Problem 20 - A Complete Resource Book in Chemistry for JEE Main

Question

Resonance structures of a molecule does not have (a) identical arrangement of atoms (b) nearly the same energy content (c) same number of paired electrons (d) identical bonding

Step-by-Step Solution

Answer

(d) identical bonding.

Step 1: Understanding Resonance Structures

Resonance structures are different Lewis structures for a molecule that depict the same arrangement of atoms. These structures are used to represent delocalization of electrons within a molecule.

Step 2: Identical Arrangement of Atoms

In resonance structures, the positions of the atoms do not change. Only the distribution of electrons among the atoms changes, while the connectivity remains constant. Therefore, resonance structures have identical arrangement of atoms.

Step 3: Energy Content Analysis

Resonance structures of a molecule contribute similarly to the overall structure, and they typically have nearly the same energy. Minor energy differences may exist but are generally negligible.

Step 4: Paired Electrons Consistency

Resonance structures maintain the same number of paired electrons, as they represent different forms of electron distribution in the same molecular framework.

Step 5: Identical Bonding Check

Resonance structures depict different electron distributions over the same atomic framework, which means the depiction of single, double, or triple bonds may vary between structures. Hence, they do not necessarily have identical bonding as seen in Lewis structures.

Step 6: Conclusion: Identify the Correct Option

After analyzing, we find that the resonance structures do not have identical bonding. Therefore, option (d) is correct.

Key Concepts

Lewis structures

Lewis structures are a method to represent molecules showing both the atomic connectivity and the electron distribution. The structure of each molecule has unique arrangements, where atoms are placed in a way that represents actual chemical bonds.

It uses dots to symbolize electrons and lines to denote chemical bonds. This method is essential for visualizing the concept of resonance structures, as it allows chemists to conceptualize different possible layouts of electron distribution without altering the positions of atoms. One key aspect of Lewis structures is that they help predict the necessary arrangement of valence electrons to satisfy each atom's requirements, most notably adhering to the octet rule, which suggests that atoms strive to have eight electrons in their valence shell.

• Lines represent bonds between atoms.
• Dots show lone pairs or non-bonding electrons.
• Illustrates the concept of resonance by showing different possible arrangements.

Using Lewis structures can be immensely instructive in determining the formal charge on each atom and helps to conceptualize the chemical nature of the molecule in question.

electron distribution

Electron distribution in molecules refers to the way electrons are spread out among the atoms in a molecule. This is crucial in the study of resonance structures, as different resonance forms show different possible distributions. This concept is crucial when visualizing how electrons might move or be shared between atoms in various configurations. For resonance structures, the atomic arrangement remains unchanged, while electrons are delocalized over the atoms. Each resonance structure is not isolated, but rather, contributes to a hybrid, which is the true representation of electron distribution in the molecule.

This hybridization results in a more stabilizing effect, decreasing the potential energy of the molecule. For many compounds, especially those involving conjugated systems, multiple resonance structures can be drawn, indicating the potential pathways of electron delocalization.

• Electrons are shared by overlapping orbitals, forming covalent bonds.
• In resonance, electrons can "move" between different positions.
• Different structures help in visualizing possible electron locations.

Overall, understanding electron distribution through resonance provides deeper insights into a molecule's reactivity and chemistry.

atomic framework

The atomic framework of a molecule refers to the fixed structural backbone made up of bonded atoms. In resonance structures, even though electron distribution varies between them, the overall atomic framework stays constant. This is an essential aspect as it ensures that the core structure, including the connectivity of atoms, is not altered.

In practice, while drawing resonance structures, the atoms remain in their precise positions and only the electrons, usually those involved in pi bonds or lone pair electrons, shift their locations. Consistently relying on the same atomic framework across different resonance forms provides clarity and negates confusion.

• The skeleton of the molecule does not change.
• Atomic positions remain fixed, guiding electron redistribution.
• Maintains structural integrity between different resonance structures.

This stability across resonance structures gives chemists a reliable way to predict the behavior of a molecule under various conditions, reinforcing the significance of a consistent atomic framework.