VSEPR Theory & Hybridization
Chemistry – COMEDK
1. Simple Explanation
| Concept | What it is | Why it matters |
|---|---|---|
| VSEPR (Valence‑Shell Electron‑Pair Repulsion) | A theory that predicts the 3‑D arrangement of atoms around a central atom by treating bonding pairs and lone pairs of electrons as “repelling” each other. | Gives the molecular shape (e.g., bent, trigonal planar) and predicts bond angles. |
| Hybridization | The mixing of atomic orbitals (s, p, d) on the central atom to form new, equivalent hybrid orbitals that host the valence electron pairs. | Explains how bond angles and molecular shapes can be derived from the types of hybrid orbitals (sp, sp², sp³, sp³d, sp³d²). |
In a nutshell, VSEPR tells where the electron pairs will sit; hybridization tells how the central atom’s orbitals adjust to accommodate those pairs.
2. Key Points
2.1 VSEPR Basics
| Step | What to do | Example |
|---|---|---|
| Count electron domains | Count bonding pairs + lone pairs around the central atom. | In H₂O: 2 bonds + 2 lone pairs = 4 domains. |
| Determine electron‑domain geometry | 1, 2, 3, 4, 5, or 6 domains → Linear, Trigonal planar, Tetrahedral, Trigonal bipyramidal, or Octahedral. | 4 domains → Tetrahedral. |
| Determine molecular geometry | Remove lone pairs from the picture. | 4 domains, 2 lone pairs → Bent shape. |
| Predict bond angles | Use ideal angles for the geometry, adjust for lone‑pair repulsion (lone pairs push bonds closer). | Bent H₂O → ~104.5° (vs. 109.5° tetrahedral). |
Rule of thumb: Lone pairs exert more repulsion than bonding pairs.
2.2 Hybridization Rules
| Hybridization | Orbits involved | Number of hybrid orbitals | Typical geometries |
|---|---|---|---|
| sp | 1 s + 1 p | 2 | Linear (180°) |
| sp² | 1 s + 2 p | 3 | Trigonal planar (120°) |
| sp³ | 1 s + 3 p | 4 | Tetrahedral (109.5°) |
| sp³d | 1 s + 3 p + 1 d | 5 | Trigonal bipyramidal (90°/120°) |
| sp³d² | 1 s + 3 p + 2 d | 6 | Octahedral (90°) |
Caveat: Hybridization is a useful mnemonic; for very small molecules (e.g., H₂) it’s not strictly necessary, and for hypervalent molecules (PF₅, SF₆) the “d‑orbitals” may not be heavily involved (just a convenient way to rationalise geometry).