Saturday, October 4, 2014

Chemistry App Nicely Helps Students See VSEPR Theory

Odyssey VSEPR (Valence Shell Electron Pair Repulsion) app shows molecular shapes


In my General Chemistry class this week we were learning about molecular shapes and VSEPR theory.  The whole point of VSEPR theory is to help us understand the 3-dimensional structure of molecules.  First we learn Lewis dot structures.  Lewis is a helpful theory but we soon see that Lewis structures have many exceptions and they don't really predict shapes.  VSEPR theory is an improvement on Lewis theory in that it does predict 3-D shapes.  The problem is that we then draw these shapes on 2-dimensional paper and the students don't really get a true picture of the 3 dimensions.  The Odyssey folks have created a neat little app that lets us get real close to the 3-D structures on the iPad.  Here is a picture of the app icon:



I have tried other apps that I really like and reviewed in earlier posts.  What is missing from the other apps is a depiction of the unshared or nonbonding electrons that are so important in influencing the shape of the molecule.  In this  Odyssey app the unshared electrons are shown:


In other apps I have used the shape of this bent molecule was clear but Odyssey VSEPR is the only app I have found that shows those two nonbonding pairs of electrons that cause this molecule to be bent.
 Also, like the other apps, this structure can be rotated to give the student a clear perspective of its 3-d structure.   I really like the way this app is arranged too.  On the right of the above picture are the choices of different molecular arrangements.  They are organized according to the number of electron groups or electron pairs in the molecule which turns out to be exactly the way I teach this concept.  So in the case of 4 electron groups the geometry is a tetrahedron.  If you want to see what it looks like when all four pairs of electrons are bonding you click on the AX4 button and the molecule looks like this:


You can see in the above picture that it is possible to highlight three atoms and get the bond angle between them which in this case is 109.47 degrees. In the molecule from the first example, which could be like water, the same electron geometry is shown (tetrahedral) but the molecular geometry is bent.  

In the bottom right of the above picture you can see that the app provides many molecules demonstrating specific example of VSEPR theory applied to molecular shape prediction.  Here is one example of the ion ICl2 -  :



I wish I had this app when I was taking chemistry the first time!  I know I keep saying that, but it makes it so easy to see and understand.  I still used my "Molymod" model kit to show the students but I do not have enough of the kits to go around, and building the molecules one at a time takes so much more time.  This app quickly lets the students see and understand the structure of the molecules.  I asked a handful of the students and they were all very positive about it. 

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