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Lewis Diagrams

Lewis diagrams (also called Lewis structures) are representations of molecules that show how valence electrons are arranged among atoms. They are a fundamental tool in chemistry for predicting molecular shape, bonding, and reactivity.

Basic Rules for Drawing Lewis Diagrams​

  1. Count total valence electrons: Sum the valence electrons of all atoms in the molecule or ion.
  2. Determine the central atom: Usually the least electronegative atom (except hydrogen).
  3. Draw single bonds: Connect the central atom to surrounding atoms with single bonds.
  4. Distribute remaining electrons: Complete octets (or duets for hydrogen) for outer atoms first.
  5. Check for multiple bonds: If the central atom does not have an octet, form double or triple bonds as needed.

Example: Water (H₂O)​

  • Total valence electrons: 2 (H) + 6 (O) = 8 electrons
  • Central atom: Oxygen
  • Single bonds: O-H and O-H
  • Remaining electrons: 4 electrons as lone pairs on oxygen
  • Lewis diagram shows two O-H bonds and two lone pairs on oxygen.

Important Concepts​

  • Octet Rule: Most atoms tend to have 8 electrons in their valence shell.
  • Formal Charge: Helps determine the most stable structure when multiple Lewis diagrams are possible.
  • Resonance: Some molecules can be represented by more than one valid Lewis structure, indicating delocalized electrons.

Applications​

  • Predicting molecular geometry using VSEPR theory
  • Understanding bond polarity and reactivity
  • Determining formal charges for ionic and covalent compounds

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