Variations From Ideal Geometry


Many factors lead to variations from the ideal bond angles of a molecular shape. Size of the atoms involved, presence of lone pairs, multiple bonds, large groups attached to the central atom, and the environment that the molecule is found in are all common factors to take into consideration. There are molecules in the database that are good examples of all of these things.


Suggested activity #1:

If students did activity #1 in the "VSEPR" topic, they will have already noticed that although there are specific angles that create an ideal tetrahedron or trigonal planar molecule, molecules of those shapes don't often possess angles of exactly 109.5 or 120. They will probably already be wondering why that is, which can lead directly into a discussion of the additional repulsion created by a lone pair, multiple bond, or an atom that is relatively large in comparison to the others in the molecule.

Giving students a few molecules to look at that display non-ideal angles and asking them to come up with an explanation would be a great exercise. It is similar to activity #2 in the "VSEPR" topic, and is based on the same principle of atomic repulsion. This exercise will also give students practice in drawing lewis structures because they will need to decide if a multiple bond is present before they can correctly explain why the molecule does conform to ideal geometry.



In the case above, the bond angle is quite a bit smaller than the 120 that would be predicted based on the hybridization (sp2, which would normally indicate a trigonal planar molecule) because of the lone pair of electrons on the nitrogen. The lone pair occupies more space around the central nucleus than a bond would because the orbital is not stretched between two nuclei as it would be in a bond. As a result, two N-O bonds are forced closer together more than they would be in a trigonal planar molecule.