A carbon atom is chiral if it is bonded to four different groups. One of the simplest chiral molecules would therefore be CFClBrI. This molecule is shown in Figure 1, along with its mirror image.

Figure 1. A chiral molecule - bromochlorofluoroiodomethane, CFClBrI, and its (nonsuperimposable) mirror image.

 

   

Figure 2. Click on the Superimpose button to show that CFClBrI is really a chiral molecule.

 

As shown on the right, any molecule that contains two or more groups that are the same (such as CFCl2Br) can be superimposed on its mirror image and therefore is not a chiral molecule.

One very important point to notice, however, is that the requirement for chirality is four different groups, not four different atoms. Therefore a carbon bonded to a chloro, bromo, methyl, and ethyl group (as in the structure on the left) is a chiral carbon, even though it is bonded to two carbon atoms.