Determining Bond Order

We now would like to discuss  a way to judge the stability of a molecule once its energy level MO diagram has been created. To do this we will evaluate the molecules' Bond Order which indicates the strength of the bond. The bond order usually corresponds to the number of bonds described by the valence bond theory. Also, the higher the Bond Order, the stronger the bond (which makes sense since triple bonds are stronger than double, and so on). The bond order can be determined using the following equation:

Bond Order= 1/2(a-b)

where a is the number of e^- in bonding Molecular Orbitals and b is the number of e^- in antibonding Molecular Orbitals.

Determining the Stability of the Molecule

If the Bond Order is Zero, then the molecule has an equal number of electrons in bonding MOs and antibonding MOs so no bonds are produced and the molecule is not stable (for example He₂). A bond order greater than zero means that more electrons occupy bonding MOs (stable) than antibonding MOs(unstable). Such a molecule would be more stable than the individual atoms and would therefore create the bonds. If the Bond Order is 1, then it is a single covalent bond. The higher the Bond Order, the more stable the molecule is. 

MO theory is often the best model to predict the bond order, bond stability or magnetic properties of a molecule or ion. The procedure is as follows:

1. Select (or draw) the appropriate MO energy level diagram.
2. Count the total number of electrons in the molecule or ion.
3. As each electron is placed in an available orbital having the lowest energy, the Pauli Exclusion Principle and Hund's Rule must be followed.
4. Calculate the bond order.
5. Use the bond order to evaluate stability.
6. Look for the presence of unpaired electrons to determine if a species is paramagnetic or diamagnetic (remember paramagnetism results from unpaired electrons within the orbitals while diamagnetism results from paired electrons within the orbitals).

Example 3.

What is the molecular orbital diagram for the diatomic neon molecule, Ne₂? How stable is the molecule? Diamagnetic or paramagnetic?

Worked Problem 

Example 4.

What is the molecular orbital diagram for the diatomic oxygen molecule, O₂? How stable is the molecule? Diamagnetic or paramagnetic?

Worked Problem

Subpages (2): Example 3 Example 4