# Resonance Chemistry

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Resonance is a way of explaining how bonding works in chemistry. It's kind of like when a few different Lewis structures come together to create one super cool molecule. In this article, we're going to talk all about resonance in chemistry. We'll start by looking at an example of resonance, and then we'll show you how to draw resonance structures. After that, we'll talk about something called dominance in resonance and how to calculate bond order. Finally, we'll give you some rules to help you understand resonance even better. So let's get started!

## 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:

Work out the molecule's total number of valence electrons. Draw the rough position of the atoms in the molecule. Join the atoms using single covalent bonds. Add electrons to the outer atoms until they have full outer shells of electrons. 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. Add the remaining electrons to the central atom. 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.

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

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.

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.

Ozone, or O3, has two major resonance structures that contribute equally to the overall hybrid structure of the molecule. The total number of valence electrons the ozone molecule has is equal to 18 - 6 electrons from each oxygen atom. The two possible Lewis structures that can be drawn for ozone are. 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 structures always involve a double bond, but the only difference between the multiple resonance structures is the position of this double bond. Resonance is caused by pi bonding, which occurs when p orbitals merge into one large overlapping region across multiple atoms in the structure. The electrons from these orbitals spread out over the overlapping region and are delocalized, decreasing the electron density and making the molecule more stable.

To summarize, molecules that show resonance can be represented by multiple alternative Lewis structures with the same arrangement of atoms but a different distribution of electrons. These alternative structures are called resonance structures and 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 learnt 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 in 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.

The transition between the resonance structures on the left and right involves the movement of a lone pair of electrons from the oxygen atom on the left to create an O=O double bond, while the original O=O double bond between the central oxygen and the oxygen atom on the right is broken and the electron pair is transferred to the oxygen atom on the right. The reverse process occurs to transition from the resonance structure on the right to the resonance structure on the left.

However, it's important to note that these diagrams can be misleading. They may imply that molecules that show resonance switch between different resonance structures, but this isn't the case. Instead, molecules that show resonance take on a hybrid molecule structure that is an average of all the resonance structures. Resonance structures are simply a way of representing these molecules and should not be taken too literally.

## Resonance structure and dominance

In some cases of resonance, one resonance structure has more influence on the overall hybrid structure than others. This dominant structure is determined using formal charges assigned to atoms, assuming that all the bonded electrons are split evenly between the two bonded atoms.

The Lewis structure with formal charges closest to zero is generally assumed to be the dominant structure. If two resonance structures have equivalent formal charges, the Lewis structure with the negative formal charge on the more electronegative atom is assumed to be the dominant structure.

For example, in the case of carbon dioxide, there are three possible resonance structures. Two of them have one oxygen atom with a formal charge of +1 and the other with a formal charge of -1, while the third has all atoms with a formal charge of +0. The structure with all atoms having a formal charge of +0 is the dominant structure.

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.

It's important to understand that when a molecule exhibits resonance, it doesn't switch between one resonance structure and another. Instead, it takes on a completely new identity that is a combination of all the resonance structures. Molecules that cannot be represented by just one Lewis structure show resonance.

Resonance is a way of describing bonding in chemistry, where several equivalent Lewis structures contribute to one overall hybrid molecule. Bond order tells us about the number of bonds between two atoms in a molecule, with a single bond having a bond order of 1 and a double bond having a bond order of 2.

To calculate bond order in a hybrid molecule, we need to draw out all of the molecule's resonance structures and work out the bond order of the chosen bond in each of them. These values are then added together and divided by the number of resonance structures. For example, the bond order of the leftmost O-O bond in ozone is 1.5, which is the average of the bond orders in the two resonance structures.

## Rules of resonance

We can put together what we've learnt so far to make up some rules of resonance: Molecules that show resonance are represented by multiple resonance structures. These must all be feasible Lewis structures. Resonance structures have the same layout of atoms but different arrangements of electrons. Resonance structures differ only in the position of their pi bonds. All sigma bonds remain unchanged. 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.

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.

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.

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.

Here are some additional key takeaways on resonance chemistry:

• Resonance is a way of describing bonding in chemistry that involves multiple equivalent Lewis structures contributing to one overall hybrid molecule.
• Hybrid molecules are unique and represent an average of all the different resonance structures of a molecule.
• Not all resonance structures contribute equally to the overall structure of a molecule, with the dominant structure having the most effect.
• Equivalent resonance structures have equal effect on the overall structure of a molecule.
• Bond order tells us about the number of bonds between two atoms in a molecule and can be calculated in hybrid molecules by adding up the bond orders across all the structures and dividing by the number of structures.

## Resonance Chemistry

What is resonance in chemistry?

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

What is a resonance structure in chemistry?

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

What causes resonance in chemistry?

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.

What is the resonance rule in chemistry?

There are a few rules when it comes to resonance in chemistry: Molecules that show resonance are represented by multiple resonance structures. These must all be feasible Lewis structures. Resonance structures have the same layout of atoms but different arrangements of electrons. Resonance structures differ only in their position of pi bonds. All sigma bonds remain unchanged. 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.

What is an example of a resonance structure?

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

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