Hückel's rule

Hückel's rule predicts that a planar, cyclic, fully conjugated molecule is aromatic when it contains 4n + 2 π electrons, where n is a nonnegative integer.

Molecular orbital theory: A framework that describes electrons as wavefunctions spread across atoms and molecules. Hückel's rule follows from the energy levels of cyclic molecular orbitals.
Cyclic molecule: A molecule whose atoms form one or more closed rings. The 4n + 2 criterion concerns a closed path of conjugated atoms.
Benzene: A six-carbon aromatic ring with six π electrons and the formula C₆H₆. Its six π electrons satisfy the rule with n equal to one.
Antiaromaticity: Destabilization associated with a planar, cyclic, fully conjugated system containing 4n π electrons. It is the destabilizing electron-count counterpart to Hückel aromaticity.
Frost circle: A polygon inscribed in a circle used to sketch the π molecular-orbital energy levels of a cyclic conjugated molecule. Its orbital diagram makes the closed-shell pattern behind the 4n + 2 count visible.
Planarity: The geometric property of lying in a single plane. A planar framework permits continuous overlap around the ring.
Cyclopentadienyl anion: A five-membered ring anion with six delocalized π electrons. Its lone pair completes a 4n + 2 count, making the ring aromatic.
Nonaromaticity: The absence of aromatic or antiaromatic behavior, often because a molecule lacks planarity or continuous conjugation. Failure of a structural condition means electron counting alone cannot establish aromaticity.
Hückel molecular orbital method: A simplified quantum-chemical method for estimating π-electron energies in conjugated molecules. This approximation supplies the orbital energies used to derive Hückel's rule.
p orbital: An atomic orbital with two lobes oriented along an axis, commonly perpendicular to a conjugated molecular framework. Adjacent p orbitals form the continuous π system the rule requires.

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