Shape Prediction

 

Suggested activity #1

A starting point for molecular shape prediction is to have students explore the database and verify the shapes by making sigma structures for the molecules they find. (click here for a discription of sigma structures and how they are used to predict molecular shape) For instance, a student could look at the trigonal planar molecules in the database, find BF3 among the results and draw a sigma structure to verify the shape that was determined by the electron diffraction technique. (This database is ideal for working with sigma structures because it doesn’t display multiple bonds that would otherwise confuse students who had not yet learned about them.)

 

 

 

Using this method, students start to see patterns in the number of electrons that give rise to various shapes, and at the same time understand that sigma structures (and later Lewis structures) are simply a way of justifying the molecular shapes that were predicted experimentally, not the other way around.
          There’s a neat website that will help students learn sigma structures by helping them count the electrons. It also calculates formal charges on the atoms, which further aids the transition from sigma to lewis structures.

http://www.stolaf.edu/depts/chemistry/courses/toolkits/121/js/lewis/