Resonance Structures (HL) (DP IB Chemistry): Revision Note

Resonance Structures

• Resonance occurs when more than one valid Lewis structure can be drawn for a molecule or ion

• This is due to delocalised electrons, where electrons are not confined to a single bond or atom

• The actual structure is not switching between forms

  • It is a resonance hybrid, where the electron density is delocalised over multiple atoms

• Resonance helps explain:

  • Bond lengths

  • Bond strength

  • Electron distribution

Example: Nitrate(V) ion, NO₃^-

• The nitrate(V) ion has 3 oxygen atoms bonded to one nitrogen atom

• Using a Lewis diagram, it appears to contain:

  • One double N=O bond

  • Two single N-O bonds

Total valence electrons = N + (3 × O) + 1 (for charge)

Total valence electrons = 5 + (3 × 6) + 1 = 24 electrons

• But, there are three equivalent structures

  • Each structure contains the double N=O bond in a different position

  • These are called resonance structures

• All three resonance structures contribute to the true structure

• The actual structure is a resonance hybrid:

  • All three N–O bond lengths are the same

    • Each is intermediate between a single and a double bond

  • The electron density is evenly delocalised across all three N–O bonds

What makes a molecule form resonance structures?

• Resonance occurs when:

  • There is a π (pi) bond that can occupy multiple positions

  • Atoms of equal electronegativity are adjacent

  • Lone pairs or double bonds can shift around the structure without breaking octet rules

• You should also be able to recognise resonance in the following structures:

Carbonate ion, CO₃^2-

• The carbonate ion is one carbon atom bonded to three oxygen atoms

• The π electrons are delocalised across all three C–O bonds

• The delocalisation explains why the actual structure is a resonance hybrid with three equal bond lengths

Benzene, C₆H₆

• Benzene is a six-carbon ring with alternating double bonds

• The π electrons are delocalised around the entire ring

• The delocalisation explains why all the C–C bonds are equal in length

Ozone, O₃

• Ozone consists of three oxygen atoms arranged in a bent shape

• The π electrons are delocalised over the two O–O bonds

• The delocalisation explains why the actual structure is a resonance hybrid with two equal bond lengths

Carboxylate anion, RCOO^-

• The carboxylate ion is one carbon atom bonded to two oxygen atoms and an R-group

• The negative charge is delocalised over both oxygen atoms

• The delocalisation explains why the actual structure is a resonance hybrid with two equal C–O bond lengths