CH301H - Principles of Chemistry I: Honors
Fall 2011, Unique 51040

Lecture Summary, 17 October 2011

Electron Configurations in Atomic Orbitals Explain Periodic Trends:  We briefly reviewed several periodic trends and saw that not only does our atomic orbital picture of the world explain the overall trends perfectly, but exceptions to those trends (such as when we are dealing with half-filled or filled orbitals), are now also explained.  

Making Molecules:  We are now going to use our atomic orbitals to form molecular orbitals, which will probide the mechanism of keeping stable electron density between two nuclei, which will in turn lower the potential energy of the system and form a molecular bond.  The technique that we are going to use to do this is called "linear combination of atomic orbitals to molecular orbitals," LCAO-MO.  Our coordinate system will be defined with the internuclear axis lying along the z-axis.  From that simple definition, we spent a lot of time drawing the structure of s and p orbitals of same and opposite phase and figuring out what the resulting structure would look like.  These are also drawn in your book, and you should spend some time getting comfortable with these images.  

We also reviewed the naming convention for these orbitals.  For each orbital, we need to ask 3 questions about the orbitals shape and phase:

   1) Does the MO have cylindrical symmetry around the internuclear (z) axis?
           yes: sigma
           no: pi

   2) Is the MO symmetric with respect to inversion?
           yes: g (for "gerade" (even))
           no: u (for "ungerade" (uneven))

   3) Is there a nodal plane perpendicular to the internuclear (z) axis in the center of the molecule?
          yes: nothing
          no: *