Benzene is a resonance hybrid mainly of two Kekule structures. Hence

Two structures make equal contribution to resonance hybrid

The correct answer is option 3. Two structures make equal contribution to resonance hybrid.

Benzene \((C_6H_6)\) is an aromatic hydrocarbon that is often represented using two resonance structures, known as Kekulé structures. These structures illustrate benzene as a six-membered ring with alternating single and double bonds. However, the true nature of benzene is more accurately described as a resonance hybrid of these two Kekulé structures. Here’s a detailed explanation:

Resonance in Benzene

Kekulé Structures:

There are two Kekulé structures for benzene:

Structure I: Alternating double and single bonds in a specific pattern.

Structure II: The pattern of double and single bonds is shifted by one position around the ring.

In each structure, there are three double bonds and three single bonds.

Resonance Hybrid:

The true structure of benzene is not accurately depicted by either of these Kekulé structures individually. Instead, benzene is a resonance hybrid, meaning that the actual electron distribution is an average of the two Kekulé structures. In the resonance hybrid, the electrons are delocalized across all six carbon atoms, creating a structure where all the carbon-carbon bonds are equivalent.

Characteristics of the Resonance Hybrid:

Bond Lengths: Experimental data shows that all the C-C bonds in benzene are of equal length (about 1.39 Å), intermediate between typical single and double bond lengths.

Bond Strengths: The bond strength is also intermediate between single and double bonds.

Stability: The delocalization of electrons across the ring provides extra stability to the benzene molecule, a phenomenon known as "resonance stabilization."

Addressing the options

1. Half the molecules correspond to one structure and half to the other structure: This is incorrect. Benzene does not exist as a mixture of two distinct structures. Each molecule of benzene is a resonance hybrid of the two Kekulé forms.

2. At low temperatures benzene can be separated into two structures: This is incorrect. Benzene cannot be separated into two different structures, even at low temperatures. The resonance hybrid nature is an intrinsic property of benzene at all temperatures.

3. Two structures make equal contribution to the resonance hybrid: This is correct. Both Kekulé structures equally contribute to the resonance hybrid, leading to a more stable, delocalized electron structure.

4. An individual benzene molecule changes back and forth between two structures: This is incorrect. Resonance does not imply that the molecule oscillates between structures. Instead, the true structure is a stable blend of the possible resonance forms.

In summary, benzene’s stability and equal bond lengths arise from the equal contribution of the two Kekulé structures to the resonance hybrid. This delocalized structure provides benzene with its unique chemical properties.