Lecture #20
Text: Chapter 14 Section 3
  CURMUDGEON GENERAL'S WARNING. These "slides" represent highlights from lecture and are neither complete nor meant to replace lecture. It is advised not to use these as a reliable means to replace missed lecture material. Do so at risk to healthy academic performance in 09-105.
Lecture Outline Quantum Theory of the Chemical Bond

Molecular orbitals (in "homonuclear diatomic molecules")

Molecular energy level diagrams

Building up electron configurations

Bond order

Re-defining how one calculates bond order within the context of molecular orbital theory.
The molecular orbital energy diagram for H2- and other isoelectronic species, all having bond order = 0.5. The electron configuration would be written as s1s2 s*1s2 s2s2
or, as in the current text, simply ss2 s*s2 ss2.
Molecular orbital energy diagrams for 1, 2, 3, and 4 electrons systems. The correspondence between bond order and bondlength and between bond order and bond energy is shown for each molecular species.
The molecular orbital energy diagram for Li2 containing all six electrons. Since antibonding electrons negate the effect of bonding electrons, the pair of s* electrons cancels out the bonding characteristics of the pair of s electrons and both pairs revert to the inner core 1s2 configurations that we expect from simpler considerations. The electron configuration could thus be written as [He][He]s2s2.
The molecular orbital energy diagram for Be2. Since the number of antibonding electrons is equal to the number of bonding electrons, there is no net bonding in this molecule and it simply breaks back up into two Be atoms.
For "diboron", B2 we get a somewhat surprising result out of the molecular orbital energy diagram and the ensuing electron configuration. The ninth and tenth electrons go into the lowest energy available orbitals. These are the p orbitals. There are two of equal energy and Hund's rule forces us to put one electron in each and with parallel spins. Thus we have bond order = 1, a single bond, but it is not a sigma bond. In fact, there is one electron in one pi-orbital and another electron in a second pi-orbital comprising this single bond.
Molecular orbital energy diagram for the lower states in molecular nitrogen, N2. The lowest two molecular orbitals return to 1s, inner core electrons on each nitrogen atom. The next two pairs, with no net bonding effect, become lone pairs that can be pictured as being in 2s valence orbitals on each nitrogen.
The valence molecular orbitals in nitrogen that give rise to the triple bond, bond order = 3.

Click the Detour sign to see an optional explanation (several slides) of why the p/s sequence changes when O2 and F2 are reached. Note, you do not need to remember this reversal of sequence. For your convenience in 09-105, you can ignore it even if it seems to apply.
The electron configuration for all 14 electrons in N2. Three variations are shown on how to represent the inner core, 1s2 electrons on each nitrogen atom. Sometimes the two pi molecular orbitals are respresented with a single p4 symbol.
Comparison of ionization energies for atomic N and molecular N2 to illustrate that the bonding molecular orbital in the latter is more stable than the atomic orbital from which it "originates". That is, combining two N atoms leads to a more stable, bound nitrogen molecule, releasing energy.
The highest occupied molecular orbitals in molecular oxygen. The bond order is 2, but is composed of a sigma bond and one net electron in each of the two p orbitals. The molecule is paramagnetic due to the two parallel electron spins.
Comparison of ionization energies for atomic O and molecular O2 to illustrate that the antibonding molecular orbital in the latter is less stable than the atomic orbital from which it "originates". That is, electrons in the antibonding orbitals "want" to release energy by returning to atomic orbitals.
The trend in bonding characteristics in going across the top row of the periodic table looking at homonuclear diatomic molecules from B2 through F2. The bond characteristics -- bondlength and bond energy -- track bond order in the manner expected.