Sunday, October 28, 2012

Balloon representations of VSEPR shapes

It seems to be very hard for students to initially picture the VSEPR shapes of molecules. They draw the Lewis dot structures and then get really hung up on trying to visualize how the Lewis doc structures correlate to the VSEPR shapes.

Here is my visual aid. BALLOONS!
It really is the best way to picture it- it is better than a model kit and better than trying to draw it three-dimensionally.
The two green balloons represent a linear molecule spread at 180 degrees. Typically without steric hindrance any two atom molecule will be a straight line like these two balloons.
The next two drawings represent the most mistaken structures. The trigonal planar (three balloons) and the tetrahedral (four balloons) are actually very, very different shapes. The trig planar is two-dimensional (flat) while the tetrahedral is three-dimensional. This is tricky because if one of the balloons in the tetrahedral is a lone pair of electrons (empty cloud without an atom) the structure becomes a trigonal pyramid. This is NOT the same thing as a trigonal planar molecule. It is still three-dimensional- it just has an empty space where one of the clouds previously had an atom.

I don't include the trigonal bipyramid and the octahedral in my class but those two structures are included just to get a preview of what people study at the next level of chemistry.

Build these structures to fully understand VSEPR! Don't mix up your trigonal pyramid with your trigonal planar molecules!