Ozone, O3, is represented by two equivalent resonance Lewis structures in which the central oxygen carries a positive formal charge and one terminal oxygen carries a negative formal charge, making the two O–O bonds equivalent in the resonance hybrid.
Valence-electron accounting
Each oxygen atom contributes 6 valence electrons. For O3, the total valence-electron count is \(3 \times 6 = 18\) electrons.
An O–O–O connectivity (oxygen as the central atom) is consistent with typical Lewis-structure conventions for triatomic molecules and supports an octet on each atom when bonding and lone pairs are placed appropriately.
Resonance description
A single structure cannot simultaneously give all atoms an octet while keeping formal charges minimized without introducing resonance. The common resonance pair has one O–O single bond and one O=O double bond. The double bond can be placed on either side of the central oxygen, producing two equivalent resonance contributors.
Formal charges and octet satisfaction
Formal charge provides a consistent bookkeeping tool for comparing resonance contributors. The formal charge on an atom is
\[ \mathrm{FC}=\left(\text{valence electrons}\right)-\left(\text{nonbonding electrons}+\frac{\text{bonding electrons}}{2}\right) \]
In each resonance form shown above, all three oxygen atoms satisfy an octet, and the formal charges are concentrated as +1 on the central oxygen and −1 on the singly bonded terminal oxygen.
| Atom position | Bonding pattern in a resonance form | Typical lone pairs | Formal charge |
|---|---|---|---|
| Terminal O (single-bonded) | One single bond | 3 lone pairs | \(-1\) |
| Central O | One single bond + one double bond | 1 lone pair | \(+1\) |
| Terminal O (double-bonded) | One double bond | 2 lone pairs | \(0\) |
Bond order, bond length, and equivalence of the O–O bonds
Resonance implies that the actual bonding is a hybrid of the contributors rather than a rapid oscillation between two fixed drawings. With one single bond and one double bond spread over two equivalent O–O connections, the average bond order per O–O bond is
\[ \text{bond order}=\frac{1+2}{2}=1.5 \]
A bond order of 1.5 is consistent with O–O bond lengths that are intermediate between typical single and double O–O bonds, and with equal measured O–O bond lengths in ozone.
Electron-domain geometry and molecular shape
The central oxygen has three electron domains in VSEPR terms: two bonding domains (each bond counts as one domain regardless of being single or double in a resonance drawing) and one lone-pair domain. Three electron domains correspond to a trigonal-planar electron-domain arrangement, and the molecular geometry is bent.
A bent shape prevents complete cancellation of bond dipoles, so ozone is polar even though all atoms are oxygen.
Common pitfalls
- O=O=O with two double bonds shown as a single, complete Lewis structure, which fails the 18-electron count unless additional electrons and charges are introduced.
- Formal charge placement reversed (negative charge on the central oxygen), which does not match the formal-charge calculation for the standard octet-satisfying structures.
- Resonance treated as two different molecules, rather than two contributors to one resonance hybrid with equivalent O–O bonds.