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/