VSEPR

Here is a great site on a VSEPR tutorial. It does require the Chime plugin to visualize and manipulate the molecules. Go to this site to download Chime. It is free and will be a big help. Here is another site with models for rotation if you have Chime. Here is a short course on VSEPR that does not require Chime.

All diatomics are linear (2 points define a straight line) and all triatomics are planar (3 points define a plane). Beyond this basic information, we need another model. One model used is the Valence-Shell Electron-Pain Repulsion Theory (VSEPR) which assumes that electrons in bonds and unshared electron pairs repel one another so that they tend to stay as far apart as possible. So VSEPR model says that the resulting molecular structures are the result of the electron pairs trying to stay as far apart as possible. Remember that the predicted structure is determined by considering the paired and unpaired electrons, but the name of the type of structure does not include the lone pair electrons.

Here is a table to help you in determining the molecular structure:

# of electron pairs about the central atomstructure
2linear
3trigonal planar
4tetrahedral
5trigonal bipyramidal
6octahedral

The other important item to help you in predicting molecular structures is the order of repulsion of the electron pairs:

lp-lp > lp-bp > bp-bp

Here is an expanded table to help in predicting the structure of molecules considering the number of bond pairs and the number of lone pairs:

valence electron pairstotal geometry# bond pairs# lone pairsmolecular geometry
2linear20linear
3trigonal planar30trigonal planar
4tetrahedral40tetrahedral
4tetrahedral31trigonal pyramidal
4tetrahedral22bent
5trigonal bipyramid50trigonal bipyramid
5trigonal bipyramid41seesaw
5trigonal bipyramid32T-shaped
5trigonal bipyramid23linear
6octahedral60octahedral
6octahedral51square pyramidal
6octahedral42square planar

We will look at a number of examples to hopefully help you get comfortable with some predictions of molecular structures.

Help Here is a brief discussion of VSEPR.

Help Here is another VSEPR model discussion site.

Help Here is a good site for VSEPR study.

Help Here is a site with examples with action!

Help Here is a quiz to take on VSEPR.

Now take a practice quiz to help you understand if you understand the basic concepts.
You must use your real name when it asks for a name.
The test will only submit when you have answers all of the questions correctly.
If you are not taking this course for credit please do not answer all the questions correctly for I don't want to be flooded with email answers to the tests.



Web Author: Dr. Leon L. Combs
Copyright 1999 by Dr. Leon L. Combs - ALL RIGHTS RESERVED