Valence State Electron Pair Repulsion Exercise
James E. Finholt
This discussion was written for instructors. It was written to provide a starting point for the development of a fully detailed set of student instructions to fit a particular course, available hardware and available software. The exercise has been tested on a computer using a Windows 95 operating system with CAChe 3.0 software.
Beginning Activity 1
Construct Lewis structures for CH4, NH3, H2O, and H2S. Use Valence Shell Electron Pair Repulsion concepts to order these species in terms of bond angle from largest to smallest. Explain the basis for your ordering.
Use the computer to build each of these molecules. Beautify each one and then optimize the geometry using AM1 parameters. Determine all bond angles. Compare the calculated results with your predictions and try to explain any differences.
Beginning Activity 2
Construct Lewis structures for SO3 and SO2. Use Valence Shell Electron Pair Repulsion concepts to predict the bond angles in SO3 and SO2 and explain the basis for your predictions.
Use the computer to build each of these molecules. Beautify each one and then optimize the geometry using AM1 parameters. Determine all bond angles. Compare the calculated results with your predictions and try to explain any differences.
Advanced Activity 1
Draw Lewis structures for H2O, OF2, H2S, and SF2. In each of the following pairs use VSEPR theory to predict which species will have the largest bond angle and explain:
a) H2O, OF2 b) H2S, SF2, c) OF2, SF2 .
Use the computer with AM1 parameters to calculate the bond angle in each of these species. Compare the calculated results with your predictions and try to explain any differences.
Advanced Activity 2
Draw Lewis structures for I3- and SF4 and predict all bond angles using VSEPR theory. Use the computer with AM1 parameters to calculate all the bond angles in each of these species. Compare the calculated results with your predictions and try to explain any differences.
Advanced Activity 3
Carry out some research to compare VSEPR theory and molecular modeling computer calculations for the prediction of molecular structure information. Try to find molecules where the two approaches give different results. It might be interesting to compare different computation methods.