Huckel’s rule is a mathematic formula that correlates the number of pi electrons with the physical structure of the cyclic molecule to determine whether it is aromatic or not. The formula reads:
N = 4n + 2
Where N represents the number of pi electrons and n is an integer. The number of pi electrons can be correlated to the number of double bonds or positive/negative charges; a double bond counts as 2 pi electrons. The formula is extracted from the Molecular Orbital Theory and if n results being a whole and positive number, then the compound can be considered aromatic. As an example, benzene has three double bonds, so 6 pi electrons. If we insert this number into the formula, we obtain:
6 = 4n + 2
4 = 4n
n = 1
As 1 is a whole and positive number, benzene can be considered an aromatic compound.