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Resonance & Lewis Structures (College Board AP® Chemistry) : Study Guide

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Written by: Martín

Reviewed by: Stewart Hird

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Resonance & Lewis Structures

  • Some molecules can be represented by more than one Lewis structure

  • Resonance structures are different variations of the same molecule, but with different arrangement of electrons

  • The resonance hybrid is the actual structure of a molecule, and it is a mixture of all its resonance structures

  • Resonance occurs because electrons are often delocalized

  • Delocalized electrons are not fixed to a single atom or covalent bond, therefore they can be spread through the molecule

  • The resonance hybrid is the most stable form of a molecule because the delocalization of electrons stabilize the structure

Resonance structures of the nitrate(V) ion

  • To draw the resonance structure of the nitrate(V) ion, the first step is to count the valence electrons that are present in the ion

  • There is one nitrogen atom, three oxygen atoms and a negative charge

    • Nitrogen in group 15 contributes with 5 valence electrons

    • Oxygen in group 16 contributes with 6 valence electrons

    • The negative charge means that an electron has been gained

 begin mathsize 16px style Valence space electrons equals straight N plus left parenthesis 3 space cross times space straight O right parenthesis plus 1 straight e to the power of minus end style

Valence space electrons equals 5 plus left parenthesis 3 cross times 6 right parenthesis plus 1 straight e to the power of minus

Valence space electrons equals 24 space straight e to the power of minus

  • After drawing an skeletal structure (N as the central atom) and placing the bonding and nonbonding electrons, three Lewis structures are possible

  • These Lewis structures consist of a double bond and two single bonds. However, a pair of electrons from the double bond and lone pairs of electrons can oscillate between three different positions

  • These oscillation allows the double bond to migrate from one part of the molecule to another

Resonance in the Nitrate(V) ion

Resonance structures in the nitrate ion

  • One the resonance structures are identified, the resonance hybrid can be drawn using dotted lines to show the position of delocalised electrons

  • The three resonance structures have more energy than the resonance hybrid of nitrate(V) ion

    • Therefore, the resonance hybrid is is the most stable and the actual structure of the nitrate(V) ion

The Nitrate(V) Ion

Resonance hybrid nitrate(V) ion

  • Some common examples of resonance are the carbonate ion, benzene, ozone, and the carboxylate ion which are showed in the table below

Species

Lewis Diagram

Resonance Hybrid

Carbonate ion, CO32-           

Three resonance structures of the carbonate ion, \(\text{CO}_3^{2-}\), showing electron movement with arrows and double-headed arrows indicating resonance.
resonance-and-formal-charge-1b

Benzene, C6H6

resonance-and-formal-charge-2a
resonance-and-formal-charge-2b

Ozone, O3

resonance-and-formal-charge-3a
resonance-and-formal-charge-3b

Carboxylate ion, RCOO-          

resonance-and-formal-charge-4a
resonance-and-formal-charge-4b

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    Martín

    Author: Martín

    Expertise: Chemistry Content Creator

    Martín, a dedicated chemistry teacher and tutor, excels in guiding students through IB, AP, and IGCSE Chemistry. As an IB Chemistry student, he came from hands-on preparation, focusing on practical exam techniques and rigorous practice. While at Universidad San Francisco de Quito, his academic journey sparked a passion for computational and physical chemistry. Martín specializes in chemistry, and he knows that SaveMyExams is the right place if he wants to have a positive impact all around the world.

    Stewart Hird

    Reviewer: Stewart Hird

    Expertise: Chemistry Lead

    Stewart has been an enthusiastic GCSE, IGCSE, A Level and IB teacher for more than 30 years in the UK as well as overseas, and has also been an examiner for IB and A Level. As a long-standing Head of Science, Stewart brings a wealth of experience to creating Topic Questions and revision materials for Save My Exams. Stewart specialises in Chemistry, but has also taught Physics and Environmental Systems and Societies.