Resonance Chemistry

What is resonance?

Some molecules can't be accurately described by just one Lewis diagram. Take ozone, O₃, for example. Let's draw its Lewis structure, using the following steps:

1. Work out the molecule's total number of valence electrons.
2. Draw the rough position of the atoms in the molecule.
3. Join the atoms using single covalent bonds.
4. Add electrons to the outer atoms until they have full outer shells of electrons.
5. Count up how many electrons you have added, and subtract this from the molecule's total number of valence electrons that you calculated earlier. This tells you how many electrons you have left.
6. Add the remaining electrons to the central atom.
7. Use lone pairs of electrons from the outer atoms to form double covalent bonds with the central atom until all atoms have complete outer shells.

First of all, oxygen is in group VI and so each atom has six valence electrons. This means that the molecule has 3(6) = 18 valence electrons.

Next, let's draw a rough version of the molecule. It consists of three oxygen atoms. We'll connect them using single covalent bonds.

Resonance in ozone. StudySmarter Originals

Add electrons to the outer two oxygen atoms until they have full outer shells. In this case, we add six electrons to each.

Resonance in ozone. StudySmarter Originals

Count up how many electrons you have added. There are two bonded pairs and six lone pairs, giving 2(2) + 6(2) = 16 electrons. We know ozone has 18 valence electrons. We, therefore, have two remaining to add to the central oxygen atom.

Resonance in ozone. StudySmarter Originals

We've now reached 18 valence electrons - we can't add any more. But oxygen still doesn't have a full outer shell - it needs two more electrons. To solve this issue, we use a lone pair of electrons from one of the outer oxygen atoms to form a double bond between itself and the central oxygen. But which outer oxygen forms the double bond? It could involve either the oxygen on the left, or the oxygen on the right. In fact, both options are equally likely. These two options have the same arrangement of atoms but a different distribution of electrons. We call them resonance structures.

Resonance in ozone. StudySmarter Originals

However, there is a problem. The two resonance structures above imply that the bonds in ozone, one double and one single, are different. We'd expect the double bond to be much shorter and stronger than the single bond. But chemical analysis tells us that the bonds in ozone are equal, meaning ozone doesn't take the form of either of the resonance structures. In fact, instead of being found as one resonance structure or the other, ozone takes on what is known as a hybrid structure. This is a structure somewhere between both of the resonance structures and is shown using a double-headed arrow. Instead of containing one single bond and one double bond, it contains two intermediate bonds that are an average of the single and the double bond. In fact, you can think of them as one-and-a-half bonds.

Resonance in ozone, including its hybrid structure. StudySmarter Originals

Here's a summary of what we've learned so far:

• Some molecules can be represented by multiple alternative Lewis structures with the same arrangement of atoms but a different distribution of electrons. These molecules show resonance.
• The alternative Lewis structures are known as resonance structures. They combine to make a hybrid molecule. The overall hybrid molecule does not switch between each structure but rather takes on a whole new identity that is a combination of all of them.

How do you draw resonance structures?

We've already learned that when you want to represent a molecule that shows resonance, you draw all of its resonance structures as Lewis diagrams with double-headed arrows between them. You might also want to add curly arrows to show the movement of electrons as the molecule 'switches' from one resonance structure to another. Let's see how this applies to ozone, O₃.

Electron movement in resonance. StudySmarter Originals

To get from the resonance structure on the left to the resonance structure on the right, a lone pair of electrons from the oxygen atom on the left is used to create an O=O double bond. At the same time, the original O=O double bond found between the central oxygen and the oxygen atom on the right is broken and the electron pair is transferred over to the oxygen atom on the right. To get from the resonance structure on the right to the resonance structure on the left, you do the reverse.

However, these diagrams can be misleading. They imply that molecules that show resonance spend some of their time as one resonance structure and some of their time as the other. We know that this isn't the case. Instead, molecules that show resonance take the form of a hybrid molecule: a unique structure that is an average of all of the molecule's resonance structures. Resonance structures are simply our way of trying to represent such a molecule and shouldn't be taken too literally.

Resonance structure and dominance

In some examples of resonance, the multiple resonance structures contribute equally to the overall hybrid structure. For example, earlier we looked at ozone. It can be described using two resonance structures. The overall hybrid structure is a perfect average of the two. However, in some cases, one structure has more influence than the others. We say that this structure is dominant. The dominant structure is determined using formal charges.

In general, we assume that the Lewis structure with formal charges closest to zero is the dominant structure. If two resonance structures both have equivalent formal charges, we assume that the Lewis structure with the negative formal charge on the more electronegative atom is the dominant structure.

Take a look at the three possible resonance structures of carbon dioxide, shown below. In two of the structures, shown in the middle and on the right, one of the oxygen atoms has a formal charge of +1 and the other has a formal charge of -1. In the other resonance structure, shown on the left, all atoms have a formal charge of +0. This is therefore the dominant structure.

Dominant structure in resonance. StudySmarter Originals

But if all of the resonance structures have the same formal charges, we say that they are equivalent. This is the case for ozone. In both of its resonance structures, there is one oxygen atom with a formal charge of +1, one with a formal charge of -1, and one with a formal charge of +0. These two structures contribute equally to the hybrid structure of ozone.

Equivalent structures in resonance. StudySmarter Originals

We'll say it again: it is important to note that ozone doesn't switch between one resonance structure and the other. Instead, it takes on a completely new identity that is somewhere in between the two. Just like pizzly bears are not sometimes polar bears and sometimes grizzlies, but rather a mixture of both species, ozone isn't sometimes one resonance structure and sometimes the other. You must combine both structures to form something else altogether. We say molecules that can't be represented by just one Lewis structure show resonance.

Rules of resonance

We can put together what we've learned so far to make up some rules of resonance:

1. Molecules that show resonance are represented by multiple resonance structures. These must all be feasible Lewis structures.
2. Resonance structures have the same layout of atoms but different arrangements of electrons.
3. Resonance structures differ only in the position of their pi bonds. All sigma bonds remain unchanged.
4. Resonance structures contribute to one overall hybrid molecule. Not all resonance structures contribute equally to the hybrid molecule; the more dominant structure is the one with formal charges closest to +0.

Examples of resonance

To round this article up, let's look at some further examples of resonance. First up: the nitrate ion, NO₃^-. It consists of three oxygen atoms bonded to a central nitrogen atom and has three equivalent resonance structures, which differ in their position of the N=O double bond. The N-O bond order of the resulting hybrid molecule is 1.33.

Resonance in the nitrate ion. StudySmarter Originals

Another common example of resonance is benzene, C₆H₆. Benzene consists of a ring of carbon atoms, each bonded to two other carbon atoms and one hydrogen atom. It has two resonance structures; the resulting C-C bond has a bond order of 1.5.

Resonance in benzene. commons.wikimedia.org

Finally, here's the carbonate ion, CO₃^2-. Like the nitrate ion, it has three resonance structures and the C-O bond order is 1.33.

Resonance in the carbonate ion. commons.wikimedia.org

We've reached the end of this article on resonance in chemistry. By now, you should understand what resonance is and be able to explain how resonance structures contribute to an overall hybrid molecule. You should also be able to draw resonance structures for specific molecules, determine the dominant resonance structure using formal charges and calculate bond order in resonance hybrid molecules.