VSEPR Theory
Valence Shell Electron Pair Repulsion
Molecular Shapes
Electron Geometry
Bond Angles
Understanding Molecular Structure Through Electron Repulsion
What is VSEPR Theory?
Definition
Valence Shell Electron Pair Repulsion Theory
Predicts the three-dimensional geometry of molecules based on electron pair repulsion
Historical Background
Proposed by Ronald Gillespie & Ronald Nyholm
In 1957 • Most widely used introductory model
Core Principle
Electron pairs around a central atom arrange themselves to minimize repulsion
Electron Pairs
Repel each other
Maximum Distance
Adopt positions far apart
Molecular Shape
Determines 3D geometry
Key Postulates of VSEPR Theory
1
Electron Pair Repulsion
All electron pairs repel and arrange to maximize distance
2
Repulsion Strength Order
LP-LP > LP-BP > BP-BP
Lone pairs repel more than bonding pairs
3
Multiple Bonds
Double or triple bonds count as one electron domain
Higher electron density → greater repulsion
4
Electronegativity Effects
More electronegative substituents pull electron density away
Reduced repulsion → compressed bond angles
5
Central Atom Size
Smaller central atoms bring bonding pairs closer together → increased repulsion → compressed bond angles
Maximize Distance
Minimize Repulsion
Predict Geometry
Essential Terminology
Electron Domain
Any region of electron density around the central atom
Single bond
Double bond
Triple bond
Lone pair
Steric Number (SN)
Total electron domains around central atom
SN = Bonding domains + Lone pairs
Electron Geometry
Arrangement of all electron domains (including lone pairs)
Determines overall spatial arrangement
Molecular Geometry
Arrangement of atoms only (lone pairs excluded from name)
Shows actual molecular shape
How to Determine Molecular Shape
1
Lewis Structure
Draw the complete Lewis structure showing all bonds and lone pairs
2
Count Domains
Count electron domains around the central atom
3
Steric Number
Calculate Steric Number (SN) to determine electron geometry
4
Lone Pairs
Identify lone pairs to determine molecular geometry
5
Bond Angles
Predict bond angles considering lone pair repulsion effects
Key Principle
Lone pairs compress bond angles more than bonding pairs
Remember
Each step builds on the previous one for accurate prediction
Steric Numbers 2 & 3
Linear
SN = 2
Molecular Structure
X
A
X
Bond Angle
180°
Lone Pairs
0
Geometry
Linear
Examples:
BeCl₂
CO₂
HCN
Trigonal Planar
SN = 3
0 Lone Pairs
X
A
X
Bond Angle: 120°
BF₃, SO₃
1 Lone Pair
X
A
:
Bond Angle: <120°
SO₂, O₃ (Bent)
Lone pair compresses bond angle from ideal 120°
Steric Number 4: Tetrahedral
Tetrahedral
Lone Pairs
0
Bond Angle
109.5°
Examples:
CH₄
CCl₄
SiH₄
Trigonal Pyramidal
Lone Pairs
1
Bond Angle
~107°
Examples:
NH₃
PCl₃
PH₃
Bent (V-shaped)
Lone Pairs
2
Bond Angle
~104.5°
Examples:
H₂O
H₂S
OF₂
Lone Pair Compression Effect
Each lone pair compresses bond angles by approximately 2–2.5° from the ideal tetrahedral angle of 109.5°
Steric Number 5: Trigonal Bipyramidal
Trigonal Bipyramidal
0 Lone Pairs
90° / 120°
Example:
PCl₅
Seesaw
1 Lone Pair
~90° / ~120°
Example:
SF₄
T-shaped
2 Lone Pairs
~90°
Example:
ClF₃
Linear
3 Lone Pairs
180°
Examples:
XeF₂
I₃⁻
Axial vs Equatorial Positions
Axial (2)
180° to each other, 90° to equatorial
Equatorial (3)
120° to each other, 90° to axial
Key Rule
Lone pairs always occupy equatorial positions to minimize repulsion (fewer 90° close contacts)
Steric Number 6: Octahedral
Octahedral
90°
Lone Pairs
