Tuesday, February 8, 2011

Teaching molecular polarity and its relation to VSEPR geometries

So I asked teachers on our Facebook page what they would want to know if they could ask a scientist how to explain anything. Our first question was about the concept of molecular polarity and its relation to VSEPR geometries. So I asked Jeff Levy, a teacher who has a Master’s Degree in Chemical Engineering from Carnegie Mellon University and has taught high school chemistry and physics at the Cranbrook Kingswood School in Michigan, the Horace Mann School in NYC and The American School in London about this concept and how he teaches about it. Here is his answer:

 In a nutshell the process goes like this:
1.  Find the molecular geometry using VSEPR theory
2.  Use electronegativity to decide if any bonds are polar
3.  Use the geometry to decide if the polar bond vectors will add or cancel each other out.

VSEPR means this: electrons repel each other (that’s the E and the R) because of the electrostatic force.  This repulsion dictates the arrangement or geometry of atoms within molecules.  When you’re talking about molecular geometry then you only need to consider the bonding pairs and unbonded or “lone” pairs (that’s the P) of electrons that are located in the outer or “valence” shell (that’s the V and S) of each atom.  Valence Shell Electron Pair Repulsion.

When teaching this I always started with gum drops and tooth picks and asked the kids “place X toothpicks in your gumdrop in such a way that all X of your toothpicks point as far away from each other as possible.”  Eventually they come up with the proper geometries (once they start to think in 3D).  I also demonstrated with balloons (tie 4 balloons together at the knot and they naturally form a tetrahedral geometry.)

And then, after you’ve figured out where the atoms are arranged in the molecule, you look at their relative electronegativities and decide where charges will aggregate and if the whole molecule will be polar.

There is a great Molecular Shapes interactive on the Teachers' Domain website,
and a great website that includes many different molecular models that you can rotate and interact with.

Please let us know if you have additional questions!

1 comment:

  1. Related documents can be downloaded at this URL: