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Resonance or mesomerism is a way of describing delocalized electrons within certain molecules or polyatomic ions where the bonding cannot be expressed by one single Lewis structure.
If you can draw more than one Lewis structure that is equivalent to each other, then it displays resonance. This usually occurs in molecules that have double bonds and single bonds. For example, benzyne has double bonds on every other bond between two carbons (and single bonds for the other carbons). Another way of drawing benzyne would be putting the double bonds as the single bonds in the previous structure and single bonds as double bonds. These structures are equivalent, and therefore benzyne displays resonance.
To add on, resonance can have different bonds as mentioned earlier. They can have partial bonds meaning that they could be between a single and double bond or between a double and triple bond.
Resonance structures are two forms of a molecule where the chemical connectivity is the same but the electrons are distributed differently around the structure. Resonance occurs when electrons can flow through neighboring pi systems. It is also known as a mesomerism and it is a way of describing delocalized electrons.
An important thing to remember about resonance structures, along with the above definitions, is that all of the bonds in the Lewis Structure have the same length. This displays that drawing resonance is just a way of showing that all of the bonds have the same length no matter where they are placed.
Just to reiterate, resonance structures are basically lewis structures that can be written in multiple forms, the difference being the arrangement of the bonds between each resonance structure. The chemical properties are all the same.
Another important thing to note is the formal charges present within these structures. If you are trying to determine if something has resonance, check to see if the formal charges make sense when changing a bond. For example, a formal charge of +1 for chlorine in a possible resonance hybrid is highly unlikely since chlorine has a high electron affinity and high ionization energy. This should be a signal that there may not be resonance.
Resonance basically means that the lewis structure of a molecule can be written in different forms, but the one that is the most appropriate to use would be the one that has a formal charge of 0, because it means it is most stable. The general lewis structure of molecules with resonance would be the combination of all or the resonance hybrid as it is called.
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