in today's video we're going to take a look at hydrocarbons and specifically at alkanes first though it's worth understanding that this whole area of chemistry is known as organic chemistry which just means that it's all about compounds that contain carbon you'll see over and over again in chemistry that carbon is a really useful element for making large compounds because each atom of carbon can form four strong bonds with the most common atoms to be bonded to being either other carbon atoms or hydrogen atoms now lots of molecules only contain these two elements and collectively we refer to all of these molecules as hydrocarbons which we can define as any compound that's formed from carbon and hydrogen only so this molecule which is called butane is a hydrocarbon but this similar molecule called butanol isn't a hydrocarbon because it also has an oxygen atom the most simple type of hydrocarbons are a group called alkanes and you need to know the first four in the series the smallest is methane which contains just a single carbon atom bonded to four hydrogens because remember carbons always have four bonds then we have ethane with two carbons and six hydrogens propane which is c3h8 and butane which is c4h10 now if you look at all of these compounds you'll see that they grow by one carbon and two hydrogens each time but otherwise they're pretty much the same we call groups of organic compounds like this homologous series and it means that they all have similar properties and so react in a similar way it also means that we can write a general formula which describes the entire series and in the case of alkanes the general formula is cn h2n plus 2. this might look a bit complicated but all it means is that if the compound contains n carbon atoms then it will contain two times n plus two more atoms of hydrogen so if we had an alkane with three carbon atoms like propane then n would be three so it would have to have two times three plus two hydrogen atoms so eight hydrogens which would give it the formula c3h8 now you only need to remember these first four but you could be asked to use this general formula to work out the molecular formula of much larger ones for example octane is an alkane with eight carbon atoms what is this molecular formula well we know that for octane n will be eight because that's how many carbons it has so it must have two times eight plus two hydrogens which gives us a 16 plus 2 or 18. so its molecular formula would be c8h18 before we finish we just need to mention that one of the key features of alkanes is that they're saturated compounds which means that every carbon atom has four single covalent bonds there aren't any double bonds for example if we were to take this molecule of propane and change one of these single bonds to a double bond then each of the carbons involved would have to lose a hydrogen and what would have left would no longer be an alkane instead we'd class it as an alkene which we'll see when we take a look at cracking hydrocarbons in a later video in our next video though we'll stick with alkanes by taking a look at their properties and seeing how to write equations for their combustion anyway that's all for now so if you enjoyed it then do tell your friends about us and we'll see you next time