Is cyclobutadiene (4 π electrons) aromatic? What about benzene (6)? Step 1: Hückel rule: aromatic needs 4n+2 π electrons in a planar ring. Step 2: Cyclobutadiene has 4 = 4n (n=1) → antiaromatic, unstable. It dimerises instantly. Step 3: Benzene has 6 = 4(1)+2 → aromatic, extra stable by 2β (~150 kJ/mol). Step 4: This explains why benzene resists addition reactions that alkenes undergo easily.