 Here is butane. So butane has one, two, three, four carbons, and it has two, three, four, five, six, seven, eight, nine, ten hydrogens. So therefore its molecular formula would be C4H10. Now with a nice amount, I don't want to change the molecular formula. And if we're using the model kits, there's the easiest way to make sure that we know that we've got isomers, is if we change this model in some way but don't add or subtract anything from it, because then we know we've got the same number of atoms of each type in our final product. So one of the ways that we can do this, and if you're looking at chain isomers, the simplest way to think about this is we're actually changing the length of the chain. So we're going to move something somewhere to change the length of the chain. So in this case the simplest way for me to do that is to remove a methyl group off the end and substitute it for one of my now middle hydrogens. If I do that, you can see that I haven't actually changed the number or type of any of the atoms. So I still have 1, 2, 3, 4 carbons and I still have my 3, 4, 5, 6, 7, 8, 9, 10 hydrogens. So the formula is still the same but now we've got a different compound. Now when we name this compound the longest chain is 1, 2, 3 carbons in it, not 4. So this now becomes a propane but there's this little group above the way that I've oriented it which is a methyl group so this would be methyl propane. So it has the same molecular formula as butane but it has a different chain length. We now have an additional side branch we didn't have on butane and we have a smaller main chain. So now we have only 3 carbons in the main chain rather than 4.