Delocalized Molecular Orbitals
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We had discussed in terms of Lewis structures the concept of resonance.
We can now apply it to the VB theory.
Let's take the formate ion as an example again.
In the VB theory, pi bonds would be formed by the overlapping of all three p orbitals in the carbon and oxygen atoms.
This theory states that the electrons are shared by all three atoms.
Since the electrons are not required to stay "localized" betweeen just two nuclei, we can say that the bond is delocalized.
This model is easier to permit a single description to the electronic structure.
Instead of writing the double arrow indicating a resonance structure pi bonds which extend over three or more nuclei can be indicated with dashed lines, as so:
The benzene ring, if you remember, is a six sided hydrocarbon ring drawn with alternating single and double bonds.
It's resonance structure is drawn like this:
In the benzene ring, the pi bonds are delocalized and form around the entire ring, forming a torus (doughnut-shaped) orbital above and below the sigma bond framework.
Click on the image for the VRML version (requires VRML enabled browser).
To simplify the drawing of a benzene molecule, a ring is drawn inside the center of a hexagon to show that the bonds are delocalized.