Chemistry: Resonance Structures

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Let me recap for you. We've used Lewis Dot Structures in order to make some predictions about molecules. All we needed to know was the number of valence electrons, and then right now you still need to be told how the atoms are connected together, but it allowed us to make some predictions. For instance, we predicted that Oxygen, O[2], should have a double bond, and Nitrogen, N[2], should have a triple bond. We really haven't talked about the ramifications of that except to say that double bonds are typically shorter and stronger than single bonds, and triple bonds are typically yet shorter still and stronger still. But there's a weakness to the model, and the weakness is exposed quite clearly when we look at the Ozone molecule, so let's take a look.
Here's I've written a Lewis Dot Structure for Ozone, and you can convince yourself that it satisfies the Octet Rule for all atoms. It's a perfectly good Lewis Dot Structure. What it suggests is that there should be a single bond between the left-hand Oxygen and the center Oxygen, and a double bond between the right-hand Oxygen and the center Oxygen. And based on this idea that double bonds should be shorter and stronger, we predict that the Ozone molecule should look like this, where we have a short bond here--and maybe I exaggerated it a little, but a short bond between the right-hand Oxygen and the center Oxygen, and a longer bond between the left-hand Oxygen and the center Oxygen. And it's going to turn out that in fact, the Oxygen lives right in the middle, so this picture doesn't seem to be enough to predict what reality is. And remember that's what this Lewis model is all about, is that we're trying to come up with a framework in which we can make some predictions so we can understand something more about molecules.
But there's another issue, and the other issue is that I drew the red Lewis Dot Structure and it looks just like the purple one here. But suppose I came back the next day and, forgetting what I had written, I wrote this Lewis Dot Structure. What's different about it? Well, the Oxygens are in exactly the same places but instead of putting the double bond between the right-hand Oxygen and the center Oxygen, I put the double bond between the left-hand Oxygen and the center Oxygen. This is still a perfectly valid Lewis Dot Structure. It satisfies the Octet Rule for all atoms. But it's clearly not exactly the same in the sense that we'd make the opposite geometry prediction, for one, that the double bond should be between the left-hand Oxygen and the center Oxygen so that this molecule would be distorted but in the other direction. And as the old saying goes, the truth lies somewhere in-between the two; that is that both of these contribute to the overall picture. And that's what we call a "resonance hybrid." In other words, a resonance hybrid is the combination of two resonance structures that together complete the picture, or at least complete more of the picture of what's going on in the molecule.
So what are resonance structures? Resonance structures are Lewis Dot Structures in which we don't move the nuclei but we put the electrons in differently such that it still satisfies the rules about octet, but the distribution of electrons is a little bit different. And as an aside, these both actually contribute equally. We're going to see some examples where Lewis Dot Structures are not exactly equivalent and so they don't contribute equally. But before we do that, let's look at some more examples.
The first example I want to show you is the Nitrate anion, which is in red right here. Nitrate, you know, is used in fertilizers. Ammonium Nitrate is a fertilizer. And what we have is the double bond starting out between the right-hand Oxygen and the Nitrogen. Here's the double bond between the left-hand Oxygen and the Nitrogen, and here's the double bond between the top Oxygen and the Nitrogen. Now, there's no actual distinction between left, right and top, but for ease of conversation I'm going to refer to left, right and top.
Now, each of these is equivalent to each other. So in other words, there are three equally contributing resonance structures, but we write down this double-headed arrow to indicate that these are resonance structures. And you can take a closer look and convince yourself that the Octet Rule is satisfied for all the atoms. To simplify it because we have all these dots, I've also written it down in the shorthand notation, and here's the shorthand notation. And then you can really see like this that we've got the double bond between the Nitrogen and the right Oxygen; here's the left and here it is on top.
