Question

A molecule has two bonds of equal length, but its Lewis structure shows one single bond and one double bond. This is best explained by:

• A. Resonance
• B. Hybridisation
• C. Polarisation
• D. Ionization

Hint:

Resonance leads to: \[ \boxed{ \begin{aligned} &\text{Equal bond lengths}\\ &\text{Fractional bond order}\\ &\text{Delocalisation of electrons}\\ &\text{Greater molecular stability} \end{aligned} } \] Common examples: \[ \boxed{O_3,\;CO_3^{2-},\;NO_3^{-},\;NO_2^{-},\;C_6H_6} \]

Answer 1

The Correct Option is A

Concept: Resonance is the phenomenon in which a molecule cannot be represented accurately by a single Lewis structure. Instead, it is represented by two or more contributing (canonical) structures. The actual molecule is a resonance hybrid of all contributing structures. As a result,

• the electrons become delocalised,

• bond lengths become equal,

• bond order becomes fractional.

Step 1: Analyse the given information.
The Lewis structure shows

• one single bond,

• one double bond.
However, experimentally both bonds have the same length. This indicates that electrons are not confined to one bond.

Step 2: Explain using resonance.
Because of resonance, the bonding electrons are delocalised over the entire molecule. Therefore, both bonds become equivalent. A common example is the carbonate ion, \[ CO_3^{2-}, \] where all three C--O bonds have equal lengths. Similarly, \[ O_3 \] and \[ NO_2^{-} \] also exhibit equal bond lengths due to resonance.

Step 3: Choose the correct option.
Equal bond lengths despite different Lewis structures can only be explained by \[ \boxed{\text{Resonance}.} \] Hence, \[ \boxed{\textbf{Option (A)}} \] is the correct answer.