Problem 108 - General Chemistry: Principles and Modern Applications

Question

Construct a concept map that describes the interconnection between valence- bond theory and molecular orbital theory in the description of resonance structures.

Step-by-Step Solution

Answer

The valence bond theory and molecular orbital theory interconnect in the description of resonance structures through the idea of electron delocalization. While the valence bond theory can describe basic electron sharing, the molecular orbital theory gives us more detailed insight into electron behavior including delocalization, permitting a comprehensive understanding of resonance in molecules. A concept map would showcase these theories and their interconnections in the depiction of resonance.

Step 1: Understanding Valence Bond Theory

The valence bond theory describes that bonding occurs as a result of electron pair sharing between the atoms, producting a molecular structure that is a composite of the atomic orbitals from the bonding atoms.

Step 2: Understanding Molecular Orbital Theory

While, the Molecular Orbital Theory proposes that atomic orbitals from the bonding atoms combine to form new molecular orbitals that span over the entire molecule, both bonding and anti-bonding molecular orbitals.

Step 3: Defining the Concept of Resonance

Resonance is a way of describing delocalized electrons within certain molecules or polyatomic ions where the bonding cannot be expressed by a single Lewis structure. Rather, a molecule with resonance is actually a hybrid of different structures.

Step 4: Interconnection between the Theories

Valence bond theory and molecular orbital theory both offer a depiction of how the electrons are arranged in a molecule, and can be used synergistically to explain the concept of resonance. While valence bond theory can explain the localized electron-sharing, molecular orbital theory extends this idea to include delocalized electrons, thus allowing a better insight into resonance phenomena.

Step 5: Constructing a Concept Map

Now, based on the understanding of both the theories and resonance, a concept map is constructed. This concept map should denote Valence Bond Theory and Molecular Orbital Theory as two individual nodes. Resonance should be an interconnected node drawing features from both the theories. This explains the interconnected nature of these theories in describing resonance structures.

Key Concepts

Molecular Orbital Theory

Molecular Orbital Theory offers a unique view of chemical bonding that differs from other approaches like Valence Bond Theory. This theory suggests that when atoms bond, their atomic orbitals combine to form new molecular orbitals. These orbitals belong to the entire molecule rather than to individual atoms.
The molecular orbitals can be either bonding or anti-bonding.

• Bonding orbitals are lower in energy and help hold the atoms together.
• Anti-bonding orbitals are higher in energy and can, under certain conditions, repel the atoms apart.

The significance of molecular orbital theory lies in its ability to describe electron delocalization. This characteristic greatly enhances our understanding of phenomena such as resonance, where electrons are not localized between atoms but spread over several atoms in a molecule.

Resonance Structures

Resonance structures are a way to represent certain molecules where electron pairs are not confined to a single atom or bond. Instead, the electrons are spread across multiple bonds or atoms, creating a resonance hybrid.
Typically, a molecule with resonance cannot be adequately described by a single Lewis structure. Instead, it must be depicted by two or more resonance structures that blend to form this hybrid.

• Each resonance structure follows normal Lewis structure rules but represents a different configuration of electron placement.
• In the resonance hybrid, the actual distribution of electrons is a blend or average of these resonance structures, resulting in greater stability for the molecule.

This concept is critical because it enables chemists to conceptualize molecules that have more distributed electron density, which often helps in stabilizing the molecule due to electron delocalization.

Electron Delocalization

Electron delocalization is a fundamental concept that explains how electrons are not confined to a single bond or location but can move over a range of atoms. This movement is a cornerstone of the concept of resonance and helps explain why molecules with resonance structures are more stable than those without.
In molecules exhibiting electron delocalization:

• Electrons are free to move across multiple bonds or atoms, which spreads the electron density over a larger area.
• This distribution can reduce potential energy and thereby enhance the stability of the molecule.

From a molecular orbital perspective, delocalization allows electrons to occupy orbitals that encompass several atoms, thus ensuring greater overlap and stronger bonds. Recognizing electron delocalization is crucial for understanding why resonance structures contribute significantly to a stable and lower energy state for molecules.