(2351) S2.2.11 Resonance structures (HL)

Introduction to Resonance Structures

• Resonance Structures: Occur when a molecule has more than one valid Lewis structure due to multiple bond positions.

• Multiple Bonds: Can include double or triple covalent bonds.

Carbonate Ion Example

• Resonance Structures of Carbonate Ion:

  • Three resonance structures exist with varying positions of the carbon to oxygen double bond.

  • Each structure represents a valid Lewis structure.

• Actual Structure: The true structure is a resonance hybrid, where:

  • Bond lengths and strengths are identical across bonds.

  • Bonds are intermediate in nature—shorter than single bonds but longer than double bonds, and stronger than single bonds but weaker than double bonds.

Further Examples of Resonance Structures

• Nitrate Ion (NO3-):

  • Has three resonance structures with the double bond in different positions.

• Ozone Molecule (O3):

  • Contains two Lewis structures with the double bond in alternate positions.

• Ethanoate Ion:

  • Exhibits two possible Lewis structures with a varying position of the carbon to oxygen double bond.

• Benzene (C6H6):

  • Represented by the Kekulé structure featuring alternating single and double bonds.

  • Offers two possible Lewis structures to describe its bonding arrangement.

  • An alternative representation shows delocalized electrons effectively.

Summary of Key Concepts

• Key Takeaway:

  • Resonance structures illustrate the concept of multiple valid configurations leading to varying bond placements.

  • The actual molecule or ion exists as a resonance hybrid, where bond characteristics are intermediate.

  • Delocalized Electrons: Important feature, referring to electrons shared over multiple nuclei, enhancing stability.

  • Localized electrons will be addressed in more detail in future discussions.