Question

The resonance structures (a) Differ only in the arrangement of electrons (b) Differ in number of paired and unpaired electrons (c) Differ largely in their energy content (d) Do not lie in the same plane

Short answer

(a) Differ only in the arrangement of electrons.

Step-by-step solution

Understanding Resonance Structures

Resonance structures are different ways to draw the same molecule showing different possible arrangements of electrons. Each resonance structure depicts a possible arrangement of electrons while maintaining the same skeletal framework of the molecule.

Analyzing Options

Examine each option: (a) Resonance structures must have the same arrangement of atoms and only differ in electron positioning. (b) Resonance structures must have the same number of paired and unpaired electrons because they represent the same molecule with the same electron count. (c) Ideally, resonance structures should have similar energy content as they represent potential forms of the same molecule. (d) All resonance structures typically lie in the same plane as they represent the same molecule.

Correct Answer Deduction

After analyzing each option, it should be clear that the correct characteristic of resonance structures is that they differ only in the arrangement of electrons, but all other factors like pairing of electrons, energy content, and molecular framework should remain constant.

Key concepts

Electron Arrangement in Resonance Structures

Resonance structures are fascinating because they show different ways electrons can be arranged in a molecule without changing the actual connections between the atoms. This means that while electrons, such as those in π bonds, may shift positions, the overall skeleton—the molecular framework—of the molecule remains the same. For example, in a molecule like benzene, resonance structures depict the possible location shifts of electrons around the ring from double to single bonds and vice versa. This helps in understanding the delocalization of electrons, a significant factor in the stability of molecules. When thinking about electron arrangement in resonance, remember:

• Atoms do not move, only electrons do.
• The number of electrons stays the same across all resonance structures.
• Each valid structure contributes to the overall properties of the molecule.

These properties give us a clearer picture of what the molecule might actually look like in reality, which is often a hybrid of all valid resonance structures.

Paired and Unpaired Electrons in Resonance

An interesting concept in resonance structures is the idea of paired and unpaired electrons. Regardless of the complexity of electron movement within different resonance forms, the total number of electrons must remain constant, and so must the count of paired and unpaired electrons. This requirement ensures that all resonance structures of a molecule reflect the same electron count and symmetry. For example:

• If a molecule contains paired electrons in one structure, they should remain paired in all other resonance forms.
• Similarly, any unpaired electrons in a free radical must persist across all resonance structures.

This concept is critical because it confirms that even though the electrons may shift positions, the fundamental electron distribution in terms of pairing does not change. This uniformity across resonance structures underscores the idea that they are simply different expressions of the same molecular reality.

Energy Content of Resonance Structures

When examining resonance structures, one might wonder if they differ in energy content. In an ideal scenario, all resonance contributors have similar energy levels, as they are representations of the same molecule. However, due to variations in electron distribution, some structures may be more stable than others, lending more "weight" to the hybrid structure. Here's what you need to know:

• More stable resonance structures have lower energy and contribute more to the true structure.
• Structures with complete octets or more optimal electron arrangements typically experience lower energy.
• The overall energy of the actual molecule is an average, often lower than any one resonance structure alone.

This concept clarifies why resonance structures are crucial; they provide insight into the molecule's most stable form by demonstrating potential electron configurations.

Molecular Framework in Resonance

A key aspect of resonance is the rigid molecular framework. Resonance structures do not alter the positions of atoms; they only show different depictions of electron distribution. This aspect maintains the molecule's integrity while displaying multiple electronic configurations. The framework of the molecule acts as an anchor around which the electrons can shift. Here are some important points to understand:

• The atoms' positions remain stationary in all resonance forms.
• The geometry of the molecule influences the ability of electrons to delocalize.
• Because the framework does not change, resonance is about showing alternative electron paths within the same structural confines.

This concept is essential, as it illustrates that resonance structures are not depictions of different molecules, but rather microscopic arrangements within the same, consistent atomic layout.