Valence Bond Theory - Sigma Framework
Overview
Valence Bond (VB) Theory describes chemical bonding as the overlap of atomic orbitals, forming bonds between atoms.
Key Concepts
- Orbital Overlap: A bond forms when atomic orbitals of two atoms overlap.
- Sigma ($\sigma$) Bonds: Strongest type of covalent bond, formed by end-to-end (head-on) orbital overlap.
- Hybridization: Mixing of atomic orbitals to form hybrid orbitals for bonding.
Types of Hybridization and Geometry
| Hybridization | Orbitals Mixed | Geometry | Example |
|---|---|---|---|
| sp | 1 s + 1 p | Linear (180°) | BeCl2 |
| sp2 | 1 s + 2 p | Trigonal planar (120°) | BF3 |
| sp3 | 1 s + 3 p | Tetrahedral (109.5°) | CH4 |
| sp3d | 1 s + 3 p + 1 d | Trigonal bipyramidal | PCl5 |
| sp3d2 | 1 s + 3 p + 2 d | Octahedral | SF6 |
Formation of Sigma Bonds
- s-s Overlap: Two s orbitals overlap (e.g., H2 molecule).
- s-p Overlap: An s and a p orbital overlap (e.g., HCl).
- p-p Overlap: Two p orbitals overlap head-on (e.g., Cl2).
- Hybrid Orbital Overlap: Hybridized orbitals form stronger sigma bonds (e.g., CH4).
Properties of Sigma Bonds
- Stronger than $\pi$ bonds due to maximum overlap.
- Allow free rotation around the bond axis.
- Present in all single bonds (e.g., C-C, C-H in methane).
Applications
- Explains molecular geometries based on orbital overlap.
- Predicts bond strength and stability in molecules.
- Helps understand chemical reactivity, e.g., why C-H bonds are strong but reactive in combustion.