You are no doubt an expert at drawing out the structures for simple molecules like those shown below. In these molecules there are covalent bonds between the individual atoms and as you would expect these covalent bonds involve the sharing of a pair of electrons. The electrons in these covalent bonds are held firmly in place between nuclei of the two atoms involved in the bond.

simple molecules which do not undergo resonance

However there are other molecules such as benzene which cannot be accurately drawn using one single structure like that for methane, water or carbon dioxide shown above. The bonding electrons in benzene are not held in place between two nuclei as in the simple molecules above but they are delocalised. These delocalised electrons are found in a variety of molecules, not just benzene. Delocalised electrons are generally pi(π) electrons that are free to move over more than two atoms. This leads to a bit of a problem! How do we draw the structure of benzene or any other molecule with delocalised electrons if it is not possible to draw the position of the bonds holding the molecule together?

In a molecule such as ethene which contain one carbon carbon double covalent bond (C=C). The double covalent bond between the carbon atoms consists of a sigma bond formed by the complete overlap of atomic orbitals and a pi(π) bond formed by the partial overlap of p-orbitals on the carbon atoms. The two electrons in each of the sigma and the pi(π) bond are held firmly between two carbon atoms. This makes it easy to draw out a molecular structure like the one above, we can easily show where the valency electrons are in the molecule by simply looking at the positions of the covalent bonds. However in a molecule such as benzene the pi(π) electrons are spread over the whole molecule, that is to say they are delocalised in clouds of electron density above and below the flat planar ring of carbon atoms, this is shown below.

structure of benzene

To try and draw the structure of a benzene molecule we often draw out two separate structures, as shown below. However none of these structures represents the actual structure of a benzene molecule. The best that we can say is that the actual structure of a benzene molecule is a combination of the two structures shown below. The two structures shown below for benzene were proposed by Kekulé, he suggested, incorrectly that the two structures of benzene shifted back and forth between the two so that it was impossible to isolate any one particular structure. However his idea was wrong and we now know that the actual structure is a combination of both structures, we would say that the two structures are in resonance with each other.

It is important to be aware that it is due to the limitations in the method we use to draw out the structures of molecules that stops us accurately drawing the real structure of benzene. So how would you describe the structure of a benzene molecule? Well you would have to say that its actual structure is a combination or a composite of the two different resonance forms. It is worth mentioning that the only difference between these two resonance forms is in the position of the C=C. The only problem is that all the carbon-carbon bond lengths in a benzene molecule are all the same length and are intermediate in length between carbon carbon single bond and carbon carbon double bonds in length, so the C=C and C-C bonds in the molecules shown below do not actually exist!

resonance hybrid structures for benzene

It is not only in benzene that we encounter the problem of accurately drawing the structure of a particular molecule or ion. If it is possible to describe the structure of a substance in such a way that the only difference between them is in the position of the electrons, usually the pi(π) electrons then the actual structure will be a composite or hybrid of the separate resonance structures. We would say that the actual structure is a resonance hybrid of all the separate structures. Resonance is the movement or delocalisation of electrons, usually in pi bonds, that is bonds which result from the partial overlap of p-orbitals, within molecules and ions. The nuclei of the atoms involved in resonance do not move.

The carbonate ion (CO32-)

The carbonate ion (CO32-) is another example of a molecule that cannot be drawn out using a single structure. Since carbon atoms make four bonds and oxygen atoms form two covalent bonds we might initially try to draw out the structure of the carbonate ion as a molecule containing two C-O and one C=O, with two of the oxygen forming and ion with a negative charge as shown below. However all the C-O bond lengths in the carbonate ion are the same length, so it cannot consist of single C-O bonds and a C=O bond. The actual structure of the carbonate ion will be a composite of the three resonance structure shown below:

resonance hybrid structures for the carbonate ion

To move from one resonance form to another it is simply a matter of moving an electron pair. In the diagram below the red arrows show the movement of a pair of electrons as one resonance form is changed into another in the forward direction. However you should bear in mind that these electron shifts DO NOT actually take place, remember that the electrons in the ACTUAL carbonate ion are delocalised. The drawing of these resonance structures is simply a limitation of the method used to draw the structures of these ions and molecules.

carbonate ion is a resonance stabilised ion

Tips on drawing out resonance structures

If you are unsure on how to start drawing resonance structures for molecules or ions a good place to begin is with drawing out Lewis structures. A Lewis structure should show the positions of the valency electrons within the molecule or ion. So for the carbonate ion we have:

How to draw resonance structures for the carbonate ion

Aromatic amines

As another example consider aromatic amines. Aliphatic amines and ammonia are good bases due to the presence of the lone pair of electrons on the nitrogen atom which is able to form dative covalent bonds with hydrogen ion (H+) in acids. As a simple example consider ammonia (NH3) which can use it lone pair of electrons to form a dative covalent bond to a hydrogen ion, as shown below:

ammonia is a good base because it can use its lone pair to form a dative bond with a hydrogen ion from an acid

However aromatic amines, such as phenylamine or aniline are poor bases. In an aromatic amine the nitrogen atom is bonded directly to an aromatic ring, this means that the lone pair of electrons on the nitrogen atom can be delocalised through the aromatic ring and so are not readily available to form dative covalent bonds in the same way aliphatic amines can. This is outlined in the diagram below:

aromatic amines are poor bases because the nitrogen lone pair are 
deloclaised through the aromatic ring

The delocalisation of the pi(π) electrons will lead to the same problems we had above when trying to draw the structure of a benzene molecule. Delocalisation implies that the electrons are not in fixed positions so trying to draw molecular structures and bonds will not be possible. Instead we have to try and show the structure of aniline by drawing a series of resonance structures, shown below. Recall that the actual structure of aniline will be a mix or composite of all the individual resonance structures.

model showing the delocalised electrons in a benzene ring

Key Points

Practice questions

Check your understanding - Questions resonance