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.