Resonance | Molecular and ionic compound structure and properties | AP Chemistry | Khan Academy

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 the Lewis structure for a nitrate anion.

All right, well, we've done this many times. The first step is to just account for the valence electrons. Nitrogen has one, two, three, four, five valence electrons in its outer shell. In that second shell, if it's a neutral free nitrogen atom, so we have five valence electrons there. Oxygen has one, two, three, four, five, six valence electrons. But if you have three oxygens, you're going to have six times three.

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 one 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. 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 nitrogen in the center right over there and around it, let's put three oxygens—one, two, three oxygens. Let's put a single bond between them.

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 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 octet. So, let's do that. 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 two, four, six 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 another covalent bond. So, let's do that.

Let me just erase this pair right over here, and I'm just going to turn that into another covalent bond. This is looking pretty good. We have eight valence electrons around each of the oxygens, and now we have eight for the nitrogen—two, four, six, eight.

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.

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. 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.

An interesting question is: why is that? 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.

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. 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. Another valid Lewis structure could look like this. So, nitrogen bonded to 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.

Whenever we see a situation where we have three valid Lewis structures, we call this resonance. Resonance, resonance, resonance! We'll put an arrow—these two-way arrows—between these structures. When you hear the word resonance, it sometimes conjures up this image that you're bouncing back; you're resonating between these structures.

But that's actually not right. The right way to think about it is these different ways of visualizing the nitrate contribute to a resonance hybrid, which is really the true way that the nitrate exists.

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. Then you would show the bond between nitrogen and each of these oxygens as a hybrid between someplace between a single bond and a double bond.

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—that you actually have three bonds that are someplace in between a single 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 an anion. So, this is the idea of resonance: you have multiple valid Lewis structures—they 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, 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.