The number of structural isomers that can be produced rises rapidly with the number of carbon atoms.. for example, the alkane with 4 carbon atoms has 2 isomers; the one with 7 has 9 structures ...C 10 has 75...and C20... has 366319!
visit
http://www.bodmas.org/vanilla
apply for membership
and post your comments/revision questions etc
Although this is written for OCR Chemistry, it contains lots of useful stuff
Just be selective as to waht you look at, remember that there is much more on the web site than is needed for Principles 2.
http://www.knockhardy.org.uk/sci.htm
hope you find it helpful..