CH301H - Principles of Chemistry I: Honors
Fall 2012, Unique 51390

Lecture Summary, 18 September 2012

VSEPR:  Valence shell electron pair repulsion (VSEPR) theory is a way to describe molecular structure based only on the way atoms are connected in a bond.  Provided we are able to identify a correct Lewis dot structure (i.e. correct number of bonds connecting correct atoms and correct number of lone pairs on correct atoms identified), then we can use VSEPR to figure out how atoms and lone pairs will arrange themselves around a central bond.  

Steps for VSEPR:

   1.  Write a correct Lewis dot structure
   2.  Find the steric number of the central atom.  SN = # of bonded atoms + # of lone pairs.
   3.  Identify the shape associated with that SN:

                 SN = 2    linear
                          3     trigonal planar
                          4     tetrahedral
                          5     trigonal bipyramidal
                          6     octahedral

The shape determines the angle between adjacent bonds around the central atom.

   4.  If there are lone pairs, figure out where they need to go, and how that will distort the shape of the molecule.  

VSEPR does a very good job of predicting the structure around central atoms, particular for molecules whose central atom is C, N, and O, and for trigonal bipyramidal and octahedral molecules.  Like Lewis structures, however, VSEPR is only a formulism, and it isn't too difficult to find molecules whose experimentally determined shape is quite different from what would be predicted by VSEPR.