atom. Compare the atomic numbers of the attached atoms to work out the various priorities. Notice that the atoms with the higher priorities are both on the same side of the double bond. That counts as the (Z)- isomer. The second isomer obviously still has the same atoms at each end, but this time the higher priority atoms are on opposite sides of the double bond. That's the (E)- isomer. What about the more familiar examples like 1,2-dichloroethene or but-2-ene? Here's 1,2-dichloroethene. Think about the priority of the two groups on the first carbon of the left-hand isomer. Chlorine has a higher atomic number than hydrogen, and so has the higher priority. That, of course, is equally true of all the other carbon atoms in these two isomers. In the first isomer, the higher priority groups are on opposite sides of the bond. That must be the (E)- isomer. The other one, with the higher priority groups on the same side, is the (Z)- isomer. And now but-2-ene . . . This adds the slight complication that you haven't got a single atom attached to the double bond, but a group of atoms. That isn't a problem. Concentrate on the atom directly attached to the double bond - in this case the carbon in the CH 3 group. For this simple case, you can ignore the hydrogen atoms in the CH 3 group entirely. However, with more complicated groups you may have to worry about atoms not directly attached to