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Worked example: Lewis diagram of formaldehyde (CH₂O)

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Video transcript

what we're going to do in this video is get a little bit more practice constructing Lewis diagrams and in particular we're going to try to construct the Lewis diagram for formaldehyde formaldehyde has one carbon two hydrogen's and an oxygen ch2o so pause this video and have a go at it try to construct a valid lewis structure or Lewis diagram for formaldehyde all right now let's do this together now the first step and we saw this in a previous video we want to think about all of the valence electrons for this molecule so we want to account account for valence electrons now the reason why we want to do that is so that while we're trying to create this structure we are making use of all of the valence electrons and to figure out how many total valence electrons we have we can look at a periodic table of elements we can see that carbon it's in that second row in that second period so its second shell is its outer shell and in that shell it has 1 2 3 4 valence electrons so carbon has 4 valence electrons a neutral free hydrogen atom is going to have one valence electron but we have two of them here so it's going to be 2 times 1 and then oxygen it also is in the second period and in its second shell it has 1 2 3 4 5 6 valence electrons and so the total valence electrons in this molecule are going to be 4 plus 2 which is 6 plus 6 which is equal to 12 valence electrons now the next step is to try to draw structure try to draw draw single bonds I'll say single bonds and a key question is what do we think is going to be our central atom and the rule of thumb is the least electronegative atom that is not hydrogen is a good candidate for our central atom so we can rule out hydrogen so between carbon and oxygen we know that oxygen is one of the most electronegative atoms more one of the most electronegative elements on the periodic table of elements it's very close to fluorine and so carbon is a good for the central atom so let's put the carbon right over here and then let's put these other atoms around it let's we could call them terminal atoms so let's put our oxygen right over there and then we have two hydrogen's hydrogen there a hydrogen there and let me draw the bonds so that's a single bond that accounts for two valence electrons that accounts for two valence electrons that accounts for two valence electrons so I've just used two four six valence electrons so if I subtract six valence electrons I am now left with six valence electrons six valence electrons so the next step is allocate the remaining valence electrons trying to get to the octet rule for atoms that are not hydrogen and then for hydrogen trying to get it to have two valence electrons so allocate allocate remaining remaining valence electrons alright so let's start with this oxygen this oxygen already has these two electrons that it's sharing hanging around so in order to get to the octet rule it needs six more so let's give it six electrons so one two three four five six well I've just used up the remaining six valence electrons so I don't really have any more to play with but let's see how the other atoms are feeling so hydrogen here it's able to share these two electrons that are in this covalent bond so it's feeling good it can kind of pretend that it has a full outer shell because its outer shell is just that one that first shell that's filled with two electrons same thing for this other hydrogen so at least the terminal atoms the oxygen and the two hydrogen's are feeling like they have a full outer shell but in the fourth step we're going to look at our central atom so let's focus on the central atom central atom and do we need more bonds or do we need to do something interesting here and what we see is that carbon it's able to have two four six electrons hanging around it but it would love to have 8 carbon would love to have a full outer shell so how could we do that well we could add more bonds work of the bonds come from well it would come from some lone pair of electrons well the only lone pairs of electrons are hanging around this oxygen so what if we were to take say this lone pair of electrons and then construct another covalent bond with that then our Lewis diagram will look like this I will actually redraw it so you have a carbon you have your three original covalent bonds you had a hydrogen a hydrogen and then you had your oxygen right over here and now we've formed a new covalent bond just like this and then you have these two other lone pairs around the oxygen so let me draw that so two and then another two around the oxygen and this is looking pretty good because the oxygen it still has eight electrons hanging around for in lone pairs and then for the in this double bond that it is sharing the hydrogens still have two electrons hanging around they're able to share the electrons in each of these covalent bonds and now the carbon is participating in you could think of it as four covalent bonds two single bonds and one double bond so each of those have two electrons associated with it so it has eight electrons hanging around so this is looking really good as a legitimate lewis structure or a Lewis diagram for formaldehyde