VSEPR: Valence Shell Electron Pair Repulsion theory, allows one to predict molecular structure. If molecular structure can be predicted then molecular properties, like polarity, can be predicted.


To predict the structure of a molecule we simply need to arrange the pairs of electrons to minimize the repulsion between them.


1. First determine the steric number by counting the number lone pairs and bonds (or atoms) around the central atom.

2. The pairs of electrons must be distributed in space to minimize pair replusions. However, it is important to note that lone pairs take up more space than bonding pairs.

3. The positions of the atoms in the molecule are used to name the shape of the molecule.


Steric Number

lone pairs +
bonds

geometry of the electrons
molecular
geometry

1 pair

linear

linear

come on...its linear!
2 pair

linear,
e- 's are 180° apart

linear

come on...its linear!
3 pair

trigonal planar,
e- 's are 120° apart

trigonal planar

V-shaped

4 pair

tetrahedral,
e- 's are 109.5° apart

tetrahedral

pyramidal

bent

5 pair

trigonal bipyramid,
e- 's 120° and 90° apart

trigonal bipyramid

see saw

T-shaped

linear

6 pair

octahedral, 90° apart

octahedral

square pyramidal

square planar

T-shaped

linear

7 pair

pentagonal bipyramidal

8 pair

square antiprismatic