[′hu̇k·əlz ¦fȯr ‚en pləs ′tü ‚rül] (organic chemistry) Aromatic (ring) compounds must have 4 n + 2 pi-bonding electrons, where n is a whole number and generally limited to n = 0 to 5. When n = 1, for example, there are six pi-electrons, as for benzene. Also known as Hückel's rule.