Topic 10: VSEPR

Suggested activity #1:

Students can get an introductory level idea of approximately what angles are associated with various molecular shapes by exploring the database. This can be done either before or after a lecture or reading discussing VSEPR theory and the ideal angles of each geometry. Before to let students discover the data on their own and give them a chance to try to explain it, or after to allow students to see that the theory is supported in the real world by experimental evidence.

 

 

Linear

Trigonal Planar

Trigonal Pyramidal

Tetrahedral

Some Angles

in the Database

C2H2 (g): 180

BrCN (g): 180

CH3CN: 178.9

NCO-: 179.8

SCN-: 179.4

 

BF3 (g):120

BBr3 (g): 120

C2H4: 121.2
          117.6
C2Cl4 (g): 122.2

 

NH3 (g):106.6

N(CH3)3: 110.6

NHF2 (g):102.9
                100
 

 
CH4 :109.5
CH3F (g):108.9
                 110
CH3I: 109.4      
           109.5
CH2Br2 :110.9
             109.1
             109.5

 

Mean of Listed Angles

179.62

120.20

105.03

 
109.60

This exercise will also lead right into a discussion of the factors that will cause the angles to be either larger or smaller than the ideal geometries. (see "variations from ideal geometry" on the topics page.)

 

Suggested activity #2:

A great exercise in thinking on the molecular level is to have students look at some molecules in the database and try to explain some of the factors that contribute to the structure. VSEPR theory does just that: it explains the orientation of the bonds based on the repulsion of other bonds and groups nearby. Students can view the bond angles in a molecule and try to explain why they might be that way based on various forces acting upon the atom and the bond.

For example, you might ask a student why they think the bond angles in the molecule pictured on the right below are larger than those in the molecule on the left.

 

 

The answer, of course, is that the large groups, and their associated electron clouds, attached to the central atom in the molecule on the right repel each other more than the hydrogens in the molecule on the left, causing a larger bond angle. Through exercises like these, students will begin to think about what an atom really is, and how replacing an atom in a molecule with something else would affect its properties.