The modern theory of aromaticity was given by Erich Huckel in 1931. According to this theory, for a compound to exhibit aromaticity, it must have the following properties:

• Delocalization of the π–electrons of the ring system.
• Planarity of the molecule. To permit sufficient or total delocalization of π–electrons, the ring must be planar to allow cyclic overlap of the p–orbitals.

Huckel Rule or (4n + 2) Rule: This rule states that for a compound to exhibit aromatic character, it should have a conjugated, planar cyclic system containing 4n + 2 (where n = 1, 2...) delocalized π– electrons forming a cyclic cloud of delocalized π–electrons above and below the plane of the molecule. This is known as Huckel rule of 4n + 2 π–electrons.

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