Here is the structure of fluorobenzene. How many structure isomers exist for difluorobenzene and for trifluorobenzene? Take a piece of paper and try to write down all of these structures. The isomers are shown in the following and I want you to think about which isomers have dipole moments and which ones do not. You can click on the "properties" file to find the computed dipole moments and to see whether your predictions are correct.