4.5 Formal Charges and Resonance | Chemistry: Atoms First 2e

Learning Objectives

Compute formal charges for atoms in any Lewis structure
Use formal charges to identify the most reasonable Lewis structure for a given molecule
Explain the concept of resonance and draw Lewis structures representing resonance forms for a given molecule

Core Concepts & Principles

When multiple valid Lewis structures can be drawn for a single molecule or polyatomic ion, chemists use formal charge as a bookkeeping tool to determine the most realistic structure.

What is Formal Charge?

Formal charge is the hypothetical charge an atom would possess if all bonding electrons were shared equally between bonded atoms. It is not an actual measured atomic charge, but rather an electronic accounting system used to evaluate structural stability.

Guidelines for Selecting the Best Lewis Structure

When evaluating competing Lewis structures for a molecule or ion, apply these priority rules:

Zero Charges Preferred: A structure where all atoms have a formal charge of zero is optimal.
Minimize Magnitudes: If nonzero formal charges are unavoidable, the structure with the smallest nonzero charges is preferred.
Opposite Signs / Proximity: Structures with adjacent formal charges of zero or opposite signs are favored over those with like charges adjacent to each other.
Electronegativity Match: Negative formal charges should reside on the most electronegative atoms, and less electronegative atoms typically occupy central positions.

Resonance and Delocalized Electrons

For many species, writing a single Lewis structure is insufficient because multiple valid structures can be drawn with identical atom connectivity.

Resonance Fundamentals

Resonance Forms: The individual valid Lewis structures that can be drawn for a molecule, separated by double-headed arrows ( $\leftrightarrow$ ).
Resonance Hybrid: The true electronic structure of the molecule, representing an exact average of all valid resonance forms.
Bonds in a resonance hybrid are identical in length and strength (e.g., intermediate between a single and double bond), rather than fluctuating between forms.

Problem-Solving Routines & Methods

Formal Charge Equation

FC = V - L - \frac{1}{2}B

Calculates the formal charge of an individual atom within a Lewis structure.

Variables & Constants

FC

=formal charge;

V

=number of valence electrons in free neutral atom;

L

=number of non-bonding lone pair electrons;

B

=number of shared bonding electrons

How to Calculate Formal Charge

1
Identify the number of valence electrons ( $V$ ) for each neutral free atom.
2
Assign all non-bonding electrons directly to their host atom as lone pair electrons ( $L$ ).
3
Split the bonding electrons evenly between the bonded atoms, assigning half ( $\frac{1}{2}B$ ) to each atom.
4
Subtract the assigned lone pair and bonding electrons from the neutral valence electron count: $FC = V - L - \frac{1}{2}B$ .
5
Verify your work: The sum of all formal charges in a neutral molecule must equal 0; in a polyatomic ion, the sum must equal the net charge of the ion.

Pro-Tip: Always double-check that your total formal charge matches the overall charge of the molecule or ion before choosing the best structure.

Practice & Concept Checks

Concept Check

Why are the two N–O bonds in the nitrite ion (

NO_2^-

) found to be identical in length and strength, even though one resonance form shows a single bond and the other shows a double bond?

Concept Check

Between two valid Lewis structures for a neutral molecule, Structure A has formal charges of (0, 0, 0) and Structure B has formal charges of (+1, 0, -1). Which structure is preferred?

Key Terms & Vocabulary

Formal ChargeChemical Bonding

The hypothetical charge an atom would have if bonding electrons were distributed evenly; calculated as valence electrons minus lone pair electrons minus half of bonding electrons.

Example: Used to evaluate the relative stability of competing Lewis structures.

Molecular StructureChemical Bonding

The specific three-dimensional arrangement or geometry of atoms within a molecule or polyatomic ion.

Example: Determined using formal charge guidelines when multiple Lewis structures are possible.

ResonanceChemical Bonding

A condition where two or more valid Lewis structures with identical atom placements can be written for a single species.

Example: Observed in molecules like ozone (O₃) and the carbonate ion (CO₃²⁻).

Resonance FormsChemical Bonding

Individual valid Lewis structures that contribute to the true electronic description of a resonance-stabilized molecule.

Example: The two distinct drawings used to represent the nitrite ion (NO₂⁻).

Resonance HybridChemical Bonding

The actual electronic structure of a molecule or ion, representing the weighted average of all valid resonance forms.

Example: The true state of NO₂⁻ where both N–O bonds are identical in length and strength.