In-Depth Notes on Bonding Theories: VBT and MOT

Valence Bond Theory (VBT)

• Definition: VBT describes the sharing of electrons between two atoms with half-filled orbitals.
• Bond Strength: Stronger bonds arise from greater overlap of orbitals, leading to an optimal internuclear distance (e.g., for diatomic hydrogen, it's 74 picometers).
• Bond Energy: The maximum bond energy occurs at this optimal distance.

Hybrid Atomic Orbitals

• Hybridization: VBT explains molecular structures and bonding angles through hybridization.
• Molecules Examples:
  • Water (H₂O) and Methane (CH₄):
  • Expected bond angles for H-O-H should be 90°, but actually, bond angles are
    around 109.5° due to tetrahedral geometry.
  • Hybrid Orbitals: 2s and 2p orbitals hybridized to form sp³ orbitals in CH₄.
• Bond Formation: Carbon typically forms four bonds, which include sigma (σ) and pi (π) bonds.

Hybridization Types

• sp³ Hybridization:
  • Example: CH₄
  • Geometry: Tetrahedral, with 109.5° angles.
• sp² Hybridization:
  • Example: C₂H₄ (Ethylene)
  • Bonds: 3 σ bonds + 1 π bond, with 120° angles.
• sp Hybridization:
  • Example: C₂H₂ (Acetylene)
  • Bonds: 2 σ bonds + 2 π bonds, linear geometry.
• sp³d Hybridization:
  • Example: PCl₅
  • Five σ bonds from sp³d hybrid orbitals.
• sp³d² Hybridization:
  • Example: SF₆
  • Six σ bonds from sp³d² hybrid orbitals.

Creation of Lewis Structures

• Counting Electron Groups:
  • Each lone pair, single bond, double bond, or triple bond counts as one electron group.
• Hybridization Type Based on Electron Groups:
  • 2 Electron Groups: sp (Linear)
  • 3 Electron Groups: sp² (Trigonal Planar)
  • 4 Electron Groups: sp³ (Tetrahedral)
  • 5 Electron Groups: sp³d (Trigonal Bipyramidal)
  • 6 Electron Groups: sp³d² (Octahedral)
• Molecular Geometry can differ from electron geometry based on lone pairs on the central atom.

Valve Shell Electron Pair Repulsion (VSEPR) Theory

• VBT Road Map:
  • Lewis Symbol → Lewis Structure → Electron Groups = #Lone Pairs + Bonding Groups → Hybridization Type → Electron Geometry → Molecular Geometry
• Molecular Geometry Types depend on lone pairs.

Summary of Hybridization Types

• Regions of Electron Density vs. Hybridization:
  • 2 → Linear → sp (180°)
  • 3 → Trigonal Planar → sp² (120°)
  • 4 → Tetrahedral → sp³ (109.5°)
  • 5 → Trigonal Bipyramidal → sp³d (120° / 90°)
  • 6 → Octahedral → sp³d² (90°)

Molecular Orbital Theory (MOT)

• Contrast with VBT:
  • VBT considers bonds localized between two atoms, while MOT views electrons as delocalized across the molecule.
  • Bonds from overlap of atomic orbitals leading to formation of molecular orbitals (σ, σ, π, π).
• Bonding and Antibonding Interactions: Determined by which orbitals are filled.
• Paramagnetism Prediction: MOT can predict that O₂ is paramagnetic due to unpaired electrons.

Bond Order and its Implications

• Bond Order Calculation: Number of bonding electrons minus antibonding electrons divided by 2.
  • Example:
  • Li₂ → Bond Order = 1
  • Be₂ → Bond Order = 0
  • B₂ → Bond Order = 1
  • C₂ → Bond Order = 2
  • N₂ → Bond Order = 3
  • O₂ → Bond Order = 2
  • F₂ → Bond Order = 1
  • Ne₂ → Bond Order = 0
• Observations:
  • As bond order increases: Bond energy increases, bond length decreases.

Molecular Orbital Diagrams for Period 2 Elements

• Important to note the energy levels of σ and π orbitals for diatomic molecules.
• Example of bond lengths for O₂ and F₂, which are shorter due to increased effective nuclear charge (Z_eff).

Additional Observations

• Molecular orbitals (bonding and antibonding) play crucial roles in determining the properties of diatomic and polyatomic molecules.
• Knowing the hybridization and electronic configuration of molecules aids in deducing their chemical behaviors and reactivity.