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Chemical formulae: compounds with both ionic and covalent bonding

A formula conveys two pieces of information; it gives the identity of the elements present, and it also tells us the ratio in which the atoms or ions are combined together. For example, the formula for glucose, C₆H₁₂O₆, tells us that there are three elements present in the compound: carbon, hydrogen and oxygen, and that in one molecule of glucose there are 6 carbon atoms, 12 hydrogen atoms and 6 oxygen atoms all linked together.

You can read the tutorial on screen or download from this link a file with the whole of a series of tutorials about formulae and equations.
 
Ionic compounds with more than two elements
Many inorganic compounds contain three or more elements, in which one or other of the ions consists of clusters of atoms held together by covalent bonds. At this stage, you do not need to be concerned about their structure, but you do need to know their formulae, because a large number of inorganic compounds are composed of these ions. The commonest are listed below:
 
Positive ions (cations)

phosphate ion, PO₄^3–
 
Ionic formulae
We can use the same rules to construct formulae as we used for simple ionic compounds. For example, potassium sulphate contains potassium ions, K⁺ and sulphate ions, SO₄^2–, and so the formula must be K⁺₂ SO₄^2–, but this is normally written without the charges as K₂SO₄.
 
In the case of a compound such as calcium hydroxide, containing Ca²⁺ ions and OH^– ions, we must write the formula as Ca(OH)₂, and not CaOH₂. Without the brackets the formula would imply that there were two hydrogen atoms but only one oxygen atom, whereas there are actually two hydroxide ions present.
 
In the Nuffield Chemistry Students’ book on page 22 you will find some questions to help you to practise the construction of some simple formulae.

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SHAUNAMAE-SNAPE
WELL DONE
23 October 2007

updated: 22 February 2007