4.1 Valence bond theory and molecular orbital theory

Chemical bonding is the glue that holds atoms together in molecules. Valence bond theory and molecular orbital theory are two approaches to understanding this glue. They explain how electrons interact to form bonds, helping us predict molecular shapes and properties.

These theories build on the basics of covalent, ionic, and metallic bonds. While valence bond theory focuses on localized electron pairs, molecular orbital theory looks at electrons spread across entire molecules. Both help us grasp how atoms stick together in different ways.

Valence Bond Theory

Fundamental Principles and Concepts

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• Valence bond theory (VBT) describes chemical bonding as the overlap of atomic orbitals to form localized electron pairs, resulting in the formation of sigma (σ) and pi (π) bonds between atoms
• VBT focuses on localized electron pairs and the overlap of atomic orbitals to explain the formation and properties of chemical bonds
• The concept of hybridization in VBT explains the mixing of atomic orbitals to form new hybrid orbitals with specific geometries, such as sp, sp², and sp³, which participate in bonding
• Hybridization allows VBT to predict and explain the geometry and bonding in molecules (BeH₂, BF₃, CH₄)

Applications and Limitations

• VBT is more intuitive and easier to visualize for simple molecules compared to molecular orbital theory
• VBT can be used to predict the geometry and hybridization of simple molecules, such as BeH₂ (linear, sp hybridization), BF₃ (trigonal planar, sp² hybridization), and CH₄ (tetrahedral, sp³ hybridization)
• VBT explains the formation of multiple bonds (double and triple bonds) in molecules like ethene (C₂H₄) and ethyne (C₂H₂) through the overlap of sp² and sp hybridized orbitals, respectively
• VBT struggles to explain the properties of molecules with unpaired electrons or those that exhibit resonance, as it primarily focuses on localized electron pairs
• VBT does not directly address paramagnetic and diamagnetic properties in molecules, which are better explained by molecular orbital theory

Molecular Orbital Theory

Fundamental Principles and Concepts

• Molecular orbital theory (MOT) describes chemical bonding as the linear combination of atomic orbitals (LCAO) to form delocalized molecular orbitals that extend over the entire molecule
• In MOT, the wave functions of atomic orbitals are combined to form bonding and antibonding molecular orbitals, with bonding orbitals having lower energy and contributing to the stability of the molecule
• The energy ordering of molecular orbitals follows specific rules, such as the aufbau principle, Hund's rule, and the Pauli exclusion principle, which govern the filling of electrons in these orbitals
• MOT provides a more accurate description of the electronic structure, especially for molecules with multiple atoms or delocalized electrons

Applications and Predictions

• MOT can be applied to construct molecular orbital diagrams for diatomic molecules, such as H₂, N₂, O₂, and F₂, and determine their bond order, stability, and magnetic properties
• The reactivity and stability of molecules can be predicted based on their frontier molecular orbitals (highest occupied molecular orbital, HOMO, and lowest unoccupied molecular orbital, LUMO) using MOT
• MOT can predict the electronic transitions and spectroscopic properties of simple molecules based on the energy differences between occupied and unoccupied molecular orbitals
• MOT effectively describes the properties of molecules with unpaired electrons or those that exhibit resonance, which VBT struggles to explain
• MOT can predict the existence of paramagnetic and diamagnetic properties in molecules based on the presence of unpaired electrons in molecular orbitals

Valence Bond vs Molecular Orbital Theory

Key Differences and Comparisons

• VBT focuses on localized electron pairs and the overlap of atomic orbitals, while MOT considers the formation of delocalized molecular orbitals that extend over the entire molecule
• VBT uses the concept of hybridization to explain the geometry and bonding in molecules, while MOT does not explicitly incorporate hybridization
• VBT is more intuitive and easier to visualize for simple molecules, while MOT provides a more accurate description of the electronic structure, especially for molecules with multiple atoms or delocalized electrons
• VBT struggles to explain the properties of molecules with unpaired electrons or those that exhibit resonance, while MOT can effectively describe these phenomena
• MOT can predict the existence of paramagnetic and diamagnetic properties in molecules based on the presence of unpaired electrons in molecular orbitals, while VBT does not directly address these properties

Strengths and Weaknesses

• VBT is more accessible and easier to apply for simple molecules, while MOT requires a deeper understanding of quantum mechanics and mathematical concepts
• VBT provides a clear picture of the localized bonding in molecules, while MOT offers a more comprehensive description of the electronic structure and properties
• MOT is better suited for explaining the properties of molecules with delocalized electrons, multiple atoms, or unusual bonding situations, while VBT is limited in these aspects
• VBT and MOT complement each other, and a combination of both theories is often used to gain a complete understanding of chemical bonding in molecules

Electronic Structure of Molecules

Molecular Orbital Analysis

• The electronic configuration of molecules can be analyzed using MOT by filling molecular orbitals according to the aufbau principle, Hund's rule, and the Pauli exclusion principle
• The bond order of molecules can be determined using MOT by calculating the difference between the number of bonding and antibonding electrons, and relate bond order to the stability and bond length of the molecule
• MOT explains the paramagnetic behavior of molecules like O₂ based on the presence of unpaired electrons in the molecular orbitals
• Photoelectron spectra of molecules can be interpreted using MOT, relating the observed ionization energies to the energies of the occupied molecular orbitals

Valence Bond Interpretation

• Resonance structures of molecules like benzene can be interpreted using VBT, explaining the delocalization of electrons and enhanced stability through resonance
• VBT provides a clear understanding of the localized bonding in molecules, particularly for simple molecules with well-defined bonding patterns
• The concept of hybridization in VBT helps predict and explain the geometry and bonding in molecules, such as the tetrahedral arrangement in CH₄ due to sp³ hybridization
• VBT offers an intuitive approach to visualizing the formation of multiple bonds (double and triple bonds) in molecules like ethene (C₂H₄) and ethyne (C₂H₂) through the overlap of hybridized orbitals