Huckel's rule

Huckel's rule is the simple counting test that tells you whether a flat, fully conjugated ring will be aromatic. It says such a ring is aromatic when its pi system holds 4n+2 pi electrons, where n is any whole number 0, 1, 2, 3 and so on. Plug in the values and you get the magic numbers: 2, 6, 10, 14, 18. A ring with one of these pi-electron counts, if it is also cyclic, planar, and fully conjugated, gets the big stability boost of aromaticity.

To use the rule, first confirm the ring is cyclic, planar, and fully conjugated (every ring atom has a p orbital in the loop), then count only the pi electrons in that loop. Each double bond inside the ring contributes two pi electrons. A lone pair counts if, and only if, it sits in a p orbital that is part of the ring's pi system. Benzene has three ring double bonds, so six pi electrons; six is 4n+2 with n=1, so benzene is aromatic. By contrast, a flat fully conjugated ring with 4n electrons, the counts 4, 8, 12, is antiaromatic, actively destabilized rather than stabilized. The contrast between 4n+2 and 4n is the whole point.

The rule has a deep reason behind it, found in molecular orbital theory: when you build the pi molecular orbitals of a ring, they come in a pattern (one lowest orbital, then pairs of equal-energy orbitals) such that a closed, fully filled, especially stable shell requires exactly 4n+2 electrons. Huckel's rule is the everyday shortcut for that MO result. In practice it is the single most useful test in this whole field; you reach for it every time you ask is this ring aromatic.