Pyridine pKaH ≈ 5.2 vs pyrrole pKaH ≈ −3.4 (8.6-unit lone-pair topology split)

Canonical pKaH split driven by lone-pair topology. Pyridine's ring nitrogen contributes two electrons to a σ lone pair *in the ring plane* (perpendicular to the π-system); protonation at this σ pair does not disturb aromaticity, so…