 Isomers are substances with the same molecular formula but different structural formulas. And I've drawn butane and isobutane on the board here. And notice I've just shown the carbons. This is one way of representing the structural formulas without having to write in all the hydrogens. Or sometimes, oftentimes, you'll see lines drawn, but we don't have hydrogens in drawn in the place. We could go in here and write in hydrogen in every place, but because carbon forms bond so readily with hydrogen, it's accepted just too long to leave it as such. And if you see nothing at the end, you just assume that a hydrogen is in its place. But I want to look at isobutane and we can draw in where the hydrogens would go. And I want to show you what I mean by being able to draw the line straight through. If we start with the first carbon, we can go all the way through to the fourth carbon without having to lift up the pen. But if we start with the first carbon here, you can see that we cannot go all the way through without having to lift up the pen coming back. And so that sort of gives you an idea as to what isomers are all about. In butane, the carbons are all in one chain, so to speak. But in isobutane, we have three carbons in a chain and the fourth carbon is attached to the second carbon. So that's how isomers work. Now, so we'll go on and we'll look at the alkenes. So alkenes contain one or more double bonds between the carbons. And so our simplest alkene then is this one, which is ethene. Eth, of course, means two, so that's showing that we have two carbons. And then the suffix, ene, recognizes that we have that double bond. And so just like members of a family may share a last name in common, members of this family of organic compounds share the suffix ene in common. So if we look at our two carbons, you can see that we have a double bond, two shared pairs of electrons between carbon one and carbon two. Now let's look at some more complicated diagrams. For example, this one is propane. Now I want you to notice something about propane. The carbon is on, the first carbon is numbered here. And here's the second carbon and here's the third carbon. But the double bond comes after the first carbon. And sometimes people get it confused and they think, well, this is an isomer because now the double bond is after the third carbon. But we always start the number closest to the double bond. So this is propane, whether we look at it this way or whether we look at it this way, it's just flipping the molecule over. So propane has one double bond and that one double bond is after the first carbon. Now let's look at butane. We spend butane so that we can get a good look at that double bond. You can see that we have a double bond after the first carbon, then a single bond between the second carbon and the third carbon and another single bond between the third carbon and the fourth carbon. So if we wanted to show butane like we showed at thing over here, it would look like this. If you look, you see I made a mistake getting happy with my hydrogen because this carbon, I've drawn it with five bonds and that's not right. We have to get rid of one of those hydrogens. Now each carbon has four bonds and you can see we have our double bond after the first carbon. So we'll go back and look at that here. And here's our double bond after the first carbon and you can see that the other bonds between the carbons are single bonds. Now consider this. This is two butane. And if you look at two butane, you can see that we have a single bond after the first carbon, then a double bond after the second carbon and then a single bond between carbon three and carbon four. And so one butane and two butane are isomers of one another. And so we can draw two butane like this. Make sure that we have all the carbons with four bonds and no more than four bonds. So this is two butane. We have to be careful not to get carried away. Sometimes people will say, well, this must be three butane. But the problem is this is one butane. Here's our first carbon, here's our second, here's our third, and here's our fourth. So this substance is the same as this one. It's just been, so here's one, two, three, four. So the point, two points that we're trying to make here. One, isomers are substances that have the same molecular formula but different structural formulas. And then alkenes contain one or more double bonds between the carbons. The simplest alkenes ethyne. Then we looked at propane and one butane and two butane.