## Molecular Orbital Diagrams

In a molecular orbital diagram that shows the bonding of two electrons (like a diatomic molecule), there are sigma and pi bonds, and the sigma and pi antibonds. Here is an example: http://www.westga.edu/~chem/courses/..._09/sld024.htm

I don't understand how to determine where to put the pi and sigma orbitals on the diagram. The order of the different molecular orbitals (the bonding pi and sigma orbitals) are different for different atoms and elements. Is there any way to know the their order other than memorizing them without making sense of them?

Recognitions:
Gold Member
First of all notice that there's an ambiguity almost only when it comes to the relative positions of $1\pi$ and $3\sigma$ (with molecules involving s- and/or p-block elements). The other MOs have pretty well-defined positions that mostly do not change qualitatively relative to each other.
So the question filters down to (if I haven't misunderstood you): how do I know if $1\pi$ should lie above or below $3\sigma$? To this end I suggest a simple rule of thumb that will work in the majority of cases. [tex]E_{1\pi} > E_{3\sigma}[/itex] is $2\pi ^*$ is unoccupied or singly occupied; else (if doubly or more occupied) it is the other way round.