Class 11 Chapter 3 Valence Bond theory and its Drawbacks
Introduction
The Valence Bond Theory (VBT), proposed by Heitler, London and developed by Pauling and Slater, explains covalent bonding as the overlap of half-filled atomic orbitals. It builds on the idea that when atomic orbitals overlap, electron pairs localize between nuclei, forming a bond.
Valence Bond Theory Fundamentals
- Orbital Overlap: Covalent bond strength depends on the extent of overlap between atomic orbitals.
- Bond Formation: Two half-filled orbitals (one from each atom) overlap and their electrons pair with opposite spins.
- Directional Bonds: Explains directional character—e.g., sp3 overlap in methane leads to tetrahedral shape.
- Resonance: Handles delocalization via resonance hybrids of multiple VB structures.
Hybridization Concept
To explain molecular geometry, VBT introduces hybrid orbitals:
| Hybridization | Orbital Mix | Geometry |
|---|---|---|
| sp | 1 s + 1 p | Linear (180°) |
| sp2 | 1 s + 2 p | Trigonal planar (120°) |
| sp3 | 1 s + 3 p | Tetrahedral (109.5°) |
| dsp2 | 1 d + 1 s + 2 p | Square planar (90°) |
| sp3d | 1 s + 3 p + 1 d | Trigonal bipyramidal |
Hybridization explains shapes of molecules like CH4, BF3, NH4+, and [Ni(CN)4]2–.
Applications of VBT
- Explains directional nature of covalent bonds and observed molecular geometries.
- Accounts for magnetic properties in molecules through unpaired electrons in orbitals.
- Describes resonance in molecules like benzene via VB structures.
Drawbacks of VBT
- Quantitative Limitations: Does not accurately predict bond energies and lengths in many cases.
- Delocalization Handling: Although resonance is invoked, VBT struggles with fully delocalized π-systems (e.g., benzene’s equal bonds).
- Integral Overlap Integral: Requires overlap integrals that are difficult to compute precisely.
- Inadequate for Transition Metal Complexes: Fails to predict actual magnetic behavior and spectra without recourse to Ligand Field / Crystal Field theories.
- Neglect of Electron Correlation: Treats electrons pairing classically and misses correlation effects captured by MO Theory.
Conclusion
Valence Bond Theory provides a useful, intuitive picture of covalent bonding and molecular shapes via orbital overlap and hybridization. However, its quantitative inaccuracies and challenges in addressing delocalization and transition metal chemistry paved the way for the more comprehensive Molecular Orbital Theory.
