AP Chemistry Unit 2 Study Notes

AP Chemistry 2.6: Resonance and formal charge

Compare valid Lewis structures and electron delocalization.

Aligned to Compound Structure and Properties from the current College Board AP Chemistry course outline. Exam weighting for this unit: 7%-9% of the multiple-choice score range listed by College Board.

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Resonance and Formal Charge Overview 6.1 What Is Resonance? 6.2 Example: Ozone 6.3 Resonance Hybrid 6.4 Delocalized Electrons 6.5 Bond Order With Resonance 6.6 Nitrate Example 6.7 Formal Charge 6.8 Example: Oxygen With Two Bonds 6.9 Oxygen With One Bond 6.10 Nitrogen With Four Bonds 6.11 Formal Charge Check 6.12 Choosing the Better Lewis Structure

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These detailed Unit 2 notes were organized from the provided study document. For further study, visit Khan Academy. All Khan Academy content is available for free at www.khanacademy.org.

Resonance and Formal Charge Overview Open

  • Khan Academy's current section includes resonance, formal charge, and comparing nonequivalent resonance structures.

6.1 What Is Resonance? Open

  • Sometimes more than one valid Lewis diagram can be drawn without changing the positions of the atoms.

    • These are called:

    • resonance structures

    • or:

    • resonance forms

  • Only electron placement changes.

  • The nuclei stay in the same places.

6.2 Example: Ozone Open

  • O₃ can be represented with a double bond on either side:

    • O=O—O

    • or

    • O—O=O

  • Neither individual structure perfectly represents the real molecule.

6.3 Resonance Hybrid Open

  • The real molecule is a resonance hybrid .

    • It does not rapidly switch back and forth between the drawings.

    • Instead, the electrons are delocalized , and the actual electron distribution is represented by a combination of the resonance structures.

  • Khan Academy describes the actual electronic structure as the resonance hybrid of the possible structures.

6.4 Delocalized Electrons Open

  • Delocalized electrons are not confined to one single bond between two particular atoms.

    • Instead, their electron density is spread across several atoms.

  • Resonance is one way of representing this.

6.5 Bond Order With Resonance Open

  • Consider two equivalent resonance structures where one bond is:

    • single in one structure

    • and:

    • double in the other.

  • The actual bonds can become equivalent.

    • Their average bond order would be:

    • (1 + 2) / 2

    • = 1.5

    • This explains why measured bond lengths may be intermediate between a normal single and double bond.

6.6 Nitrate Example Open

  • NO₃⁻ has multiple equivalent resonance structures.

  • One N—O bond is represented as double in each individual structure, but the double bond can be placed with any of the three oxygens.

    • Therefore the actual N—O bonds are equivalent.

  • Average bond order:

  • (2 + 1 + 1) / 3

    • = 4/3 ≈ 1.33

6.7 Formal Charge Open

  • Formal charge is a bookkeeping method used to evaluate Lewis structures.

    • Formula:

    • Formal charge = valence electrons − nonbonding electrons − ½(bonding electrons)

  • A quicker version:

    • FC = valence − dots − lines

    • where:

    • dots = nonbonding electrons

    • lines = number of bonds

6.8 Example: Oxygen With Two Bonds Open

  • Oxygen has:

  • 6 valence electrons.

    • Suppose it has:

    • 2 lone pairs = 4 nonbonding electrons

    • 2 bonds

  • Formal charge:

    • 6 − 4 − 2

    • = 0

6.9 Oxygen With One Bond Open

  • Suppose O has:

    • 3 lone pairs = 6 nonbonding electrons

    • 1 bond

  • FC:

    • 6 − 6 − 1

    • = −1

6.10 Nitrogen With Four Bonds Open

  • Nitrogen has:

  • 5 valence electrons.

  • If it has:

    • 0 lone-pair electrons

    • 4 bonds

  • FC:

    • 5 − 0 − 4

    • = +1

6.11 Formal Charge Check Open

  • The sum of all formal charges must equal the overall charge of the species.

  • For a neutral molecule:

    • formal charges must add to:

    • 0

  • For NO₃⁻:

    • formal charges must add to:

    • −1

  • For NH₄⁺:

    • formal charges must add to:

    • +1

    • This is an excellent way to check your work.

6.12 Choosing the Better Lewis Structure Open

  • When several nonequivalent structures are possible, generally prefer:

    • structures with smaller formal-charge magnitudes

    • less unnecessary charge separation

    • negative formal charge on the more electronegative atom when possible

  • But these are guidelines, not rules you should apply blindly.

  • Khan Academy specifically includes evaluating nonequivalent resonance structures using formal charges.