulfate, iron(III) sulfate (Stock notation). A metal with only one common charge gets no numeral — calcium chloride, not calcium(II) chloride. Older books and shop labels also use -ous and -ic: ferrous for iron(II), ferric for iron(III), cuprous and cupric for copper(I) and copper(II). Acid salts keep their hydrogen: sodium hydrogen carbonate (also called bicarbonate), potassium dihydrogen phosphate.

The same central atom can carry different numbers of oxygens: $\ce{ClO-}$, $\ce{ClO2-}$, $\ce{ClO3-}$, $\ce{ClO4-}$. Their names climb a ladder with chlorine’s oxidation state (+1, +3, +5, +7): hypochlorite, chlorite, chlorate, perchlorate, and Chinese marks the same four steps with its own prefixes. Sulfur and nitrogen have shorter ladders: sulfite (+4) and sulfate (+6), nitrite (+3) and nitrate (+5).

Acid names follow the anion: -ate becomes -ic acid (sulfate → sulfuric acid) and -ite becomes -ous acid (sulfite → sulfurous acid). An acid without oxygen is hydro-…-ic acid (hydrochloric acid). Pure HCl is a gas, hydrogen chloride; only in water is it hydrochloric acid.

Two non-metals combine without forming ions, so there is no charge balance to go by: $\ce{NO}$, $\ce{NO2}$, $\ce{N2O4}$ and $\ce{N2O5}$ are all oxides of nitrogen. The name must therefore state the atom counts, with the prefixes mono-, di-, tri-, tetra-, penta-, hexa-…: nitrogen monoxide, nitrogen dioxide, dinitrogen tetraoxide, dinitrogen pentaoxide. The first element never takes mono-; the second takes it only when the same pair forms several compounds — carbon monoxide, but simply hydrogen chloride and boron nitride. IUPAC keeps the prefix vowel (pentaoxide); many textbooks write pentoxide, and you will meet both. Water, ammonia and methane keep their trivial names.

A hydrate joins its water of crystallisation to the formula with a dot: $\ce{CuSO4.5H2O}$ is copper(II) sulfate pentahydrate (blue vitriol).