CH301H - Principles of Chemistry I: Honors
Fall 2013, Unique 52195

Lecture Summary, 23 October 2012

Making Molecules:  We are now going to use our atomic orbitals to form molecular orbitals, which will provide 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: *
          no: nothing

Homonuclear Diatomcs:  Today we spent a lot of time constructing MO diagrams ofhomonuclear diatomic molecules in period 1 and period 2.  We discussed the general rules of LCAO-MO:

  1) Determine the ground state electron configuration of each atom.
  2) AO's interact only if their energy and shape is similar.
  3) Construct MO's from the AO's, where the total number of MO's must equal the number of AO's used.
  4) Arrange the MO's in order of lowest to highest energy.
  5) Add all electrons to MO's using Aufbau (Pauli + Hund's rule).
  6) Determine the bond order of the molecule.

We made our way up to Li2, and will continue the homonuclear diatomics next week.