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Video transcript

let's see if we can draw the Lewis diagram for a nitrate anion so a nitrate anion has one nitrogen and three oxygens and it has a negative charge I'll do that in another color it has a negative charge so pause this video and see if you can draw that the Lewis structure for a nitrate anion alright well we've done this many times the first step is to just account for the valence electrons nitrogen has 1 2 3 4 5 valence electrons in its outer shell and in that second shell if it's a neutral free nitrogen atom so we have 5 valence electrons there oxygen has 1 2 3 4 5 6 valence electrons but if you have three oxygens you're going to have 6 times 3 and so if you just add up the valence electrons if these were free neutral atoms you would get 5 plus 18 which is 23 valence electrons now the next thing we have to keep in mind is this is an anion this has a negative 1 charge right over here so it's going to have one more extra electron one more extra valence electron than you would expect if these were just free atoms that were neutral so let's add one valence electron here so that gets us to 24 valence electrons and then the next step is let's try to actually draw this structure and the way we do it is we try to pick the least electronegative atom that is not hydrogen to be the central atom in this case it is nitrogen it's to the left of oxygen in that second period so let's put a nitrogen in the center right over there and around it let's put three oxygens so one two three oxygens let's put a single bond between them and so so far we and let me do that in another color so we can account for it better so I'll do them in purple so so far we have accounted for two four six valence electrons so minus six valence electrons gets us to 18 valence electrons the next step is we would try to allocate as many of these as possible to our terminal atoms the oxygens over here try to get them to a full Tett so let's do that this each of these oxygens they're already participating in one of these covalent bonds so they already have two valence electrons hanging out so let's see if we can give them each another six to get to eight so two four six two four six and then two four and six and so just like that we have allocated 18 valence electrons 6 12 18 so minus 18 valence electrons and we are now left with no further valence electrons to allocate but let's see how our atoms are doing we know the oxygens have a full octet but the nitrogen only has 2 4 6 valence electrons hanging around it would be great if there was a Lewis structure where we could have eight valence electrons for that nitrogen well one way to do that is to take one of these lone pairs from one of the oxygens and turn that into a another covalent bond so let's do that so let me just erase this pair right over here and I'm just going to turn that into another covalent bond and this is looking pretty good we have eight valence electrons around each of the oxygens and now we have eight for the nitrogen 2 4 6 8 and we have to remind ourselves that this is an anion it has a negative one charge so to finish the Lewis diagram we would just put that negative charge there and this is all well and good but if this was the only way that nitrate existed when we observed nitrate anions in the world we would expect to see one shorter bond and two longer bonds and we would expect one of the bonds to have a different energy than the other two but in the real world we don't see that we see that all of the bonds actually have the same length and they actually have the same energy and so the interesting question is why is that and one thing that you might appreciate is when I took that lone pair to create this covalent bond I could have done it with that top oxygen I could have done it with this bottom-left oxygen or I could have done it with that bottom-right oxygen and so there's actually three valid Lewis structures that we could have had not only could we have had this Lewis structure we could have had this one and I'll draw it all in yellow to save us some time where you have this nitrogen as a single bond with that top oxygen and so that top oxygen still has six electrons in lone pairs and maybe it forms a double bond with the bottom left oxygen so this bottom left oxygen only has two lone pairs one of them would have gone to form the double bond and then this oxygen would look the same so what I am drawing here is another valid Lewis structure or the double bond might have formed with this bottom right oxygen so let me draw that so another valid Lewis structure could look like this so nitrogen bonded that oxygen has three lone pairs this oxygen also has three lone pairs and now this one has the double bond and only has two lone pairs and whenever we see a situation where we have three valid Lewis structures we call this resonance resonance resonance and we'll put an arrow these two-way arrows between these structures and when you hear the word residence it sometimes conjures up this image that you're bouncing back you're resonating between these structures but that's actually not right what the right way to think about it is these different ways of visualizing the nitrate these contribute to a resonance hybrid which is really the true way that the nitrate exists and so if we wanted to draw a resonance hybrid it would look like this you have the nitrogen in the center you have your oxygens one two three I can draw our first covalent bond like that and then you would show the bond between nitrogen each of these oxygens are a hybrid between someplace between a single bond and a double bond and so instead of just one of them having the double bond and the other two having single bonds they're all somewhere in between so maybe you draw a dotted line something like that to show what the reality is is that you actually have three bonds that are someplace in between a sing and a double-bond because the electrons in this molecule are delocalized throughout and of course you want to make sure you always want to make sure that people recognize that this is a anion so this is the idea of residents you have multiple valid Lewis structures they are all contribute to a resonance hybrid which is actually what we observe we're not just bouncing between these different structures the actual observation will be a hybrid of the three now what we just drew here these three are all equivalent but in certain cases and we'll see this in future videos you don't have equivalent structures and some of them might contribute more to the resonance hybrid than others but we'll see that in future videos