Here is a brief discussion of VSEPR.
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 atom | structure |
| 2 | linear |
| 3 | trigonal planar |
| 4 | tetrahedral |
| 5 | trigonal bipyramidal |
| 6 | octahedral |
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 pairs | total geometry | # bond pairs | # lone pairs | molecular geometry |
| 2 | linear | 2 | 0 | linear |
| 3 | trigonal planar | 3 | 0 | trigonal planar |
| 4 | tetrahedral | 4 | 0 | tetrahedral |
| 4 | tetrahedral | 3 | 1 | trigonal pyramidal |
| 4 | tetrahedral | 2 | 2 | bent |
| 5 | trigonal bipyramid | 5 | 0 | trigonal bipyramid |
| 5 | trigonal bipyramid | 4 | 1 | seesaw |
| 5 | trigonal bipyramid | 3 | 2 | T-shaped |
| 5 | trigonal bipyramid | 2 | 3 | linear |
| 6 | octahedral | 6 | 0 | octahedral |
| 6 | octahedral | 5 | 1 | square pyramidal |
| 6 | octahedral | 4 | 2 | square planar |
We will look at a number of examples to hopefully help you get comfortable with some predictions of molecular structures.
Here is a brief discussion of VSEPR.
Here is another VSEPR model discussion site.
Here is a good site for VSEPR study.
Here is a site with examples with action!
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. |
