A monocyclic, approximately planar, continuously conjugated pi system with pi electrons has a closed-shell aromatic configuration in the elementary Hückel model, where is a nonnegative integer. Fused systems and heterocycles require a specified orbital and resonance description rather than counting every drawn double bond in each polygon.
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Hückel's rule is a principle in organic chemistry that provides a criterion for the stability of cyclic, planar, and fully conjugated polyene systems. According to Hückel's rule, a cyclic compound will exhibit aromatic stability if it contains \(4n + 2\) π electrons in its conjugated system, where \(n\) is a non-negative integer (0, 1, 2, etc.).