[mt2568]

Hey All -

I have a quick question...

When you are drawing the molecular orbital diagram for an element or molecule how do u know if the sigma bond is higher in energy then the pi bond or vice versa?  I've noticed that different molecules like it different ways (ie N2 verus O2).  Thanks for the time and effort guys.

matt

[Albert]

Generally speaking, a sigma molecular orbital is higher in energy then a pi one.

O₂ is particular because of Herr Hund's Rule: it is better for O₂ to have two electrons, at the same energy, with the same spin.
So, for what concers O₂, the sigma MO is 'fulfilled' before the two pi ones.

[Mitch]

Also, the degree of s-p mixing of the orbitals is cruical in knowing whether pi bond will be lower in energy than 2sigma. The way to tell is by looking at a chart of ionization energies: for oxygen the 2s orbital and 2p orbital have a reasonable difference in energies(~16.5eV) and thus not much s-p mixing will occur and the pi bond will be higher than sigma2.