 Okay, what about these two molecules? Like the last two, they're isomers. Pause the video for a second and work out their molecular formula. And I'll draw out the condensed structural formula. They both have the molecular formula C6H12, they're alkenes. So they each have six carbons and they each have one double bond, so they're clearly both a kind of hexane. But how to indicate where the double bond is in the molecule? Again, we fall back on numbering the main chain. Number the chain starting from the end closest to the double bond. Again, we're looking to get the lowest possible number into the final name. So in the first molecule, the double bond begins on carbon number one. And in the second molecule, it begins on carbon number two. We then indicate this in the name by inserting a number into the name like this. Hex one-ene and hex two-ene. The number shows that the thing that comes after it, that is the double bond indicated by the ene, starts at that numbered carbon. For simple alkenes without branches, you can also sometimes find these names written as one hexane and two hexane. But as our molecules get more complicated and we start having prefixes on the names, it's often clearer to put the number directly in front of the ene to indicate what it refers to. So here's just a little summary page of those last four molecules that we looked at. Both of those pairs of molecules were isomers. But by naming them, we can indicate the differences in their structures.