Write $H$ückel rule and give a suitable example.

(N/A) $H$ückel rule states that for a planar,cyclic,conjugated molecule to be aromatic,it must contain $(4n + 2) \ \pi$-electrons,where $n$ is an integer $(n = 0, 1, 2, ...)$.
Example: Benzene $(C_6H_6)$ contains $6 \ \pi$-electrons,which follows the $(4n + 2)$ rule for $n = 1$.