The Old Model

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, $BL$.