Valence Shell Electron Pair Repulsion (VSEPR) theory is used to predict the shape of molecules based on electron pair repulsions around a central atom.
Basic Principles
Electron pairs (bonding and nonbonding) repel each other and arrange themselves to minimize repulsions.
Lone pairs exert more repulsion than bonding pairs, affecting bond angles.
The number of electron domains (bonding pairs + lone pairs) determines molecular shape.
Electron Domain Geometries
Electron Domains
Geometry
Bond Angles
2
Linear
180°
3
Trigonal Planar
120°
4
Tetrahedral
109.5°
5
Trigonal Bipyramidal
90°, 120°
6
Octahedral
90°
Molecular Shapes and Examples
Electron Domains
Lone Pairs
Molecular Shape
Example
3
1
Bent (Angular)
SO2
4
0
Tetrahedral
CH4
4
1
Trigonal Pyramidal
NH3
5
2
T-Shaped
ClF3
6
2
Square Planar
XeF4
Predicting Molecular Shapes
Determine the Lewis structure of the molecule.
Count the electron domains around the central atom.
Determine the electron domain geometry.
Identify lone pairs and use them to adjust the molecular shape.
Use bond angles to refine the molecular geometry.
Applications
Understanding molecular polarity and dipole moments.
Predicting reactivity and physical properties of compounds.
Explaining interactions in biological systems (e.g., protein folding).