 In this video we're going to look at what an isomer is. Consider the molecule butane. The parent part of the name, but, tells you that there are four carbons and the suffix tells you that all the bonds are single, so we can draw the structure. And the formula is C4H10. However, there's another way of arranging four carbons and ten hydrogens. And it's this. In this molecule the formula is exactly the same. There are the same number and type of atoms, but the structure is different. It's T-shaped rather than a straight chain. It's a completely different molecule. So these two molecules are called isomers. They have the same number and type of atoms, but the atoms are bonded to give different structures. Let's now look at this collection of molecules and work out if any of them are isomers. So let's number them first. We'll call this one number one, two, three, four. And I'll start with number one. Now, first of all, how many carbons are there? So one, two, three, four, five. Six carbons in this molecule. And you have to imagine the hydrogen. So on this end carbon here, there are going to be one, two, three hydrogens. Four, five. This carbon here is only going to have one hydrogen because it's already got three bonds to carbon. That's six, seven, eight, nine, 10, 11, 12, 13, 14. C6H14. And in fact, you could have worked that out just by looking at the molecule because you can see that there are no double bonds in it. And if they're all single, it must be an alkane. And you know the formula for an alkane. It doesn't matter whether the alkane is branched or not. The general formula, CNH2N plus two still holds. Okay, so that's C6H14. Let's look at the second one. One, two, three, four, five, six carbons. And again, you can see it's all single bonds. So it's an alkane. So it's going to be H14. It's a good exercise if you're not quite sure of this to draw it out fully as a Lewis structure or a structural formula to convince yourself there really are 14 hydrogens in this molecule. So that means that these two molecules here are isomers. They have the same number of carbons, the same number of hydrogens, but the atoms are arranged in a different way. Looking at the third molecule, let's count the carbons. One, two, three, four, five, six. So it is also C6H14. Is it an isomer? Now if you look carefully and compare it with number one, you can see that actually it's exactly the same as number one. I've just tilted it slightly. So you've got to watch out for this. The same molecule can be drawn in different orientations and it may look slightly different. But if you carefully examine which atoms are bonded to which and they're arranged in the same way, then it's the same molecule. Lastly, looking at number four, we count up the carbons. Two, three, four, five, six, seven. C7, it's an alkane. So the number of hydrogens is two times seven plus two, which is 16. It clearly can't be an isomer because it has a different number of carbons. So it's just a different molecule.