 Okay, welcome back everybody. In this video we are going to start by talking about isomers which we talked about last week. Last week I gave a definition for what isomers are. These are isomers are molecules that have the same simple formula, but the atoms are attached in different ways. And one of the examples that I gave was butane. And this other molecule here, this is butane. The formula for this molecule is C4H10. This molecule here is called 2-methylpropane, and it is also C4H10. But if you notice the atoms are attached in different ways. So these two molecules here, this one and this one, they are isomers of each other. There is another way to define isomers and it might be a little bit better. And that is isomers are molecules that have the same formula, but they have different IUPAC names. If I showed you two molecules and I said are they isomers of each other, or are they identical? If they are identical, they will have the same formula, and they will have the same IUPAC name. If they are isomers, they will have the same formula, but they will have different IUPAC names. Maybe that's the easier thing to do, and I'll show you some examples in the next couple of slides. So here's a flow chart. If I show you two molecules and I ask whether they are isomers or identical, you basically have to ask a series of questions. You say, do they have the same formula? If they don't have the same formula, then they're completely different molecules. They're not even isomers of each other. If they do have the same formula, then you have to ask, do they have the same name? If they have the same name, then they are identical. If they don't have the same name, but they do have the same formula, then they're isomers of each other. So you can write this flow chart down and use it on a quiz or a test, whatever works for you there, or you can memorize it. So as an example, I show you these two molecules, and I say are they isomers, are they identical, or are they completely different? The first thing you do is say, do they have the same formula? Well, this one is one, two, three, four, five, C5 something. This one is C4 something. They don't even have the same formula. So the answer to the first question is no. These things are completely different molecules. We can skip that part because we already know that they can't be isomers and they can't be identical if they have different formulas. Now, here's two other molecules there. Do they have the same formula? Well, this one is C5, one, two, three, four, five, six, seven, eight, nine, ten, eleven, twelve, twelve hydrogens. This one is C5, one, two, three, four, five, six, seven, eight, nine, ten, each ten. They don't have the same formula. They are completely different. I'll show you one more example. We can skip the rest of this because we know they're completely different. Here's two molecules. Are they isomers, identical, or totally different? Well, do they have the same formula? One, two, three, four, five, six, six carbons, seven, eight, eight carbons. One, two, three, four, five, six, seven, eight, eight carbons. I will skip the calculation part, but they both have 18 hydrogens. So, they have the same formula. So, we can rule that out. The answer to the first question is yes. The second question is, do they have the same name? So, we have to name these things, right? We have to count the longest chain of carbons. That's one, two, three, four, five, six. That's six. So, the end part of that molecule's name ends in hexane. We have to number in this direction. And this is a methyl side chain. This is a methyl side street. So, it is dimethyl because there are two of them, hexane. And the methyls are located at carbon number two and carbon number three. So, that top molecule's name is two, three dimethyl hexane. We have to play the same game here. Find the longest chain of carbons. That's there. That's six. So, the end name will also be hexane. We have to number the carbons. We have to number them this way. And this is a methyl. That is also a methyl. So, it is dimethyl. And the methyls are located at carbon number two and carbon number three. So, that molecule's name is also two, three dimethyl hexane. So, both of those molecules, one on the top here, one on the bottom, they're both called two, three dimethyl hexane. Because of that, we can say, do they have the same formula? Yes, they do. Do they have the same name? Yes, they have the same name. So, these two molecules are identical to each other. If we came up with a different name between these two, then they would have been isomers of each other. But in this case, we got the same name, so they are identical. And that's the end of this video. I will see you in the next one.