Topic 2.6 Notes – Resonance and Formal Charge

1. What Resonance Is

Sometimes you can draw more than one valid Lewis structure for the same arrangement of atoms, where only the electrons move. That situation is called resonance.

Key rules:

• Atom positions stay the same.
• Total number of valence electrons stays the same.
• Only π bonds and lone pairs shift.
• Structures are connected with a double-headed arrow (↔), not an equilibrium arrow.

The real molecule is a resonance hybrid. Electrons are delocalized over multiple atoms.

A classic example is carbonate, $\ce{CO3^{2-}}$. In the diagram below, the double bond moves between the three oxygens, giving three equivalent Lewis structures.

Resonance structures of the carbonate ion, $\ce{CO3^{2-}}$

All three structures are equivalent. In reality:

• All C-O bonds are the same length.
• None is purely single or double.

Bond Order in Resonance Hybrids

When bonds are shared across positions, the bond order becomes fractional:

$$\text{Bond order} = \frac{\text{Total number of bonds between two atoms across structures}}{\text{Number of equivalent bond positions}}$$

For carbonate:

• In any one structure: one double bond and two single bonds, so 4 shared electron pairs spread over 3 C-O positions
• Bond order $= \frac{4}{3}$

That means each bond is stronger than a single bond but weaker than a double bond.

Equivalent resonance structures contribute equally. Nonequivalent ones do not.

2. When and How to Draw Resonance Structures

When Resonance Is Required

You should look for resonance when:

• A double bond can be placed in multiple equivalent positions
• A lone pair is adjacent to a π bond
• A negative charge can spread over multiple atoms
• A positive charge can shift between atoms
• You are drawing common polyatomic ions like $\ce{NO2^{-}}$, $\ce{SO3}$, $\ce{CO3^{2-}}$

If equivalent structures exist, you must draw all of them on exams.

How to Draw Resonance Structures

The process is mechanical:

1. Draw a correct Lewis structure.
2. Find a π bond or lone pair next to a π bond.
3. Move electrons only:
   • Lone pair → form double bond
   • Double bond → becomes lone pair
4. Keep the same atoms and total electrons.
5. Use brackets and charge for ions.
6. Connect with ↔.

Never move atoms. Only electrons move.

On FRQs, missing a valid resonance structure can cost a point even if everything else is correct.

3. Formal Charge and Choosing the Best Structure

Sometimes multiple Lewis structures are possible but not equivalent. That’s when formal charge helps.

What Formal Charge Is

Formal charge estimates how electrons are distributed compared to a neutral atom.

$$\text{FC} = \text{Valence e⁻} - \text{Lone e⁻} - \text{Number of bonds}$$

Quick check:
$$\text{Sum of all FC} = \text{overall charge}$$

If it doesn’t match, something is wrong.

Choosing the Best Structure

Use these criteria:

Example pattern: If one structure places a −1 on oxygen and another places −1 on carbon, the oxygen structure is better because oxygen is more electronegative.

On exams, when they say “justify,” explicitly mention formal charge and electronegativity.

4. Expanded Octets and Exceptions to the Octet Rule

Atoms in Period 3 or below (P, S, Cl, etc.) can exceed 8 electrons.

Example situation:

• All single bonds give the central atom a +1 formal charge.
• Forming one double bond reduces that charge to 0.

Often, the best structure:

• Minimizes formal charge
• Places negative charge on oxygen if present

Expanded octets are allowed for these larger atoms, but carbon, nitrogen, oxygen, and fluorine do not expand.

5. Limitations of Lewis Structures

Lewis structures are models. They have limits.

Odd-Electron Species

If a molecule has an odd number of valence electrons, one atom will not have a full octet.

Example type: $\ce{NO}$

You cannot satisfy every octet. That’s expected. These are called radicals.

What Lewis Structures Don’t Show

• Actual 3D shape (that’s VSEPR)
• Real electron motion
• Exact bond energy
• True electron density distribution

Resonance structures are not flipping back and forth. The hybrid is the real structure.