Resonance Structures: What They Are and How to Draw Them
Let's start with a simple question. When you draw a molecule like ozone (O3), you might put a double bond between the central oxygen and one of the end oxygens, and a single bond to the other. But experiments show both O–O bonds are identical — same length, same strength. So which drawing is correct?
Neither single drawing is correct. The real molecule is a hybrid of both possibilities. That's the core idea of resonance.
The Intuition: A Musical Analogy
Think of a chord played on a piano. A C major chord is made of three notes: C, E, G. No single note is the chord — the chord is the blend of all three. Similarly, a resonance hybrid is the blend of all valid Lewis structures (called resonance contributors or canonical forms) for a molecule. The real molecule is not flipping between these forms; it exists as a single, stable average.
Resonance structures are not in equilibrium. The molecule does not switch from one form to another. It is a single structure that is the weighted average of all contributors.
The Precise Definition
Resonance structures are two or more Lewis structures that differ only in the placement of electrons (pi bonds and lone pairs), never in the positions of atoms. The real molecule is described by a resonance hybrid — a superposition of all contributors.
Rules for Valid Resonance Structures
- Atoms never move. Only electrons (pi bonds, lone pairs, and sometimes sigma bonds in special cases) change positions.
- The total number of electrons stays the same. You are just redistributing them.
- Each structure must obey the octet rule (for second-period elements) and have valid formal charges.
- All structures must have the same net charge and the same number of unpaired electrons (if any).
How to Draw Resonance Structures: A Step-by-Step Method
Let's use the carbonate ion (CO32−) as our example.
Step 1: Draw the best Lewis structure
Start with the skeleton: carbon in the center, three oxygens around it. Count valence electrons: C has 4, each O has 6, plus 2 for the charge = 4+18+2=24 electrons. Place bonds and lone pairs to satisfy octets. You'll get one structure with a C=O double bond and two C–O single bonds, each single-bonded oxygen carrying a negative charge.
Step 2: Identify movable electrons
Look for pi bonds (double or triple bonds) and lone pairs that are adjacent to pi bonds or to atoms with an empty p orbital. In carbonate, the C=O pi bond and the lone pairs on the negatively charged oxygens are the movable parts.
Step 3: Push electrons using curved arrows
An arrow starts at the electron source (a pi bond or lone pair) and points to where the electrons go (to form a new pi bond or to become a lone pair). In carbonate:
- Take the pi bond from C=O and push it to become a lone pair on that oxygen.
- Simultaneously, take a lone pair from a negatively charged oxygen and push it to form a new C=O pi bond.
Step 4: Draw the new structure
After pushing, you get a second structure where the double bond is on a different oxygen. Repeat to get the third structure (all three oxygens take turns being double-bonded).
Always check that the total number of electrons and the net charge remain unchanged after each arrow push. A common mistake is to accidentally add or remove electrons.
Common Patterns to Recognize
| Pattern | Example | What moves |
|---|
| Allylic system | CH2=CH−CH2+ | Pi bond shifts, positive charge moves |
| Conjugated diene | CH2=CH−CH=CH2 | Pi bonds shift (less common in neutral molecules) |
| Carbonyl group | R2C=O | Lone pair from O forms pi bond, pi bond becomes lone pair |
| Benzene ring | C6H6 | Alternating double bonds shift around the ring |
The Most Common Mistake Beginners Make
Breaking sigma bonds. Remember: sigma bonds (single bonds between atoms) never break in resonance. Only pi bonds and lone pairs move. If you find yourself moving an atom or breaking a single bond, you are drawing a different molecule (a constitutional isomer), not a resonance structure. …