Lecture 3a: Resonance

What is resonance in chemistry?

A way of representing molecules using multiple contributing structures that differ only in electron positions, not atom positions.

Do resonance structures exist individually?

No — they are hypothetical; the real molecule is a resonance hybrid.

What is a resonance hybrid?

The actual structure of the molecule, a blend of all valid resonance contributors, more stable due to electron delocalisation.

Why is resonance important?

• Explains stability

• Affects reactivity

• Influences electron distribution

• Predicts geometry and bond lengths

What moves in resonance structures?

Only electrons (π electrons and lone pairs). Atoms do not move.

What must remain the same in all resonance structures?

• Atom positions

• Atom connectivity

• Total number of valence electrons

What rule must each resonance structure obey?

It must be a valid Lewis structure (octet rule, correct formal charges).

What bonds cannot be broken or formed when drawing resonance structures?

σ (sigma) bonds — connectivity must remain unchanged.

What makes a major resonance contributor?

• Full octets

• Minimal formal charges

• Negative charge on electronegative atoms

What makes a minor resonance contributor?

• Incomplete octets

• High formal charges

• Less favourable charge placement

What is the effect of resonance on stability?

Delocalisation lowers energy → increases stability (resonance stabilisation

Where do curved arrows start and end in resonance?

• Start: lone pair or π bond

• End: atom or bond where electrons move to form a new π bond or lone pair

What are common areas of delocalisation?

• π bonds

• Lone pairs next to π bonds or positive charges

• Conjugated systems

What are the three common electron‑movement patterns in resonance?

• Lone pair → forms a π bond

• π bond → becomes a lone pair

• π bond → shifts to form a new π bond elsewhere

Why do atoms prefer full octets in resonance structures?

Full octets lower energy and increase stability, making those contributors more significant.

What is the relationship between resonance and conjugation?

Conjugated systems allow electron delocalisation, making resonance possible and stabilising the molecule.