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Determining Most Stable Lewis Structures

Aug 13, 2025

Overview

This lecture explains how to determine the most stable or plausible Lewis structure of a molecule using formal charge calculations, with step-by-step examples for CO₂ and POCl₃.

Multiple Lewis Structures

  • Molecules like sulfate ion (SO₄²⁻) can be represented by many valid Lewis structures.
  • Out of possible structures, only one is typically the most stable or plausible.

Determining the Most Stable Lewis Structure

  • The most stable Lewis structure is found by calculating the formal charge on each atom.
  • The optimal structure has atoms with the lowest possible formal charges (ideally zero).

Formal Charge Calculation

  • Formal charge formula: Formal charge = (Number of valence electrons) – (Number of electrons surrounding the atom).
  • Electrons from bonds: single bond = 1 electron, double bond = 2 electrons, triple bond = 3 electrons (per atom).
  • Lone pairs also count toward surrounding electrons.

Example 1: CO₂ Structures

  • Structure 1: Central carbon with triple bond to one O and single bond to the other O.
    • Formal charges: O (triple bond) +1, C 0, O (single bond) –1.
  • Structure 2: Central carbon with double bonds to each O.
    • Formal charges: All atoms have a formal charge of 0.
  • Structure 2 is the most plausible as all atoms’ formal charges are minimized.

Example 2: POCl₃ Structures

  • Structure 1: All single bonds.
    • Formal charges: O –1, P +1, Cl 0 (all three).
  • Structure 2: Double bond between P and O, other bonds single.
    • Formal charges: All atoms 0.
  • Structure 2 is more plausible due to lowest formal charge on all atoms.

Guidelines for Determining Plausible Structures

  • Best structures have all atoms with zero formal charge.
  • Negative formal charges should appear on the most electronegative atoms.
  • Positive formal charges should appear on the least electronegative atoms.

Key Terms & Definitions

  • Lewis Structure — Diagram showing the arrangement of electrons in a molecule.
  • Formal Charge — The charge an atom would have if electrons in bonds were shared equally.
  • Valence Electron — Electrons in the outermost shell involved in bonding.
  • Electronegativity — An atom’s ability to attract electrons.

Action Items / Next Steps

  • Practice calculating formal charges and drawing Lewis structures for additional molecules.

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