Resonance Structures

Core Concepts

In this tutorial on resonance structures, you will learn what resonance structures are and how to find all of the possible resonance structures a molecule has. You will also learn how to identify and compare the stability among these structures.

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What are resonance structures?

resonance structure - acetate ion
Above: Resonance structures for acetate ion.

Resonance structures are various forms of the same molecule where the electrons have transferred from one region to another.

Why are resonance structures important?

Resonance structures are significant because they provide a much more realistic view of the shape of a molecule. It should be noted, each individual resonance structure is averaged into a resonance hybrid which is both the true shape of the molecule and the most stable resonance form.

How do you find a molecule’s resonance structures?

When drawing resonance structures, here are a few things you want to keep in mind:

  1. Only move the electrons (never move the atoms).
    • This includes lone pair electrons and pi bonds (remember: There is one pi bond in a double bond and two pi bonds in a triple bond.)
    • Never break a single bond (also known as a sigma bond).
  2. Work in one region at a time.
    • This means focus on the three atoms closest together and make your way down the molecule.
  3. You will want to begin in the electron-dense regions.
    • This means you will be pushing the electrons from the negative charge to the positive.
  4. Typically, you will be turning lone pairs into bonds and bonds into lone pairs.
  5. Follow the octet rule and try to stay away from large formal charges.

Example: How would you draw the resonance structures of Ozone (O3)?

What is Ozone?

First, let’s take a look at ozone. You have probably heard of the ozone layer in the earth’s stratosphere. The reason it is named “the ozone layer” is because of its high concentration of ozone, which absorbs the majority of the sun’s ultraviolet radiation. In chemistry, it is one of the most powerful oxidizing agents as it has a large standard reduction potential.

Drawing the Lewis Structure of Ozone

To find the resonance structure of ozone, we will draw the lewis structure of ozone. Since the molecular formula is O3, we know there are 18 valence electrons (oxygen has six valence electrons as 6 x 3= 18). After placing all the electrons, we will have a double bond and a single bond.

resonance structure - ozone
It helps to start by drawing a simple lewis dot structure. Then, draw in the bond lines and assign the formal charges.
Drawing the Resonance Structure

Notice there is a positive formal charge on the top oxygen and a negative on the oxygen to its left. To find the formal charges, count the number of electrons the atom owns and compare it to its valence electrons. For example, the top oxygen owns 5 electrons. 2 of the electrons are a lone pair and the other 3 come from the bonds. Remember, it owns only one of the electrons in a bond. Therefore, 6 – 5= +1. The same follows for the bottom oxygen. The bottom left has 7 electrons and 6 – 7= -1. To find the number of valence electrons, refer to the group number at the top of the periodic table.

Step 1: Transfer the electrons on the oxygen with the negative charge and turn it into a double bond.

Step 2: Break the double bond between the positive oxygen and the neutral oxygen. Place the two electrons that were once in a single bond on the oxygen to the right.

Step 3: Now add a negative charge to the oxygen on the bottom right as it now has a formal charge of -1.

Notice how the arrow separating the structures has two heads. Resonance structures use this to show the flow of electrons going back and forth. Once you cannot create a new unique resonance structure, you can stop.

How do you determine the most stable resonance structure?

Resonance structures with the lowest amount of energy are known to be the most stable. This is significant because the greater the stability of a singular structure, the more it will contribute to the resonance hybrid. The most stable structures are known as the major contributors. Meanwhile, the least stable structures are known as the minor contributors. To find out which resonance structure is the most stable, there are five main rules to follow. (Note: if two structures are equal, they will contribute equally.)

Resonance Stability Rules:

  1. The resonance hybrid is more stable than any singular structure.
  2. The structure with no charge is more stable than any structure with charges.
  3. The structure with the least number of formal charges is more stable than those with more
  4. The most stable structure will have the negative charge placed on the most electronegative atom.
  5. The most stable structure will have the positive charge placed on the least electronegative atom.
Example: What structures are the most stable?
resonance structure - finding the most stable structure

The first thing you should notice is that the negative charge is located in a different region for each structure. Going back to what we know, the most stable structure will have the negative charge on the most electronegative atom. In this case, we have two different atoms: oxygen and carbon. Which is more electronegative? If you said oxygen, you are correct. Therefore, structures 2 and 3 are more stable than 1 and 4.

Which is more stable, structure 2 or structure 3?

The answer is they are equal and, therefore, will contribute equally as major contributors.

Further Reading:

Bond Order vs Bond Length

Molecular vs Empirical Formula