Lesson 4 of 12 · 7 min
Resonance structures
NCERT § "Resonance Structures"
High above Riya's city, the air holds ozone, O₃. Draw its Lewis structure and one O–O bond comes out single, the other double. Measure the molecule and the two bonds are identical.
The lesson in notes
In short
If no one Lewis structure can describe a molecule, several structures are drawn that share the same arrangement of nuclei but place the electrons differently; the real molecule is a hybrid of them.
Ozone has two equal O–O bonds of 128 pm, between a single (148 pm) and a double (121 pm) O–O bond, which no single Lewis structure can show.
The carbonate ion has three identical C–O bonds, and CO₂ is also described by resonance.
The resonance hybrid has lower energy than any contributing structure, which stabilises the molecule.
Resonance structures do not exist separately and the molecule does not flip between them; there is no equilibrium between the canonical forms.