Hückel's rule
Notably, [18]annulene is the first annulene after benzene ([6]annulene) to be fully aromatic: its π-system contains 4"n" + 2 electrons ("n" = 4), and it is large enough to comfortably accommodate six hydrogen atoms in its interior, allowing it to adopt a planar shape, thus satisfying Hückel's rule. The discovery of aromatic stabilization for [18]annulene is historically significant for confirming earlier theoretical predictions based on molecular orbital theory, since simple versions of valence bond theory did not readily explain the 4"n" + 2 rule.