The Old Model is to consider a chemical bond as a particle whose diameter is the bond length. As such, all resonance associated with a particle is applicable to the chemical bond. For example,
\(f_{res}=0.061\cfrac{c}{a_{\psi}}\)
\(a_{\psi}=\cfrac{1}{2}BL\)
\(f_{res}=0.122\cfrac{c}{BL}\)
The radius of such a particle is half the bond length, \(BL\).
