the double bond. We'll look at that problem in a moment. Here is one of the isomers of but-2-ene: The CH 3 group has the higher priority because its carbon atom has an atomic number of 6 compared with an atomic number of 1 for the hydrogen also attached to the carbon-carbon double bond. The isomer drawn above has the two higher priority groups on opposite sides of the double bond. The compound is (E)-but-2-ene. A minor addition to the rule to allow for isotopes of, for example, hydrogen Deuterium is an isotope of hydrogen having a relative atomic mass of 2. It still has only 1 proton, and so still has an atomic number of 1. However, it isn't the same as an atom of "ordinary" hydrogen, and so these two compounds are geometric isomers: The hydrogen and deuterium have the same atomic number - so on that basis, they would have the same priority. In a case like that, the one with the higher relative atomic mass has the higher priority. So in these isomers, the deuterium and chlorine are the higher priority groups on each end of the double bond. That means that the left-hand isomer in the last diagram is the (E)- form, and the right-hand one the (Z)-. Extending the rules to more complicated molecules If you are reading this because you are doing a course for 16 - 18 year olds such as UK A level, you may well not need to know much