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Current time:0:00Total duration:6:49

Video transcript

let's say we're given the molecular formula c3h8 oh and we're asked to draw a lewis dot structure so on the left here is one possible Lewis dot structure that you can draw that has that molecular formula there are three carbons 1 2 3 there's one oxygen right here and if you count up the hydrogen's you will get 8 and this Lewis dot structure let me go and write that this is a lewis dot structure here this one shows all of the bonds all right so all the bonds are drawn in but takes a lot of time to draw in all of the bonds and so we could represent this molecule in different ways we could condense this Lewis structure a little bit so this is equal to the structure I'm about to draw and we'll focus in on the carbon that I just circled there in red so that carbon is this one right here that carbon is bonded to an OHA so we could say this is an OHA here put in lone pairs of electrons on the oxygen and that carbon is also bonded to this hydrogen so let me draw on that hydrogen on the right side that carbon in red is bonded to this carbon in magenta and the carbon in magenta is bonded to three other hydrogen's so we could represent that as a ch3 so I could write ch3 here and the carbon in red is this one and the carbon in magenta is this one on the left side the carbon in red is bonded to another carbon in blue and the carbon in blue is bonded to three hydrogen's so there's another ch3 on the left side so let me draw that in so we have a ch3 on the left and the carbon in blue is directly bonded to the carbon in red so this is called a partially condensed structure so this is a partially condensed partially condensed structure we haven't shown all of the bonds here but this structure has the same information as the lewis structure on the left it's the same molecule it's just a different way to represent that molecule we can keep going we could go for a fully condensed structure so let's do that focus in on the carbon in red so this one right here let me draw in that carbon over here so that's that carbon that carbon is bonded to two ch3 groups there's a ch3 group on the right so there's a ch3 group on the right and there's a ch3 group on the left so I could write ch3 ch3 and then I could write a 2 here which indicates there are two ch3 groups bonded to directly bonded to the carbon in red what else is bonded to the carbon in red there's a hydrogen so I'll put that in so the carbon is bonded to a hydrogen the carbon is also bonded to an OHA so I will write in here and OH H this is the fully condensed version so this is completely condensed and notice there are no bonds shown right there are no bonds drawn in here you have to infer you have to infer the bonding from the condensed alright let's start with the condensed and go all the way to a lewis structure so we'll start with a condensed and then we go to partially condensed structure and then we'll go to a full lewis structure just to get some more practice here so I'll draw in a condensed one so we have ch3 3 and then Co ch3 alright let's turn that into a partially condensed structure so this carbon in red right here we're going to start with that carbon so I'll start drawing in that carbon right here what is bonded to that carbon well we have ch3 groups and we have three of them so there are three ch3 groups directly bonded to that carbon so let me draw them in so here's one ch3 group here is another ch3 group and then finally here is the third ch3 group so this carbon in red over here is this carbon the carbon in red is also bonded to an oxygen alright so we need to draw in an oxygen next so now we have our oxygen notice the carbon in red now has an octet of electrons around it the oxygen is bonded to another ch3 group so the oxygen is bonded to another ch3 let's draw that in so we have our ch3 and since we're doing a partially condensed I won't draw in those bonds we have ch3 like that I could put in my lone pairs of electrons on the oxygen to give the oxygen and octet of electrons and now we have our partially condensed structure if you want to expand it even more and draw the full Lewis structure again we start with the carbon in red so here's the carbon in red and that carbon is bonded to another carbon and this carbon is bonded to three hydrogen's so I draw on those three hydrogen's so this ch3 group that I just drew right is this one alright next we have a ch3 group on the left side so I need to draw in a ch3 on the left hopefully I have enough room to do that I'll squeeze it in here so we have our hydrogen's and that's our second ch3 group so let me circle it in in green here so here's a ch3 and then finally we have we have let me make this blue down here we have another ch3 group so I'll draw that one in so we have another ch3 we'll make room for all these hydrogen's here and that's the one in blue so we're drawing out all of the bonds now in our full lewis structure next we have an oxygen so we have an oxygen right in here with two lone pairs of electrons on the oxygen the oxygen is bonded to another ch3 so let me let me let me pick a color here for that one so we have another ch3 on the right and let's draw it in there's a carbon with three bonds to hydrogen so that's our last ch3 group so let me circle it this one right here is this one so that's an important skill being able to go from a condensed to a partially condensed to a full Lewis dot structure and also going the opposite direction going from Lewis to partially condensed and finally to condensed and usually you'll only see these used for small molecules right it's obviously it's obviously easy to work with when you have small molecules when you have large molecules this doesn't work very well and you'll see almost exclusively bond line structures you use to represent larger organic molecules and we'll look at those in the next video