Expanded Valence

 

          As you have probably noticed, there are many more than the basic 6 shapes in the database. The first step to teaching those shapes is to explain the concept of “expanded valence.” First you teach the octet rule, and then you tell them when its ok to break it, and what molecules look like when you do.

 

 

Suggested activity #1

          Have the students look at some molecules in the database that exhibit expanded valence. The “expanded valence” database automatically allows you to search these molecules, but does not include the very complex shapes that are included in the “full database.” We suggest that you choose some molecules from the list of shapes and use them as an introduction to the concept of expanded valence.

 

octahedral                            

square planar                   

trigonal bipyramidal

t-shaped

see-saw

 

You can have the students make note of which atoms often have more than 8 electrons around them, and then tell them the general rule for determining if an atom is allowed to exhibit expanded valence. (Row 3 and lower on the periodic table.)

 

 

Suggested activity #2

          Students can make paper models of the “expanded valence” shapes. It is often easier to connect the name with the shape if the student can hold it in his/her hand and see how the “see-saw” shape can pivot like a see-saw on their desk.