For a molecule to count as aromatic, it must meet several conditions at once. It has to be cyclic, planar (flat), and fully conjugated, meaning every atom in the ring has a p orbital that overlaps continuously around the loop. When these structural rules are met, Huckel’s rule provides the final test: the ring must contain 4n + 2 pi electrons in that shared system.
Benzene is the classic example. Its ring has six pi electrons, and plugging n = 1 into 4n + 2 gives exactly 6. These electrons spread out evenly over all six carbons instead of staying in fixed double bonds, which lowers the molecule’s energy and makes benzene far less reactive than a normal alkene. Other counts that work are 2, 6, 10, and 14 pi electrons.
Rings with 4n pi electrons (4, 8, 12, and so on) are called antiaromatic and are actually less stable than an open-chain version would be. Cyclobutadiene, with four pi electrons, is so unstable that it can barely be isolated. A molecule that is not cyclic, not planar, or not fully conjugated is simply nonaromatic and follows ordinary reactivity rules.
The rule also works for charged rings and rings that contain atoms other than carbon. The cyclopentadienyl anion and the tropylium cation each carry six pi electrons and are aromatic, and heterocycles such as pyridine, furan, and pyrrole are aromatic because a lone pair or extra electron brings the count to six.