Let's look at Acetate anion. Acetate is an organic molecule. It's the conjugate base of Acetic Acid, which is vinegar, and you're going to encounter that later on. We have a double bond between the Carbon and this Oxygen, or alternatively the double bond between the Carbon and this Oxygen. Again, we're not moving any of the nuclei around. It's that we're moving the electrons around but still satisfying the Octet Rule. And again, in the shorthand notation, which I have here, we're looking at the Carbon-Oxygen double bond here versus the Carbon-Oxygen double bond here.
One final example on this page, Sulfur Dioxide--and remember, the Lewis Dot Structure for Sulfur Dioxide is related to that for Ozone because Sulfur and Oxygen are in the same column in the Periodic Table, so this Lewis Dot Structure ought to look really familiar to you based on the Ozone one that we showed you on the previous page--but remember that Sulfur, being in the third row in the Periodic Table, we're allowed to expand octets. And if we're allowed to expand the octet, then we can have something like this as a resonance structure, where the Sulfur has 10 electrons. Oxygen still has eight because Oxygen, being in the second row, has to satisfy the Octet Rule. But Sulfur can have 10, and so we have this kind of resonance structure as well.
Now, it ought to be clear that these are somehow equivalent to each other, and these three are somehow equivalent to each other. And these two are equivalent to each other but they're, in some fundamental way, different from the third. So in other words, the two blues are equal contributors to the overall picture of what's going on, but the purple is different from the blues. It contributes--and we don't understand how much it contributes, but we need it in order to complete the picture.
Now, an analogy that I realize I just left out is the idea of a centaur being a resonance hybrid of a man and a horse. If you've ever seen "Xena, Warrior Princess" on television, there are centaurs. They were sort of characters in Greek myths that from the waist-up were men and then the back end was a horse, so four legs and a tail. And we call it a resonance hybrid because it isn't really a horse and it isn't really a man but it has characteristics and aspects of both, and that's why we say that it's a hybrid.
So, when I was finishing with SO[2], I was saying that purple doesn't contribute the same as blue, and that gets us to the idea that we can have resonance structures that are inequivalent. So here's Nitrous Oxide. You may have been to the dentist before and the dentist offers you laughing gas. That's Nitrous Oxide, and it looks like this. And again, we have three resonance structures here. They all satisfy the Octet Rule. And they're inequivalent because the molecule is not symmetrical, so we don't know which one of these contributes. Maybe they all contribute. Maybe one is more important than the other. We don't have the tools yet to do that. We're going to see that later on.
And similarly we have Fulminate anion. Fulminate of Mercury or Mercury Fulminate is the material in blasting caps, so this is a very unstable molecule. But we can still write resonance structures for it that satisfy the Octet Rule. And then we have Cyanate anion here. You can take your time and look these over and convince yourself that they satisfy the Octet Rule and that the bond orders, or the places where the single bonds and double bonds and triple bonds are, are moving about in these structures.
Now, one last point I wanted to make is that you have to be clear that you're not allowed to move the atoms around when you're talking about resonance structures. So we have Fulminate and Cyanate here, and they both consist of Nitrogen, Carbon and Oxygen. But here Oxygen is connected to Nitrogen, and here Carbon is connected to Oxygen, so these are not the same molecule. These are entirely different molecules or entirely different ions. So they're not resonance structures of each other. So in other words, we wouldn't put the double-headed arrow here as well. We have double-headed arrows here and here because these are resonance structures of each other, and then we have double-headed arrows here because these are resonance structures of each other, but between the blue and the green these are entirely different molecules. So you're not allowed to move atoms around when you're creating new resonance structures. Leave the atoms where they are; move the dots, move the electrons.
Okay, so what are resonance structures? Resonance structures are different ways that we can put the electrons into a Lewis Dot Structure. They still satisfy the Octet Rule or, in the case of beyond the third row, you can start talking about expanded octets. But we don't move the nuclei around. And the reason why we need resonance structures is because we need them to complete the picture. In other words, sometimes one Lewis Structure just isn't enough to complete the picture of what's going on.
Chemical Bonding: Fundamental Concepts
Resonance Structures and Formal Charge
Resonance Structures Page [1 of 2]