Cornell Note + Wikimedia: VSEPR Theory and Molecular Geometry

Cues / Keywords

Steric number
Lone pair repulsion
Electron geometry vs. molecular shape
Bond angle compression

Main Notes

VSEPR (Valence Shell Electron Pair Repulsion) Theory predicts molecular geometry by minimising repulsion between electron pairs (bonding + lone pairs) around the central atom.

Key rule: Lone pairs occupy more space than bond pairs. Repulsion order: $\text{lp-lp} > \text{lp-bp} > \text{bp-bp}$

Steric Number (SN) Calculation: $\text{SN} = \text{(no. of } \sigma\text{ bonds on central atom)} + \text{(no. of lone pairs on central atom)}$

From SN to hybridization to molecular shape:

SN	Hybridization	Electron Geometry	Molecular Shape (0 lp)	Example
2	sp	Linear	Linear (180°)	$CO_{2}$ , $BeCl_{2}$
3	$sp^{2}$	Trigonal planar	Trigonal planar (120°)	$BF_{3}$ , $SO_{3}$
4	$sp^{3}$	Tetrahedral	Tetrahedral (109.5°)	$CH_{4}$ , $CCl_{4}$
5	$sp^{3}$ d	Trig. bipyramidal	TBP (90°/120°)	$PCl_{5}$
6	$sp^{3}$$d^{2}$	Octahedral	Octahedral (90°)	$SF_{6}$

Effect of lone pairs (SN=4 examples):

Lone Pairs	Shape	Bond Angle	Example
0	Tetrahedral	109.5°	$CH_{4}$
1	Trigonal pyramidal	~107°	$NH_{3}$
2	Bent	~104.5°	$H_{2}O$

Critical NEET point: Always distinguish between electron geometry (based on ALL electron pairs) and molecular shape (based on BOND PAIRS only).

Visual Reference

VSEPR Molecular Geometries

Linear C X X 180° $BeCl_{2}$ , $CO_{2}$ , $CS_{2}$

Bent (V-shaped) O H H

lone pairs 104.5° $H_{2}O$ , $SO_{2}$ , $O_{3}$

Trigonal Planar B F F F 120° $BF_{3}$ , $AlCl_{3}$ , $SO_{3}$

Tetrahedral C

H H H H 109.5° $CH_{4}$, $CCl_{4}$, $NH_{4}^{+}$

Trigonal Bipyramidal P

Cl Cl Cl Cl Cl 90° 120° $PCl_{5}$, $PF_{5}$

Octahedral S

F F F F F F 90° $SF_{6}$, [Co($NH_{3}$)_{6}]^{3+}

Source: Wikimedia Commons — VSEPR geometry chart

Summary / Recall

VSEPR uses SN to find hybridization and electron geometry. Removing lone pairs gives molecular shape. Lone pairs compress bond angles in the order: lp-lp > lp-bp > bp-bp.