Halogens and the free energy of activation in a resonant benzene system If we look at the right-hand-side table here: http://en.wikipedia.org/wiki/Hammett_equation, we notice something particular.  It appears that the free energy of activation (for example, measured in terms of the acidicity of a carboxyl group on a benzene ring), and with respect to substituents in the para- position on the benzene ring, increases as one moves down column 17 on the periodic table.

Why is this happening?  Shouldn't we expect the opposite to be true?  Consider that, as we travel down column 17 corresponding to the halogens, electronegativity decreases and the probability of resonance contributions via pi-bonding should increase.
Resonance between a carbon and a halogen atom decreases as we move down the series $\ce{F, Cl, Br, I}$.  This is because overlap is most effective between orbitals of the same size.  Therefor, carbon-fluorine overlap, both involving $\ce{2p-2p}$ orbitals should be much stronger than carbon $\ce{2p}$ orbital overlap with the $\ce{3p, 4p, etc_{.}}$ orbitals in the larger halogens.  Consequently carbon-halogen resonance becomes less significant as we move down the halogen series.  If we now consider that 1) the resonance and inductive effects for the para-halogens are in opposition to each other (tend to counteract each other) and 2) the resonance effect decreases as we move down the halogen series, then the order of halogens in the Table you mentioned makes sense.