Why does the lone pair on nitrogen behave differently in pyrrole and pyridine?

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heterocyclic compound There are many different sulfur-containing heterocycles. One of the best known is thiophene, C₄H₄S, derivatives of which occur as plant pigments and as other natural products such as biotin.

The behavior of the lone pair on nitrogen in pyrrole and pyridine differs fundamentally due to how each atom participates in its respective aromatic system.1

In pyrrole, the nitrogen atom is "pyrrole-like." It is bonded to two carbon atoms and one hydrogen atom. Its lone pair of electrons resides in an orbital perpendicular to the ring plane, where it actively participates in the cyclic conjugated system. By contributing these two electrons to the aromatic sextet, the nitrogen atom donates electron density to the ring carbons.1

In contrast, the nitrogen atom in pyridine is "pyridine-like." It is bonded to only two carbon atoms via a double bond. Its lone pair is not involved in the aromatic sextet. Because nitrogen has a higher electronegativity than carbon, it tends to attract electron density toward itself rather than donating it, which is the opposite of the behavior observed in pyrrole.1