VSEPR Theory

Overview

Valence Shell Electron Pair Repulsion (VSEPR) theory predicts molecular geometry based on the repulsion between electron pairs around the central atom. Electron pairs arrange themselves to minimize repulsion.

Key Concepts

Electron Domains

Any of the following count as one electron domain:

• Single bond
• Double bond
• Triple bond
• Lone pair

Steric Number

$$\text{Steric Number} = \text{Bonding domains} + \text{Lone pair domains}$$

Electron Geometry vs Molecular Geometry

• Electron Geometry: Arrangement of ALL electron domains
• Molecular Geometry: Arrangement of ATOMS only (what we observe)

VSEPR Shapes

2 Electron Domains

Bonding	Lone Pairs	Electron Geometry	Molecular Geometry	Bond Angle	Example
2	0	Linear	Linear	180°	CO₂, BeCl₂

3 Electron Domains

Bonding	Lone Pairs	Electron Geometry	Molecular Geometry	Bond Angle	Example
3	0	Trigonal planar	Trigonal planar	120°	BF₃
2	1	Trigonal planar	Bent	<120°	SO₂

4 Electron Domains

Bonding	Lone Pairs	Electron Geometry	Molecular Geometry	Bond Angle	Example
4	0	Tetrahedral	Tetrahedral	109.5°	CH₄
3	1	Tetrahedral	Trigonal pyramidal	~107°	NH₃
2	2	Tetrahedral	Bent	~104.5°	H₂O

5 Electron Domains

Bonding	Lone Pairs	Electron Geometry	Molecular Geometry	Bond Angle	Example
5	0	Trigonal bipyramidal	Trigonal bipyramidal	90°, 120°	PCl₅
4	1	Trigonal bipyramidal	Seesaw	~90°, ~120°	SF₄
3	2	Trigonal bipyramidal	T-shaped	~90°	ClF₃
2	3	Trigonal bipyramidal	Linear	180°	XeF₂

6 Electron Domains

Bonding	Lone Pairs	Electron Geometry	Molecular Geometry	Bond Angle	Example
6	0	Octahedral	Octahedral	90°	SF₆
5	1	Octahedral	Square pyramidal	~90°	BrF₅
4	2	Octahedral	Square planar	90°	XeF₄

Repulsion Strength

$$\text{LP-LP} > \text{LP-BP} > \text{BP-BP}$$

This explains why:

• Lone pairs occupy more space
• Bond angles decrease with more lone pairs

Effect of Lone Pairs on Bond Angles

Molecule	Geometry	Predicted Angle	Actual Angle
CH₄	Tetrahedral	109.5°	109.5°
NH₃	Trigonal pyramidal	109.5°	107°
H₂O	Bent	109.5°	104.5°

Determining Molecular Polarity

Step 1: Draw Lewis structure

Step 2: Determine molecular geometry

Step 3: Identify bond dipoles

Step 4: Check if dipoles cancel

Polar Molecules

• Asymmetric geometry
• Bond dipoles don't cancel
• Examples: H₂O, NH₃, HCl

Nonpolar Molecules

• Symmetric geometry
• Bond dipoles cancel
• Examples: CO₂, CH₄, BF₃

Examples

Carbon Dioxide (CO₂)

O=C=O

Electron domains: 2
Molecular geometry: Linear
Bond angle: 180°
Polarity: Nonpolar (dipoles cancel)

Water (H₂O)

    O
   / \
  H   H

Electron domains: 4 (2 bonding + 2 lone pairs)
Electron geometry: Tetrahedral
Molecular geometry: Bent
Bond angle: 104.5°
Polarity: Polar

Ammonia (NH₃)

    N
   /|\
  H H H

Electron domains: 4 (3 bonding + 1 lone pair)
Electron geometry: Tetrahedral
Molecular geometry: Trigonal pyramidal
Bond angle: 107°
Polarity: Polar

Methane (CH₄)

      H
      |
  H—C—H
      |
      H

Electron domains: 4 (all bonding)
Molecular geometry: Tetrahedral
Bond angle: 109.5°
Polarity: Nonpolar

Quick Reference

Steric #	0 LP	1 LP	2 LP	3 LP
2	Linear	-	-	-
3	Trig. planar	Bent	-	-
4	Tetrahedral	Trig. pyramidal	Bent	-
5	Trig. bipyramidal	Seesaw	T-shaped	Linear
6	Octahedral	Sq. pyramidal	Sq. planar	-