Hückel's Rule
The rule that a planar cyclic conjugated system is aromatic if it contains 4n + 2 pi electrons.
Formula / equation
4n + 2 π electrons
Unit
Dimensionless
In depth
The rule follows from molecular orbital theory: 4n + 2 electrons exactly fill all the bonding orbitals of a cyclic system. Rings with 4n pi electrons are antiaromatic and markedly destabilised, which is why cyclobutadiene is so elusive.
Examples in the real world
Benzene's six pi electrons satisfy the rule with n = 1; the cyclopentadienyl anion does so as well.