Chapter 7: Lewis Structures and Resonance

Chapter 7: Lewis Structures and Related Concepts

Topics Covered

• Review of Lewis structures
• Drawing Lewis structures
• Understanding and calculating formal charge
• Lewis structure exceptions
• Resonance in molecules

Lewis Structures

• Definition: Lewis structure is a graphical representation of molecular bonding. It shows the arrangement of valence electrons among atoms in a molecule.
  • Steps to Draw Lewis Structures:
  1. Determine the total number of valence electrons.
  2. Draw the skeleton structure of the molecule (connect the atoms).
  3. Distribute remaining electrons to satisfy octet rule (or duet rule for H).
  4. Ensure central atom has appropriate electron pairs and considers double or triple bonds if necessary.
  5. Adjust as needed to minimize formal charges.

Drawing Lewis Structures with Formal Charges

• Formal Charge (FC): A concept to identify the hypothetical charge of an atom in a molecule. Helps in predicting the most stable structure.
  • Formula: FC = ext{Valence electrons} - ext{Non-bonding electrons} - rac{1}{2} ext{Bonding electrons}
• Guidelines for Formal Charges:
  1. All formal charges should be 0 if possible.
  2. The structure with the smallest number of non-zero formal charges is preferred.
  3. Adjacent charges should be 0 or of opposite signs.
  4. Favorable formal charges on more electronegative atoms.

Exceptions to Lewis Structures

• Odd Electron Molecules: Molecules with an odd number of electrons (e.g., NO) may have unpaired electrons leading to free radicals.
• Electron Deficient Compounds: Certain compounds (e.g., BeH2 and BF3) do not follow the octet rule; boron has less than an octet.
• Hypervalent Molecules: Molecules like SF6 can hold more than 8 electrons due to d-orbitals being available in 3rd period and beyond.

Resonance

• Definition: Resonance occurs when a molecule can be represented by two or more valid Lewis structures (resonance structures). The actual structure is an average of these forms.
• Example of Resonance:
  • For nitrate ion ($NO_3^-$): There are multiple ways to arrange double and single bonds among the oxygen atoms and the nitrogen.
• Rules for Drawing Resonance:
  1. Keep the arrangement of atoms the same in all structures.
  2. Electrons can be redistributed to form different bonds.
  3. The contribution of each resonance structure to the overall hybrid structure can vary, being based on the stability of each resonance form.

Problem Examples

• HCN: Draw the Lewis structure and determine formal charges.
• PCl5: Analyze how phosphorus expands its valence shell.
• CO2: Examine the multiple resonance forms and the formal charges.

Key Points

• When drawing Lewis structures, account for formal charges to achieve the most stable configuration.
• Understand when to apply resonance and confirm the functionality of the structures drawn.
• Frequent reference to electronegativity will aid in determining potential bond types and stability in Lewis structures.
• Familiarize yourself with the valence electron count to ensure the accuracy of drawn structures and their associated formal charges, especially for complex molecules.