0
All positions equivalent
Example:
SF₆
Square Pyramidal
~90°
Lone Pairs
1
One lone pair
Examples:
BrF₅
IF₅
Square Planar
90°
Lone Pairs
2
Trans positions
Examples:
XeF₄
ICl₄⁻
Key Principle
All six positions are equivalent in octahedral geometry
Lone Pair Positioning
Two lone pairs prefer trans positions (180° apart) to maximize separation
Lone Pairs: The Invisible Architects
Why Lone Pairs Repel More
Lone pairs attracted to one nucleus only
Spread over wider cone of space
Bonding pairs constrained by two nuclei
Compression Effect
Lone pairs push bonding pairs closer together
Each lone pair compresses angles by ~2–2.5°
Lone pairs excluded from molecular geometry name
Tetrahedral Series: Bond Angle Compression
CH₄
109.5°
0 Lone Pairs
Baseline
NH₃
107°
1 Lone Pair
−2.5°
H₂O
104.5°
2 Lone Pairs
−5°
Key Insight
Each lone pair adds ~2–2.5° compression
Multiple Bonds & Electronegativity
Multiple Bond Effects
Repulsion Strength
Triple > Double > Single
Example: Formaldehyde (H₂C=O)
∠H–C–H
≈ 116°
Compressed
∠H–C=O
≈ 122°
Expanded
Electronegativity Effects
More electronegative substituents pull electron density away from central atom
Bonding pairs repel less → angles compress
Comparison: NH₃ vs NF₃
NH₃
107.3°
NF₃
102.1°
N–F bonds pull electron density → reduced repulsion
Multiple Bonds
Act as one domain but carry more electron density
Electronegativity
Higher electronegativity reduces bonding pair repulsion
Combined Effects
Multiple factors can work together to compress angles
Geometry Determines Polarity
Nonpolar Molecules
Symmetric geometry with no lone pairs
Bond dipoles cancel out
No net dipole moment
CO₂
Linear
BF₃
Trig. Planar
SF₆
Octahedral
Polar Molecules
Lone pairs present or asymmetric geometry
Bond dipoles do not cancel
Net dipole moment present
H₂O
Bent
NH₃
Trig. Pyramidal
CHCl₃
Tetrahedral*
Molecular Polarity Summary
CO₂
Linear
Nonpolar
H₂O
Bent
Polar
BF₃
Trig. Planar
Nonpolar
NH₃
Trig. Pyramidal
Polar
CH₄
Tetrahedral
Nonpolar
XeF₄
Square Planar
Nonpolar
Applying VSEPR Theory
CO₂
Central Atom
C
Steric Number
2
Lone Pairs
0
Geometry
Linear
180°
SO₂
Central Atom
S
Steric Number
3
Lone Pairs
1
Geometry
Bent
~119°
NH₃
Central Atom
N
Steric Number
4
Lone Pairs
1
Geometry
Trig. Pyramidal
~107°
ClF₃
Central Atom
Cl
Steric Number
5
Lone Pairs
2
Geometry
T-shaped
~87.5°
XeF₄
Central Atom
Xe
Steric Number
6
Lone Pairs
2
Geometry
Square Planar
90°
Key Insight
Each molecule's geometry is determined by its steric number and number of lone pairs on the central atom
Scope & Practical Use
Limitations
Transition Metal Complexes
Fails here; Crystal Field Theory required
Approximate Values
Bond angles are estimated, not precise
Anomalous Cases
Li₂O predicted bent but actually linear
Resonance & Delocalization
Not accounted for by VSEPR theory
Applications
Reactivity Prediction
Shape controls steric accessibility
Molecular Polarity
Determines solubility, boiling points
Drug Design
3D shape crucial for enzyme active sites
Materials Science
Geometry dictates crystal packing
Key Takeaways
Core Principle
Electron pairs repel and arrange to maximize distance
Steric Number
Determines electron geometry and basic shape
Lone Pairs
Sculpt molecular geometry by compressing bond angles
Bridge to Quantum
Essential qualitative tool for understanding structure
From Simple Principle to Complex Understanding
VSEPR theory delivers powerful molecular predictions from a single idea: electron pairs repel and spread apart