[Lencer]

Hey guys,

I am reading the sparknotes on MO diagrams, (http://www.sparknotes.com/chemistry/bonding/molecularorbital/section1.html), and I am a little confused. For their example for B2 (diagram 1.5) and F2 (diagram 1.6), I was wondering why is B2's pi bonds are lower in energy than sigma bonds, while the F2's pi bonds are higher in energy?

Thank you, 

Lencer

[enahs]

Did you read the paragraph of text between the two diagrams?

[TheBigF]

The text between these diagrams offers one possible, but not entirely accurate explanation (IMHO).

The swapping of energies for low Z numbers arises from a repulsion between sigma(1) and sigma(2) when the energy difference between the atomic s and p orbitals is small.

When the atomic number (Z) is low (or more importantly the nuclear charge is low) the energy gap between the s and p orbitals is small. As Z increases the s orbital energy falls more rapidly than the p orbital one. The net effect is that the s-p energy gap  increases as Z increases - this is shown on the diagrams. This reduces the repulsive interaction between sigma(1) and sigma(2) molecular orbitals, until at Z=7, and beyond, the expected "normal" ordering of molecular orbitals is observed.

[AWK]

Hey guys,

I am reading the sparknotes on MO diagrams, (http://www.sparknotes.com/chemistry/bonding/molecularorbital/section1.html), and I am a little confused. For their example for B2 (diagram 1.5) and F2 (diagram 1.6), I was wondering why is B2's pi bonds are lower in energy than sigma bonds, while the F2's pi bonds are higher in energy?

Thank you, 

Lencer

These are real diagrams based on calculations. You can memmorize them, but not predict basing on your knowledge