This website
offers an interesting way to approach the subject of multiple bonds (moving
from sigma structures to lewis structures). Since multiple bond theory was
developed simply as a way to explain the uncharacteristically short bonds found
in some molecules, using the data in this website, students can gain an
accurate appreciation of multiple bonds by approaching the subject from the
same angle: data first, theory second.
We also decided not to show double bonds in the molecules in the
database, which is helpful for two reasons. 1. It does not complicate things
for beginning chemistry students who haven’t learned multiple bonds yet and 2.
Multiple bonds are theoretical interpretations of the experimental data. We
prefer, for the purposes of this website, to include as few of our
interpretations of the data as possible so that the students can come to their
own conclusions, rather than be force-fed ours.
Suggested Activity #1
We
suggest having students explore the database, taking special note of variation
in bond length between molecules. The "bonds" search was developed
especially for that purpose. Once the "bonds" search is selected, the
red histogram on the left can be used to narrow down the search to show only
molecules that contain bond lengths within a certain range. It is difficult to
see the pattern if the whole database is searched, so we suggest using the
“entry level” database and restricting the search to C-C bonds. When the
results come up, you can sort by bond length, and then, as you scroll down you
can see that triple bonds are the shortest, followed by double and then finally
all the single bonds show up. Students should be making sigma structures of
some molecules on the list, and then a classroom discussion could follow about
why some of the bonds are so much shorter than others. At that point, the octet
rule for the central atom should be introduced.
The same website that is helpful in learning sigma structures can be used for lewis structures.
http://www.stolaf.edu/depts/chemistry/courses/toolkits/121/js/lewis/
sigma structure |
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lewis structure |
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sigma structure |
lewis structure |
Suggested activity #2:
Along with the concept of double bonds comes the concept of
resonance: the idea that some bonds do not have complete single or multiple
bond character. This website can also be used to demonstrate that such
molecules have bonds that are intermediate in length, basically that they are
measurably in between a single and a double bond. The “cp database” option is a
good place to start looking for molecules to use as examples of this. (All
hydrogens are shown in the structures. Remember that if there are only 3 bonds
to a carbon atom, it is extremely likely that one of the there is a double bond
somewhere. For instance, if there is a 6 member carbon ring with only one
hydrogen on each carbon, there are 3 double bonds in the ring being shared
equally by all the atoms.)
Some good molecules to look
at are:
·
Benzene
·
carbonates