 Let's start with the first two. When you have a molecule that you need to name, first identify and name the longest continuous carbon chain that's in that molecule, and then identify and name any groups that are attached to that chain. Let's start by looking at the isomers that have the formula C4H10. I've put them here both in a full structural formula and a stick structure. The first molecule, the linear one, is easy. It's a straight chain with four carbons and it's an alkan, so it's called butane. What about the second isomer? Well, the first naming rule was to find and name the longest continuous carbon chain. In this molecule, the longest chain is three carbons long. And what's the name for a single bonded chain that's three carbons long? Propane. Okay, but what about this little branch? The naming of branches, also called substituents, follows the same pattern as the alkanes. Except this time we put YL, all on the end of the word, to indicate that it's a branch and not a whole molecule. But the parent part of the name follows exactly the same pattern according to the number of carbons involved. So a branch that's one carbon long is methyl and a branch that's two carbons long is ethyl and so on. I'll draw a couple of these out to make it clearer. Overall, or taken in a group, these substituents are called alkyl. We've replaced the ane of alkanes or the ane of alkenes with the isle suffix, or ILL, to show that we're talking about branches. So back to our second isomer. The naming rules say that we should find and name the longest chain in the molecule, so that's propane. And our list of branches says that a one carbon branch, like we have here, should be methyl. So the name of this molecule is methyl propane. So naming helps us avoid the ambiguity in formulae. Just giving the molecular formula doesn't tell you the structure of the molecule, but using its name gives you all the information that you need to draw it out.