Getting Started
The Lewis structure model provides a powerful, simple way to visualize how atoms are connected and how valence electrons are distributed within a molecule. However, this model sometimes falls short. For many molecules and polyatomic ions, a single Lewis diagram cannot accurately represent the true electronic structure, forcing us to refine our model to account for electron sharing across multiple bonds or to choose the best possible diagram among several valid options.
What You Should Be Able to Do
After working through this section, you should be able to:
Draw multiple equivalent Lewis structures for a molecule or ion and describe the resulting resonance hybrid.
Calculate the formal charge on each atom within a given Lewis structure.
Use formal charge principles to evaluate and select the most plausible Lewis structure from a set of non-equivalent options.
Explain why the Lewis model is insufficient for molecules containing an odd number of valence electrons.
Key Concepts & Analysis
Drawing the most accurate Lewis structure is a decision-making process. When an initial drawing isn't sufficient, we use the concepts of resonance and formal charge to refine our model and select the best possible representation.
Inputs & Preconditions: The starting point is always the molecular formula and the correct total count of valence electrons for all atoms in the species. A prerequisite is the ability to draw a basic, valid Lewis structure that satisfies the octet rule for as many atoms as possible.
Key Steps / Mechanism:
Draw an Initial Plausible Lewis Structure: Arrange the atoms with the least electronegative atom typically in the center, and distribute the valence electrons to form bonds and lone pairs, aiming to satisfy the octet rule.
Evaluate the Structure for Alternatives: Once you have a valid structure, ask two critical questions:
Equivalence Check: Can I move electron pairs (from lone pairs to bonds, or vice versa) to create a new structure with the exact same arrangement of atoms and the same number of bonds, just in a different location? If yes, the molecule exhibits resonance.
Plausibility Check: Can I draw a different, non-equivalent structure (e.g., with a double bond instead of a triple bond) that also largely satisfies the octet rule? If yes, you must use formal charge to determine which structure is best.
Step 3A: Applying Resonance (For Equivalent Structures):
When multiple equivalent structures exist, the actual molecule is not any single one of them. It is a resonance hybrid—an average of all contributing structures. For example, the nitrate ion, NO₃⁻, has 24 valence electrons. We can draw three equivalent Lewis structures: