Names of ionic compounds

I. Compounds of great ionic character

Two types of compounds fall into this category: 1. those consisting of a metal combined with a nonmetal (e.g. NaCl, Ag₂S, PbO) and 2. compounds containing polyatomic ions, except for the oxoacids (e.g. CaSO₄, NH₄NO₃, KCN, but excluding H₂SO₄, HNO₃ etc.). For the sake of naming compounds, they will be classified as ionic compounds in this tutorial.

To name an ionic compound, one should name the cation first, then, name the anion (with the word 'ion' omitted). It is not necessary to indicate the number of cations and anions in the compound because it is understood that the total positive charges carried by the cations must equal the total negative charges carried by the anions. A few examples are listed below:

KI	potassium ion + iodide ion = potassium iodide
CoCl₂	cobalt(II) ion + two chloride ions = cobalt(II) chloride
CoCl₃cobalt(III)	ion + three chloride ions = cobalt(III) chloride
Hg₂Cl₂	mercury(I) ion + two chloride ions = mercury(I) chloride or mercurous chloride
AgNO₃	silver ion + nitrate ion = silver nitrate

It is not called silver(I) nitrate because Ag⁺ is the only stable ion of silver.

(NH₄)₂S	two ammonium ions + sulfide ion = ammonium sulfide
Al(HCO₃)₃	aluminum ion + bicarbonate ion = aluminum bicarbonate or aluminum hydrogen carbonate

Some ionic compounds incorporate water molecules in their structure. These compounds are called hydrates. To name the hydrates, the number of waters of hydration is indicated by a Greek prefix following the name of the compound. For example, CuSO₄·5H₂O is called copper(II) sulfate pentahydrate.

Since the number of cations and anions in a molecule is not specified in the name of an ionic compound, to figure out the molecular formula from the name sometimes is not straightforward. The following examples show how this can be achieved in a systematic matter:

Example 1: Give the molecular formula of aluminum sulfide.

Solution:

i) Since aluminum is a metal and sulfur is a nonmetal, this compound is classified as an ionic compound.

ii) The cation, aluminum ion, is: Al³⁺ (if you forget the charge of the aluminum ion, look up the position of Al in the periodic chart).

iii) The anion, sulfide, is: S^{2^-} (the –ide suffix indicates that it is a simple anion)

iv) How many Al³⁺ should combine with the appropriate number of S^{2^-} such that the molecule carries no net charge? Al₂O₃ is the answer.

Example 2: Give the molecular formula of vanadium(III) phosphate.

Solution:

i) You may not recognize that vanadium is a metal. However, the suffix –ate in the word 'phosphate' is the hint of an oxoanion, a polyatomic ion. You know that this compound is classified as an ionic compound.

ii) The cation, vanadium(III) = V³⁺

iii) The anion, phosphate = PO₄³⁺

iv) How many V³⁺ should combine with the appropriate number of PO₄^{3^-} such that the molecule carries no net charge? VPO₄ is the answer.

Example 3: Give the molecular formula of ammonium sulfate.

Solution:

i) Both ammonium and sulfate are polyatomic ions, again, this compound is classified as an ionic compound.

ii) The cation is ammonium ion = NH₄⁺

iii) The anion is sulfate ion = SO₄^{2^-}

iv) The molecular formula of the compound is (NH₄)₂SO₄, because it takes two groups of NH₄⁺ to combine with one SO₄^{2^-} to give a molecule that carries no charge.