Untitled - Kelly Walsh High School

Molecular Geometry and Hybridization 151
overlap of one sp hybrid orbital and two p-orbitals on one carbon with the same
on the other carbon. This results in one sigma bond (overlap of the sp hybrid
orbitals) and two pi bonds (overlap of two sets of p-orbitals).
10-3 Molecular Orbital (MO) Theory
Another covalent bonding model is molecular orbital (MO) theory. In MO theory,
atomic orbitals on the individual atoms combine to form molecular orbitals
(MOs). These are not hybrid orbitals. An MO covers the entire molecule.
Molecular orbitals have definite shapes and energies. The combination of two
atomic orbitals produces two MOs. (The total number of orbitals never
changes.) One of the MOs is a bonding MO. The other is an antibonding MO.
The bonding MO has a lower energy than the original atomic orbitals. The antibonding
MO has a higher energy. Lower energy orbitals are more stable than
higher energy orbitals.
Once the MO forms, electrons enter. We add electrons using the same rules we
used for electron configurations. The lower energy orbitals fill first. There are a
maximum of two electrons per orbital. Orbitals of equal energy will half-fill
orbitals before pairing electrons. When two s atomic orbitals combine two
sigma (s) MOs form. One is sigma bonding (s). The other is sigma antibonding
(s*). Figure 10-5 shows the MO diagram for H 2.
Figure 10-5 Molecular orbital diagram of H 2
Note that the two electrons (one from each hydrogen atom) have both gone
into the sigma bonding MO. We can determine the bonding situation in MO
theory by calculating the MO bond order. The MO bond order is the number