Huckel proposed that aromaticity is a function of electronic structure. A compound may be aromatic, if it obeys the following rules
i. The molecule must be co-planar
ii. Complete delocalization of π electron in the ring
iii. Presence of (4n+2) π electrons in the ring where n is an integer (n=0,1,2….)
This is known as Huckel’s rule.
Some of the examples for Huckel rule