But the commonest of all is the benzene ring, C6H6, which has a special symbol of its own.
Note: Explaining exactly what this structure means needs more space than is available here. It is explained in full in two pages on the structure of benzene elsewhere in this site. It
would probably be better not to follow this link unless you are actively interested in benzene chemistry at the moment - it will lead you off into quite deep water!
Deciding which sort of formula to use
There's no easy, all-embracing answer to this problem. It
depends more than anything else on experience - a feeling that a particular way of writing a formula is best for the situation you are dealing with.
Don't worry about this - as you do more and more organic
chemistry, you will probably find it will come naturally. You'll get so used to writing formulae in reaction mechanisms, or for the structures for isomers, or in simple chemical equations, that you won't even think about it.
There are, however, a few guidelines that you should follow. What does your syllabus say?
Different examiners will have different preferences. Check first with your syllabus. If you've down-loaded a copy of your syllabus from your examiners' web site, it is easy to check what they say
they want. Use the \to search the organic section(s) of the syllabus for the word \
You should also check recent exam papers and (particulary) mark schemes to find out what sort of formula the examiners really prefer in given situations. You could also look at any support material published by your examiners.
Note: If you are working to a UK-based syllabus and haven't got a copy of that syllabus and recent exam papers, follow this link to find out how to get them.
What if you still aren't sure?
Draw the most detailed formula that you can fit into the space available. If in doubt, draw a fully displayed formula. You would never lose marks for giving too much detail.
Apart from the most trivial cases (for example, burning
hydrocarbons), never use a molecular formula. Always show the detail around the important part(s) of a molecule. For example, the important part of an ethene molecule is the carbon-carbon double bond - so write (at the very least) CH2=CH2 and not C2H4. Where a particular way of drawing a structure is important, this will always be pointed out where it arises