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Lewis Structures: Resonance

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    5130

    • A Lewis Structure is a very simplified representation of the valence shell electrons in a molecule. It is used to show how the electrons are arranged around individual atoms in a molecule. Electrons are shown as "dots" or for bonding electrons as a line between the two atoms. The goal is to obtain the "best" electron configuration, i.e. the octet rule and formal charges need to be satisfied.

    Resonance

    Many Lewis structures can exist in more than one form. This is due to multiple equivalent bonding patterns being possible. We will discuss resonance in terms of several examples given below:

    CH3CO2-




    Lewis Structures

    resonance structures

    average structure

    There are two equivalent Lewis structures possible for the acetate anion. The only difference is which specific oxygen has the formal negative charge, the other has to have a double bond to the C atom. The "average structure" represents a "time averaged" molecule, each O atom is double bonded to the C atom half the time, and singly bonded with a formal negative charge the other half.

    NO3-




    Lewis Structures

    resonance structures

    average structures

    There are three equivalent Lewis structures possible for the nitrate anion. The only difference is which specific oxygens have the formal negative charges, the remaining one has to have a double bond to the N atom. The "average structure" represents a "time averaged" molecule, each O atom is double bonded to the N atom 1/3 the time, and singly bonded with a formal negative charge 2/3 of the time. Note: the N atom always has a formal charge of +1.

    Benzene

    Kekule representation of benzene

    Lewis Structures

    resonant hybrid

    resonance structures

    There are two equivalent Lewis structures possible for benzene. The only difference is where the double bonds are located. The resonant hybrid represents the "time averaged" structure, where there are double bonds between C atoms half the time, and single bonds the other half.

    Why worry about the different structures? They represent different things.

    • The Lewis structures are better when you are looking at a chemical reaction, where you need to know which atom is charged or where the double (or triple) bond is.
    • The resonant hybrid or "averaged structure" is better if you are trying to describe the physical properties of a molecule. These properties rely on the "time averaged" electron distribution in the molecule, which is better represented as the average structure.