VSEPR: Lone-Pair Repulsion & Bond-Angle Compression

Key Question Raised: “When we have these pairs on our structure, what happens to the bond angle?”
- Central theme: how the presence of lone (non-bonding) electron pairs affects molecular bond angles.
VSEPR Framework Reminder
- Electron pairs (bonding and non-bonding) arrange themselves to minimize repulsion.
- Ideal geometries assume only bonding pairs; lone pairs introduce additional repulsive forces.
Hierarchy of Repulsive Strength (strongest → weakest)
- $\text{LP–LP} > \text{LP–BP} > \text{BP–BP}$
- Greater repulsion ⇒ greater compression of adjacent bond angles.

General Observation: “As a result of this repulsion, bond angles become excessively smaller than the ideal values.”
Typical Deviations
- Tetrahedral baseline: $109.5^{\circ}$ .
- One lone pair (e.g.
- $\mathrm{NH_3}$ , trigonal-pyramidal): $\approx 107^{\circ}$ .
- Two lone pairs (e.g.
- $\mathrm{H_2O}$ , bent): $\approx 104.5^{\circ}$ .
Qualitative Trend: each additional lone pair pushes bonded atoms closer together, further reducing the bond angle.

Bond angles are diagnostic: deviations signal lone pairs or multiple lone pairs on the central atom.
Practical uses
- Predicting molecular shape and polarity.
- Rationalizing reactivity (sites of highest electron density often drive chemical behavior).
Visualization Tip
- Imagine each lone pair as an “elbow” taking up more space than a bond, forcing other atoms inward.

Builds directly on the VSEPR postulate covered earlier: “Regions of electron density arrange to minimize repulsion.”
Reinforces the importance of distinguishing electron-domain geometry (e.g., tetrahedral) from molecular geometry (e.g., trigonal pyramidal, bent).

Ideal tetrahedral angle: $109.5^{\circ}$ .
Empirical values:
- $\mathrm{NH_3}$ : $107^{\circ}$ .
- $\mathrm{H_2O}$ : $104.5^{\circ}$ .