Before I start this question, I am aware tha
- Electrons in all molecules are delocalized to some extent
- Delocalization is a the phenomenon and resonance is an attempt to explain it.
When I asked this question from my professor, this is the explanation he gave me:
In VBT, it is difficult to approximate the molecular orbital with
localized bonding orbitals, so the electronic configuration is a weighted average of the infinite possible electronic configurations i.e Ψ= Aψ1 +Bψ2+Cψ3......
For most molecules, the most stable lewis structure has the majority of the contribution and can sufficiently approximate the actual electronic by itself. However, for some molecules, there are multiple equally-stable lewis structures with an equal and significant contribution and each of them has to be taken into account. For e.g, for nitrate ion, Ψ=1/3 (ψ1 +ψ2 +ψ3) where ψ1,2,3 represent the 3 equal lewis structures. As a result, the electrons MUST be delocalised between the appropriate bonds
This explanation, in simple words, argues that since the 3 lewis strucutes are identical/indistinguishable, the electron density must be equal and thus delocalised between the bonds by symmetry.
However, this doesn't really explain the physical reasoning behind delocalization. Why is (significant) delocalization more energetically stable for some molecules but not for others? And why dpes delocalization always occur in molecules with some kind of symmetry between the possible atoms for a double/triple bond?