Short Answer

Some molecules show resonance because a single Lewis structure cannot correctly represent how their electrons are arranged. These molecules have electrons that can be placed in more than one position, especially when double bonds or lone pairs can shift. As a result, multiple valid Lewis structures, called resonance structures, are needed.

Resonance occurs to stabilise the molecule by spreading or delocalising electrons across several atoms. This electron delocalisation reduces charge concentration, increases stability, and makes bond lengths more equal. The actual molecule is a combination of all resonance structures known as a resonance hybrid.

Detailed Explanation :

Why Some Molecules Show Resonance

Some molecules show resonance because their bonding and electron arrangements cannot be accurately described by a single Lewis structure. Instead of electrons being fixed between two atoms, in these molecules electrons are delocalised, meaning they are spread out across multiple atoms. This delocalisation gives the molecule special stability and unique bonding characteristics that cannot be shown with just one structure.

Resonance is not a movement of atoms or switching back and forth between structures. Rather, the molecule exists as a resonance hybrid, which is a blend or average of all possible resonance forms. Understanding why molecules show resonance helps explain their stability, equal bond lengths, charge distribution, and reactivity.

1. Presence of Multiple Valid Lewis Structures

A molecule shows resonance when more than one valid Lewis structure can be drawn for it.
This happens when:

• Atoms follow bonding rules
• The total number of electrons remains the same
• The arrangement of atoms does not change
• Only the positions of electrons vary

For example, in ozone (O₃), the double bond can be drawn on either the left or the right oxygen. Both structures are correct, so resonance must be considered.

2. Electron Delocalisation

Resonance occurs because electrons can be shared by more than two atoms.
This is called electron delocalisation.

Delocalisation is possible when:

• There is a double bond next to a single bond
• Lone pairs can shift into bonding positions
• Empty or partially filled orbitals are available
• Charges can be distributed over several atoms

This spreading out of electrons lowers the energy of the molecule and increases stability.

For example:
In the nitrate ion (NO₃⁻), the double bond can be placed between nitrogen and any of the three oxygen atoms. Electrons are delocalised over all three oxygens.

3. Presence of Conjugated Systems

Many molecules with alternating single and double bonds (called conjugated systems) show resonance.
Examples include:

• Benzene (C₆H₆)
• Butadiene (C₄H₆)

In these molecules, electrons in π-bonds overlap over several atoms, allowing electrons to shift freely and form resonance structures. Conjugation increases stability and affects molecular shape, reactivity, and colour.

4. Distribution of Charge

Resonance helps spread out negative or positive charges over multiple atoms.
A single Lewis structure may show a charge concentrated on one atom, which is unstable. By spreading this charge across several atoms, the molecule becomes more stable.

For example:
In the carbonate ion (CO₃²⁻), the −2 charge is not confined to one oxygen. Instead, resonance distributes the charge across all three oxygen atoms.

This reduced charge concentration makes the ion much more stable.

5. Stability Through Lower Energy

Resonance stabilises a molecule because delocalised electrons:

• Reduce electron–electron repulsion
• Spread energy evenly across the molecule
• Allow atoms to share the bonding responsibilities
• Create intermediate bond strengths

The resonance hybrid always has lower energy than any individual resonance structure.

For example:
Benzene has equal carbon–carbon bond lengths due to resonance, making it extremely stable compared to other hydrocarbons with alternating single and double bonds.

6. Octet Completion and Better Bonding Arrangements

Sometimes a single Lewis structure does not allow all atoms to complete their octet. Resonance provides alternative structures in which the octet rule can be satisfied or better approximated.

For example:
In NO₂, one structure gives nitrogen only 6 electrons, but a resonance structure provides a double bond, completing the octet more effectively.

Resonance therefore helps explain correct electronic arrangements that simple structures cannot.

Examples of Molecules Showing Resonance

• Ozone (O₃) – double bond shifts between two oxygens
• Nitrate ion (NO₃⁻) – three equal resonance structures
• Carbonate ion (CO₃²⁻) – delocalisation across three oxygens
• Benzene (C₆H₆) – electrons spread evenly in a ring
• Acetate ion (CH₃COO⁻) – negative charge shared by two oxygens

Each example shows electrons not belonging to just one bond or atom.

Conclusion

Molecules show resonance when their electrons can be placed in more than one valid arrangement, resulting in multiple Lewis structures. Resonance occurs due to electron delocalisation, conjugated systems, and the need to distribute charges or complete octets. The actual molecule exists as a resonance hybrid, which is more stable than any single structure. Resonance is essential for understanding the stability, structure, and behaviour of many important molecules.