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Resonance Chemistry

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Resonance Chemistry

Pizzly bears are a rare hybrid animal, a cross between a polar bear and a grizzly bear. They've been successfully bred in captivity for years and have also been found in the wild: the first sighting of a wild pizzly was confirmed in 2006. But although pizzly bears are made up of two different species of bear, polar and grizzly, they are their own unique organism. You don't see them as sometimes a polar bear and sometimes a grizzly. Instead, they are a completely different bear. This is similar to resonance structures in chemistry.

Resonance is a way of describing bonding in chemistry. It describes how several equivalent Lewis structures contribute to one overall hybrid molecule.

  • This article is about resonance in chemistry.
  • We'll look at an example of resonance before discovering how to draw resonance structures.
  • We'll then explore dominance in resonance and look at bond order calculations.
  • After that, we'll use our knowledge to create some resonance rules.
  • We'll finish with some further examples of resonance.

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.

This is just a quick summary of how to draw a Lewis structure. For a more detailed look, check out the article "Lewis Structures".

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 ozone Lewis structure StudySmarterResonance 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 ozone Lewis structure StudySmarterResonance 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 ozone Lewis structure StudySmarterResonance 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 ozone Lewis structure StudySmarterResonance 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 Resonance in ozone and its hybrid structure StudySmarterResonance in ozone, including its hybrid structure. StudySmarter Originals

Resonance structures always involve a double bond. The only difference between the multiple resonance structures is the position of this double bond.

The causes of resonance

Resonance is caused by pi bonding. You might know that single bonds are always sigma bonds. They're formed by the head-on overlapping of atomic orbitals, such as s, p or sp hybrid orbitals. In contrast, pi bonds are formed by the sideways overlapping of p orbitals. But when it comes to molecules that show resonance, instead of occurring between just two atoms, you find pi bonding across multiple atoms in the structure. Their p orbitals merge into one large overlapping region. The electrons from these orbitals spread out over the overlapping region and don't belong to any one specific atom. We say that they are delocalized. When a molecule delocalizes its electrons, it decreases its electron density, which helps it become more stable.

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.

Resonance ozone electron movement in resonance structure StudySmarterElectron 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.

Formal charges are charges assigned to atoms, assuming that all the bonded electrons are split evenly between the two bonded atoms.

We have a whole article dedicated to formal charges, where you can find out how to calculate them for all sorts of molecules. Head over to "Formal Charges" for more.

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.

Resonance dominant structure in resonance StudySmarterDominant 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.

Resonance equivalent structures in resonance StudySmarterEquivalent 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.

Resonance is a way of describing bonding in chemistry. It describes how several equivalent Lewis structures contribute to one overall hybrid molecule.

Resonance and bond order calculations

Bond order tells you about the number of bonds between two atoms in a molecule. For example, a single bond has a bond order of 1 and a double bond has a bond order of 2. Here's how you calculate the bond order of a particular bond in a hybrid molecule:

  1. Draw out all of the molecule's resonance structures.
  2. Work out the bond order of your chosen bond in each of the resonance structures and add these together.
  3. Divide your total bond number by the number of resonance structures.

For example, let's try and find the bond order of the leftmost O-O bond in ozone, shown above. This bond in the left-hand resonance structure has a bond order of 1, whilst in the right-hand resonance structure, it has a bond order of 2. The overall bond order is therefore .

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 nitrate ion lewis diagram StudySmarterResonance 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 Resonance in benzene StudySmarterResonance in benzene. commons.wikimedia.org

Finally, here's the carbonate ion, CO32-. Like the nitrate ion, it has three resonance structures and the C-O bond order is 1.33.

Resonance Resonance in carbonate ion StudySmarterResonance 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.

Resonance Chemistry - Key takeaways

  • Some molecules can be described by multiple Lewis diagrams which contribute to one overall hybrid molecule. This is known as resonance.

  • Hybrid molecules are unique molecules. They are an average of all the different resonance structures of a molecule.

  • Not all resonance structures contribute equally to the overall structure of a molecule. The resonance structure with the most effect is known as the dominant structure. Resonance structures with equal effect are known as equivalent.

  • To calculate the bond order in hybrid molecules with equivalent resonance structures, add up the bond orders across all of the structures and divide by the number of structures.

Frequently Asked Questions about Resonance Chemistry

Resonance is a way of describing bonding in chemistry. It describes how several equivalent Lewis structures contribute to one overall hybrid molecule.

A resonance structure is one of multiple Lewis diagrams for the same molecule. Overall, they show the bonding within the molecule.

Resonance is caused by the overlapping of multiple p orbitals. This is part of a pi bond and forms one large merged region, which helps the molecule spread out its electron density and become more stable. The electrons aren't associated with any one atom and are instead delocalized. 

There are a few rules when it comes to resonance in chemistry:

  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 their position of 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 molecules that show resonance are ozone, the nitrate ion and benzene.

Final Resonance Chemistry Quiz

Question

What is resonance?

Show answer

Answer

Resonance is a way of describing bonding in chemistry. It describes how several equivalent Lewis structures contribute to one overall hybrid molecule.

Show question

Question

Compare isomerism and resonance.

Show answer

Answer

Isomers differ in their arrangement of atoms. Resonance structures have the same arrangement of atoms but different arrangements of electrons.

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Question

True or false? Molecules that show resonance spend half of their time as one resonance structure and half of their time as the other.

Show answer

Answer

False

Show question

Question

True or false? All of the bonds in ozone are equal.

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Answer

True

Show question

Question

Explain why ozone has no dominant resonance structure.

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Answer

The resonance structures of ozone have the same formal charges, meaning that they are equivalent. Therefore, they contribute equally to the overall hybrid molecule.

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Question

Which of the following symbols is used to show resonance?

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Answer

A double headed arrow

Show question

Question

True or false? Resonance is caused by delocalized pi bond electrons.

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Answer

True

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Question

How do formal charges relate to dominance in resonance?

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Answer

The Lewis structure with formal charges closest to zero is the more dominant structure.

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Question

A molecule has two resonance structures. Structure A has a formal charge of -1 on a sulfur atom and +0 on a nitrogen atom. Structure B has a formal charge of +0 on the sulfur atom and -1 on the nitrogen atom. All other formal charges are the same. Which is the dominant structure?

Show answer

Answer

Structure B

Show question

Question

A molecule has two resonance structures. Structure A has a formal charge of +0 on both carbon atoms, whilst structure B has a formal charge of +1 on one of the carbon atoms and -1 on the other. All other formal charges are the same. Which is the dominant structure?

Show answer

Answer

Structure A

Show question

Question

True or false? Resonance structures have different arrangements of sigma bonds.

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Answer

False

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