 Allotropes are interesting, but they're limited by the fact that you only have one type of atom to work with. Carbon really comes into its own when you start letting it bond with other elements. It forms, in fact, the basis of organic chemistry, which describes a vast range of molecules, all based on carbon, and the most complex of which are the molecules that we, as living creatures, are built from. We'll deal with organic chemistry in much more detail in Year 12, but for now we're just going to look at a couple of groups of compounds in the simplest family of carbon compounds called the hydrocarbons. So to summarize hydrocarbons, hydrocarbons are compounds that are principally composed of carbon and hydrogen, certainly the ones that we're going to look at for now are just composed of carbon and hydrogen. They've got simple structures based on the fact that carbon forms four covalent bonds, and they form a variety of families according to differences in their structures. They also include most modern fuels like oil, petrol, and natural gas. These are all hydrocarbons. The two simplest hydrocarbon families are the alkanes and the alkenes, and these have different chemical properties because of a difference in their bonding, and that's what we're going to look at now. Alkanes are the simplest molecules you can make from just carbon and hydrogen, and you already know the simplest alkane. It's methane. So I'm going to draw the Lewis structure for methane. Lewis structures for hydrocarbons are very easy. As long as you're just dealing with carbon and hydrogen, there are no lone pairs, and you just need to follow the rule that carbon will always form four bonds and hydrogen will always form one. The next in the series of alkanes is called ethane, and you make it by taking two carbon atoms, joining them with a single bond and then filling up their remaining three bonds each with hydrogen. So its overall formula is C2H6. Next is propane. Now we have three carbons, again joined by single bonds, and again we fill up all the unused bonds that the carbon atoms could make with hydrogen. That's three for the end carbon, two for the middle, and three for the other end carbon. So the formula for propane is C3H8. And next is butane, four carbons. Fill them up with hydrogens, and that gives you C4H10. Now the alkanes series continues. Each time you add one carbon to the molecule and you fill up the remaining bonds with hydrogen, the molecule gets longer and longer. After the first four, the names are easier to remember because they're the same as the geometrical shapes. A pentagon has five sides, for instance, and pentane has five carbons. And then you've got hexane with six, and heptane with seven, and octane, nonane, decane, and so on. I'll stop there, but there isn't really any limit to how long you can make the molecule. In fact, the really long ones are polymers, which I'll mention in a little while. The other family we're going to look at is alkanes. These are also made just from carbon and hydrogen, and they also follow a series where each molecule is one carbon longer than the last. But the crucial difference between alkanes and alkanes is that alkanes have at least one carbon-carbon double bond in the molecule. We're only going to look at the alkanes that have exactly one double bond at the end of the molecule here. But there are variations on this theme that we'll explore next year. So I'm going to skip the first one. You'll see why in a second. Let's go straight to ethene. The eth part of the name tells you that there are two carbons. That's just like ethane in the alkanes. So we put down the two carbons, and then, because it's an alkene, we join them up with a double bond. That's the defining feature of the alkene. Now we simply fill in the remaining bonds with hydrogens. Each carbon forms four bonds, and two have already been taken up by the double bond, so each carbon gets two hydrogens. And the formula for ethene is C2H4. Now let's go back to methane. Can you see that it isn't possible to have a methane molecule? The defining feature, remember, was a double bond between two carbon atoms. But methane would have only one carbon, so it just doesn't exist. Okay, so the next one in the series after ethene is propene with three carbons. So we draw them out, and we join the first two by a double bond, and the next by a single. Remember, we said that we were going to focus on alkanes with just one double bond here. And then we fill in the remaining bonds with hydrogens. That gives us C3H6. Next, we've got butane, four carbons. Join the first two by a double bond, and then fill in the hydrogens. And that gives us C4H8. And the series continues with pentene and hexene and so on. For practice, it's a good idea to try drawing out these larger alkanes and alkanes.