According to Hückel's rule, a planar cyclic compound is aromatic if it has:

**Hückel's rule**: a planar, fully conjugated, cyclic molecule is aromatic if and only if it has $(4n + 2)$ $\pi$ electrons, where $n$ is a non-negative integer. Examples: - Benzene: 6 $\pi$ electrons ($n=1$). Aromatic. - Naphthalene: 10 $\pi$ electrons ($n=2$). Aromatic. - Cyclobutadiene: 4 $\pi$ electrons (not $4n+2$). **Anti-aromatic**, highly unstable. - Cyclopropenyl cation: 2 $\pi$ electrons ($n=0$). Aromatic. Aromaticity gives extra stability (resonance stabilisation energy) and shifts properties: NMR ring current, no addition reactions at room temperature, etc.