Short on time? This page is the recap. For explanations and worked examples, use the full study guide.
Recap
- Resonance structures have the same skeleton and electron count; only lone pairs and multiple-bond pairs move. Join them with ↔.
- When the structures are equivalent (formate, nitrite, nitrate, carbonate), the real species is a resonance hybrid: one structure, not a switch. The resonance bonds are identical in length and energy, and any charge is shared.
- When the structures are not equivalent (N₂O), they contribute unequally; formal charge ranks them.
- Species with an odd number of electrons (NO₂, ClO₂) cannot give every atom an octet. This is a limitation of the Lewis model.
Key relationships
| Relationship | Use it to |
|---|---|
| FC = V − N − ½B (V valence, N nonbonding electrons, B bonding electrons) | find the formal charge of any atom |
| Shortcut: FC = V − dots − lines | do the same quickly from a drawing |
| Sum of FCs = overall charge | check your diagram |
| Average bond order = bonds to the equivalent atoms ÷ number of positions | compare bond lengths and strengths (higher order → shorter, stronger) |
| Atom (usual pattern) | FC 0 | FC +1 | FC −1 |
|---|---|---|---|
| C | 4 bonds | 3 bonds, 0 lone pairs | 3 bonds, 1 lone pair |
| N | 3 bonds, 1 lone pair | 4 bonds | 2 bonds, 2 lone pairs |
| O | 2 bonds, 2 lone pairs | 3 bonds, 1 lone pair | 1 bond, 3 lone pairs |
Choosing the best diagram
- Octets for C, N, O and F (never more than 8).
- Formal charges as close to zero as possible.
- Negative formal charge on the most electronegative atom.
- No like charges on neighbouring atoms.
If the octet rule and formal charge disagree (for example SO₄²⁻), name the criterion you use.
Assumptions behind the model
- Formal charge assumes bonding electrons are shared exactly equally.
- Lewis diagrams assume electrons are in fixed pairs; resonance is the patch for electrons that are really spread out.
Mistakes to avoid
- Saying the molecule “flips” between structures.
- Moving atoms between “resonance structures”.
- Assuming every resonance bond order is 1.5 (nitrate is 4/3).
- Formal charges that do not add up to the ion’s charge.
- Giving N or O more than 8 electrons to lower formal charges.
Quick self-check
- Formal charge on S in H₂S? (6 − 4 − 2 = 0)
- Average C–O bond order in CO₃²⁻? (4 ÷ 3 = 1.33)
- Formal charge on N in NH₄⁺? (+1)
- Why can’t NO have an octet on both atoms? (It has 11 valence electrons, an odd number)
Next: practice questions.