I need help understanding isomers.

Firstly the sort of isomer that you are referring to is a structural isomer. As you will see there are other kinds of isomers as well.

C3H7Cl is a monochlorinated saturated hydrocarbon corresponding to the unsubstituted hydrocarbon C3H8. So it is a propane. In the case of propane, there can be only two monohalogenated isomers, one with the halogen (chlorine in this example) on a terminal carbon and the other with chlorine on the middle carbon. Are you with me so far?

CH3-CH2-CH2-Cl 1-chloropropane

or

CH3-CH(Cl)-CH3 2-chloropropane

Note that the 1-carbon and the 3-carbon are equivalent so there is no 3-chloropropane, you always number the substituted carbon as low as possible.

In both cases these have the same Hill formula C3H7Cl and by definition, two different structures with the same Hill (empirical) formula are structural isomers.

Saturated means all single bonds (sigma conds) and hydrocarbons of this sort have the general formula CnH2n+2.

C1 methane
C2 rthane
C3 propane
C4 butane
C5 pentane
C6 hexane
C7 heptane
C8 octane

You may have heard of octane in relation to gasoline/petrol so now you know what it is.

Methane and ethane of course can have no monosubstituted isomers, propane is the simplest example your teacher could provide. Be happy he did not ask you to draw all possible isomers of the chlorohydrocarbon C5H11Cl for example.structural

How to draw isomers:

[Edited on 18-3-2009 by Sauron]