The four-part test
Run through these in order. Cyclic — closed ring. Planar — all the ring atoms in the same plane. Fully conjugated — every ring atom contributes a p-orbital perpendicular to the ring; no sp³ carbons interrupting the loop. 4n + 2 π electrons — count π electrons in the conjugated system, plug in integers for n.
Counting π electrons
Each double bond inside the ring contributes 2 π electrons. A lone pair on a heteroatom that's in a p-orbital perpendicular to the ring also contributes 2. Pyrrole has 4 from two double bonds plus 2 from the nitrogen lone pair = 6 → aromatic. Pyridine has 6 from three double bonds; nitrogen's lone pair sits in an sp² orbital in the plane and doesn't count → aromatic.
Antiaromatic vs nonaromatic
A ring that meets the first three criteria but has 4n π electrons (4, 8, 12…) is antiaromatic — actively destabilized. If the ring fails any of the first three (sp³ carbon in the ring, ring puckers out of plane), it's nonaromatic — neither stabilized nor destabilized.
Cations and anions
The cyclopentadienyl anion has 6 π electrons (4 from two double bonds + 2 from the lone pair on the carbanion) → aromatic. The cyclopentadienyl cation has 4 → antiaromatic. The cycloheptatrienyl (tropylium) cation has 6 → aromatic, which is why it's a remarkably stable carbocation.
Draw this on the whiteboard
Open the OChem Board whiteboard — benzene rings, wedge/dash bonds, and a clickable periodic table built in. No account needed.