[Music] hi and welcome back to freesciencelessons.co.uk by the end of this video you should be able to describe the structure of alkenes you should then be able to explain why alkenes are unsaturated and finally you should be able to draw the displayed formula of alkenes and this is for triple chemistry students only in the last video we started looking at alkene molecules in this video and the next couple of videos we're going to be looking at alkenes in much more detail we've already seen the alkene ethene the molecular formula of ethene is c2h4 i'm showing you here the displayed formula of ethene now the displayed formula shows us all of the atoms and the covalent bonds as we saw alkenes have at least one double covalent bond between the carbon atoms and in the case of ethene we can see that here now a key fact that you need to learn is that alkenes have the general formula cnh2n what this means is that if we know the number of carbon atoms in other words n we can calculate the number of hydrogen atoms to do that we simply multiply the number of carbon atoms by two as you can see ethene has two carbon atoms two multiplied by two is four and that's the number of hydrogen atoms in ethene propine is an alkene with three carbon atoms multiplying three by two tells us that propine also has six hydrogen atoms and we can see those here and the molecular formula of propine is c3h6 coming up i'll give you some questions to try yourself and we'll explore why alkenes are unsaturated molecules okay here's a sample question for you butene has four carbon atoms work out the molecular formula of butane and draw the displayed formula so pause the video now and try this yourself okay as we said butane has four carbon atoms multiplying four by two tells us that butane has eight hydrogen atoms so the molecular formula butane is c4h8 and here's the displayed formula of butane now i should point out that in this case i've put the double bond in the center of the molecule however at gcse the position of the double bond is not important so you can put it between any two adjacent carbon atoms here's another question for you pentene has five carbon atoms work out the molecular formula of pentine and draw the displayed formula so pause the video now and try this yourself okay pantene has five carbon atoms multiplying five by two tells us that pentene has ten hydrogen atoms so the molecular formula of pentine is c5h10 and i'm showing you the displayed formula of pentene here and again remember that in gcse you can put the double bond between any two adjacent carbon atoms now it's really important that you learn the structures of ethene propine butene and pentine because you could be asked them in your exam now there is one final fact about alkenes that you need to learn scientists say that alkene molecules are unsaturated that's because alkenes have got two fewer hydrogen atoms than the alkane with the same number of carbon atoms so i'm showing you the alkane ethane and the alkene ethene both ethane and ethene have got two carbon atoms however ethane has six hydrogen atoms whereas ethene only has four hydrogen atoms so as you can see alkenes have got two fewer hydrogen atoms than the equivalent alkyne remember you'll find plenty of questions on alkenes in myovision workbook and you can get that by clicking on the link above