# Comparing acidity of anilinium ion and para-fluoro anilium ion

Which of these compounds - anilinium ion and para-fluoro anilium ion - is more acidic?

I thought the answer would the be first one since $$\ce{-F}$$ is an ortho-para activating group, so the electron density at the carbon at which $$\ce{-NH3+}$$ is attached would be more. I know that $$\ce{-F}$$ has a strong inductive effect, but since this is para with respect to $$\ce{-NH3+}$$, I thought I could say +M dominates. However the answer given was the second one. Why is this so and when can I neglect the inductive effect of halogen substituents in benzene to favour the mesomeric effect caused by them?

The conjugate bases are aniline and para-fluoro aniline respectively. The $\ce{-NH2}$ group shows +M (mesomeric effect), so on drawing out the resonance structures we find that the second resonance structure is more stable as the resulting negative charge is inductively withdrawn by fluorine group.