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Page 105 of 232 Experiment 9: VSEPR 5
6 General Chemistry I and II Lab Manual 0
5 Trigonal Bipyramidal Trigonal Bipyramidal^
Trigonal Bipyramidal See-Saw
Trigonal Bipyramidal T-Shaped
Trigonal Bipyramidal Linear*
Trigonal Bipyramidal Linear
Linear * These shapes are highly symmetrical if all atoms bonded to the central element are identical.
+ These shapes are highly symmetrical if diametrically opposed elements are identical.
^ Trigonal Bipyramidal is a highly symmetrical shape if both elements on the axis are identical
and all three elements in the plane are identical.
VSEPR: Some Examples and Shapes
Let's take a look at some of these shapes. Doing this will also prove to be useful in
showing how to use VESPR. The three-dimensional figures below were generated using
Chem3D. Here at Dakota State University, we use HyperChem, which will be the last portion of
Example 1: COH2
First, we determine the Lewis-Dot structure. Hopefully, by now you are relatively skilled at this.
The Lewis-Dot structure would look as follows: Now we determine the central element, which in this case is fairly obviously carbon. To
determine the set number, we notice that carbon has no lone pair electrons (we ignore the lone
pairs on the oxygen because they are not associated with the central element). We further notice
that there are three atoms attached directly to carbon (even though there is a double bond to
oxygen, it is only one element). Therefore, the set number is 3.
Looking at our table, we see that the parent structure for this molecule is trigonal planar.
Looking further, with no lone pair electrons, the molecular shape is also trigonal planar. Here is
how it looks from a couple of different viewpoints:
Dakota State University Page 106 of 232 Experiment 9: VSEPR General Chemistry I and II Lab Manual We probably could have drawn the Lewis-Dot structure in this case the way it would really look,
since the molecule, as can be seen, is confined to a plane.
Example 2: CH4
Here we have a classic example of VSEPR. Again, we start with the Lewis-Dot structure: Again, it is obvious that the central element is carbon. To get the set number, there are (again)
no lone pairs, and four atoms attached to the central atom, giving us a set number of 4. Looking
at the table, set number four is tetrahedral, and with no lone pairs, the molecular shape is
tetrahedral. Here is how it looks: Dakota State University Page 107 of 232 Experiment 9: VSEPR General Chemistry I and II Lab Manual This looks quite different from our Lewis-Dot rendering. Each hydrogen is evenly distributed
about the carbon in three dimensional space, with angles of about 110o . In our Lewis-Dot
structure, it looked as if the molecule would be square planar. This point cannot be stressed too...
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