2.1.2 (a) Writing formulae of Ionic compounds

Syllabus

Formulae and equations

(a) the writing of formulae of ionic compounds from ionic charges, including:

(i) prediction of ionic charge from the position of an element in the periodic table

(ii) recall of the names and formulae for the following ions: NO₃^–, CO₃^2–, SO₄^2–, OH^–, NH₄⁺, Zn²⁺ & Ag⁺

What does this mean?

Predicting charges of ions from position in the Periodic Table.

Nothing has changed from GCSE.

Atoms "need" a full outer shell.

Atoms of elements in Group 1 have electronic structures that end with 1.

Li (2.1), Na (2.8.1), K (2.8.8.1) etc.

The easiest way to achieve this is to lose an electron.

Hence Group 1 elements only ever form 1+ cations.

In the same way, all Group 2 elements will form 2+ ions.

And Aluminium and Gallium (Grp 3) will form 3+ ions - other Group 3 elements are rarely in A level questions.

Group 4 elements don't usually form ions.

Group 5 elements have electronic structures ending with 5 - these need to gain 3 electrons.

So Group 5 elements form 3- anions.

Similarly, Group 6 elements form 2- anions.

And Group 7 elements form 1- anions.

Group 0 (Grp 8) elements already have full outer shells so don't form ions.

Transition elements (in the middle block - the D block) can usually form form more than one ion eg Fe³⁺ and Fe²⁺. So if these are referred to in writing the examiner will use oxidation numbers to tell you which ion they mean, in this case Fe (III) or Fe (II) respectively.

The only transition ions you are expected to know the charge of are Ag⁺ and Zn²⁺ because those are the ions that these two elements invariably make.

Compound ions

Later on in the course you will learn how to write the formula of some compound ions from their names and oxidation numbers.

But you really need to know the following;

Nitrate = NO₃^–, Carbonate = CO₃^2–, Sulphate = SO₄^2–, Hydroxide = OH^–, Ammonium = NH₄⁺

^{Other Nitrate and Sulphate ions exist but will always be followed with an oxidation number - of which, more later.}

^{Without an oxidation number, the exam board mean the common ion shown right.}

Writing formulae of ionic compounds.

^{If the charges of the anion and cation}^match^{then the formula is 1 to 1.}

eg K⁺ & Cl^- makes KCl

eg Ca²⁺ & O^2- makes CaO

eg Al³⁺ & N^3- makes AlN

eg Ga³⁺ & PO₄^3- makes GaPO₄

^{If the charges of the anion and cation}^{don't match}^{then the formula is calculated as shown below.}

If you need more than one compound ion then it is placed in a bracket.

eg. Ca²⁺ & NO₃^- makes Ca(NO₃)₂

eg. NH₄⁺ & CO₃^2-makes (NH₄)₂CO₃

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