Nomenclature of Inorganic Compounds

Nomenclature stands for system of naming. In chemistry, the study of a system of naming chemical compounds has been designed by I.U.P.A.C (international union of pure and applied chemistry). We shall be looking at naming of inorganic compounds here.

This system enables the composition of most chemical substances to be stated directly from their names.

Binary compounds

These are compounds containing only two elements. The naming of such compounds indicates the nature of the elements. It ends in -ide. Example, H₂S - hydrogen sulphide, and CuO – copper(II) oxide.

However, there are certain exceptions, such as water, H₂O, methane, CH₄, ammonia, NH₃ and other non-metallic hydrides.

Other exceptions are compounds of three elements which have their names ending in –ide. Example, the hydroxides (NaOH and KOH); acids salts of hydrogen sulphide (e.g. sodium hydrogen sulphide, NaHS); salts like ammonium chloride, NH₄Cl (here the ammonium group, NH₄⁺ is treated as an element).

Acid radicals

There are groups of atoms known as acid radicals. These radicals are combinations of other non-metallic elements and oxygen – they combine with hydrogen to form acids. Their naming indicates the number of oxygen atoms contained, the valency or oxidation number of the central or key element, and ends in -ate.

The oxidation number of the key element is written last. Example, – CO₃^2- - trioxocarbonate(IV); - SO₃^2- - trioxosulphate(IV), NO₃^- - trioxonitrate(V) and – SO₄^2- - tetraoxosulphate(VI).

When metallic elements incorporate into acid radicals, the same procedure is followed. Example, MnO₄^- - tetraoxomanganate(VII); Cr₂O₇^2- - heptaoxodichromate(VI).

Note: "tri" stands for 3; "tetra" 4; "hepta" 7; and "oxo" stands for oxygen.

Metal compounds

When we have a metal (of fixed oxidation state) and an acid radical, forming only one kind of salt, the name of the salt is simply the name of the metal together with that of the acid radical. Example, NaNO₃ - sodium trioxonitrate(V); Al₂(SO₄)₃ – aluminium tetraoxosulphate(VI) and MgCl₂ – magnesium chloride.

On the other hand, when metals (of variable oxidation state) combine with acid radicals to form more than one kind of salt, then, the naming is done by including the oxidation state or valency of the metal in the particular compound. Example, FeCl₂ – iron(II) chloride, FeCl₃- iron(III) chloride; FeSO₄ – iron(II) tetraoxosulphate(VI) and Fe₂(SO₄) – iron(III) tetraoxosulphate(VI).