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Resonance and Formal Charge

Resonance and formal charge are key concepts in understanding the stability and electron distribution of molecules. They help chemists determine the most accurate Lewis structures and predict chemical behavior.

Resonance​

  • Definition: Resonance occurs when a molecule can be represented by two or more valid Lewis structures (resonance structures) that differ only in the placement of electrons, not atoms.
  • Purpose: Resonance structures depict delocalized electrons, which are spread over multiple atoms, stabilizing the molecule.

Example: Ozone (O₃)​

  • Ozone has two major resonance structures with one double bond and one single bond between the oxygen atoms.
  • The true structure is a hybrid, with bond lengths intermediate between a single and double bond.

Formal Charge​

  • Definition: Formal charge is a bookkeeping tool used to determine the electron distribution within a molecule.
  • Formula:
    Formal Charge = (Valence electrons in free atom) − (Nonbonding electrons) − ½(Bonding electrons)

Rules and Tips​

  • The most stable Lewis structure usually has formal charges closest to zero.
  • Negative formal charges should reside on the more electronegative atoms.
  • Resonance structures help distribute formal charges evenly across the molecule.

Importance​

  • Helps predict stability of molecules and ions.
  • Guides correct placement of bonds and electrons in Lewis structures.
  • Provides insight into reactivity and chemical behavior of molecules.

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