Electronegativity effect on bond lengths

Inductive effects, of course, propagate down a chain. Thus, while a-chloroacetic acid is much stronger than acetic acid itself, P-chloropropionic acid is somewhat stronger than propionic acid, and y-chlorobutyric acid is only marginally stronger than butyric acid. 

as is usual in organic chemistry, we discover an effect, and we work out a rationalization for that effect. And organic chemistry made iimnense advances over the last couple of hundred years in just this way. But now we are in a position where we can go a little bit further. If we have in fact a correct understanding of this effect, then 

So then how, exactly, are we going to formulate the electronegativity effect in molecular mechanics And how are we going to show that our formulation is indeed correct and accurate in a predictive sense  

Continuing down the table, sulfur causes a negligible shortening, while silicon causes a substantial lengthening (h-0.009 A) of the carbon-carbon bond to which it is attached. 

Perusing down Table 6.1 a little further, if we look at the effect of the different atoms previously discussed on shortening the C-H bond lengths, the shortenings tend to be smaller but in a similar order. It is also of interest that putting a second fluorine onto a molecule, attached to the same atom as the first fluorine, causes a mutual shortening of both fluorines by quite a large amount (0.030 A). 

---