Structure: Aromatic rings containing one or more nitrogen, oxygen, or sulfur atoms in place of a ring carbon (e.g., pyridine, furan, pyrrole) Hybridization: sp² throughout the ring, maintaining the delocalized π system Geometry: Planar Polarity: Varies with the heteroatom; ring nitrogen lone pairs may or may not be part of the aromatic π system depending on whether they are conjugated into the ring Typical reactivity: Retains aromatic character and undergoes substitution rather than addition; the heteroatom strongly influences reactivity and where substitution occurs (Chapter 17). Widespread in pharmaceuticals and biomolecules.

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