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Current time:0:00Total duration:5:14

Video transcript

let's say we're asked to draw all the structural isomers that have the molecular formula c5h12 the word isomer means same parts and so we're talking about the same number of atoms all of our structural isomers are going to have five carbons and 12 hydrogens our isomers are going to differ and how those atoms are connected to each other so they differ in terms of their structure and that's why we call them structural isomers you can also call them constitutional isomers so we need five carbons so for our first isomer we could just draw five carbons in a chain so here are my five carbons in a chain and you should have already seen the video on bond line structures before you watch this one so let's draw out those five carbons and let's double check and make sure we have the correct number of hydrogen's the carbon on the far left has three hydrogen's so here we have our three hydrogen's next carbon has two same with the next carbon so two for this one two for the next carbon and finally three hydrogen's for the last carbon so let's count up everything and make sure we have the correct molecular formula so we have 1 2 3 4 5 carbons so that's c5 and then we should have 12 hydrogen's here's 3 plus 2 gives us 5 plus 2 gives us 7 plus 2 gives us 9 and then we need we have 3 more for a total of 12 so c5h12 is the molecular formula for this compound all right let's draw another structural isomer that has the same molecular formula so instead of drawing five carbons in a chain now we have to draw four so let's start by drawing four carbons we need a total of five carbons so we need to show the fifth carbon branching off of our chain so we could show the fifth carbon branching off of our chain here so let's draw in those five carbons so here we have our five carbons let's count up hydrogen's carbon on the left has three so three hydrogen's here three hydrogen's on this top carbon there's only one hydrogen on this carbon two hydrogen's on this one and finally three hydrogen on this carbon so let's count up our atoms so let's use red for this one we have 1 2 3 4 5 carbons so that's c5 and then for hydrogen's we have 3 here plus 3 gives us 6 plus 1 gives us 7 plus 2 gives us 9 and 3 more for a total of 12 so c5h12 is the molecular formula for this compound so these two these two drawings represent two different molecules both these molecules have the molecular formula c5h12 but they differ in terms of how those atoms are connected they turn they differ in terms of their structure so we call them structural isomers of each other all right to draw another structural isomer some students might say oh well we could start with four carbons in our chain again and this time instead of showing instead of showing a branch off of this carbon we could show a branch off of this carbon and so a student might draw this structure and say okay there's a different structural isomer but actually these just two different ways to represent the same molecule if you analyze that second structure that we just drew all right the connections are the same we have a CH right here bonded to a ch3 bonded to a ch3 and bonded to a ch2 and the ch2 is bonded to a ch3 that's the same that's the same structure as what we drew out over here so it looks like it's a different structure all right it's a different drawing than the one up here but actually this is just two different ways to represent the same molecule alright so we have two structural isomers so far let's think about one more so we can no longer do four carbons in our chain so we go down to three carbons so we start with three carbons in our chain we know we need a total of five carbons so we need to show to more carbons added to our chain and we would have to add those two carbons to our central carbon like that let's draw out all of our carbons here and let's add in our hydrogen's so this carbon would have three hydrogen's same with this arvin and the same with this one and finally the same for this carbon the carbon in the center right this carbon the center here already has four bonds so it doesn't have any hydrogen's on it let's count up everything let's count our carbons first 1 2 3 4 5 carbons so c5 and then we have three hydrogen's plus 3 is 6 plus 3 is 9 plus 3 is 12 so c5h12 is the molecular formula for this compound and this is a another structural isomer so it's a different molecule from the other two so we have a total of three structural isomers that have the molecular formula c5h12