Introduction No lone pairs One Lone Pair Two Lone Pairs Three Lone Pairs
2 Electron Pairs 2 Bonding Electron Pairs      
3 Electron Pairs 3 Bonding Electron Pairs      
4 Electron Pairs 4 Bonding Electron Pairs 3 Bonding Electron Pairs 2 Bonding Electron Pairs  
5 Electron Pairs 5 Bonding Electron Pairs 4 Bonding Electron Pairs 3 Bonding Electron Pairs 2 Bonding Electron Pairs
6 Electron Pairs 6 Bonding Electron Pairs 5 Bonding Electron Pairs 4 Bonding Electron Pairs  

VSEPR Model

Electronic Structure and Molecular Shape

Most of the time it is impossible to predict the molecular geometry of a compound based solely on the number of atoms that surround a central atom. For example BF3 has a trigonal planar structure, ammonia NH3 has a trigonal pyramidal geometry and iodine trichloride ICl3 has a T-shaped molecular shape.
BF3 formula
BF3 picture
NH3 formula
nh3 picture
ICl3 formula
ICl3 picture

The VSEPR (Valance Shell Electron Pair Repulsion) theory is based on the principle that electron pairs around an atom repel each other. This is true for bonding electron pairs and lone pairs, which are not involved in bonding. It gives us the possibility to predict and understand the structure of molecules and ions that consist of a central atom and a different number of it surrounding atoms.

The starting point is the Lewis dot structure of the compound. The number of electron pairs around the central atom determines the electron pair geometry. Lone pairs and bonding electron pairs are placed around the central atom.

The final step is to leave out lone pairs and determine the molecular geometry or molecular shape.

 

©2005 Paul Kiprof, All Rights Reserved
Comments: pkiprof@d.umn.edu
Updated: December 2005