Simple aromatic rings

Simple aromatic rings, also known as simple arenes or simple aromatics, are aromatic organic compounds. They consist only of a conjugated planar ring system with delocalized pi electron clouds. Many simple aromatic rings have trivial names. They are usually found as substructures of more complex molecules ("substituted aromatics"). Typical simple aromatic compounds are benzene, indole, and cyclotetradecaheptaene.
Simple aromatic rings with non-carbon ring atoms are called "heterocyclic". For example, rings can include oxygen, nitrogen, or sulfur. Molecules with one ring are called monocyclic as in benzene. Molecules with two rings are called bicyclic as in naphthalene. Molecules with more than two rings are called polycyclic as in anthracene. Simple monocyclic aromatic rings are usually five-membered rings like pyrrole or six-membered rings like pyridine. Fused aromatic rings consist of monocyclic rings that share their connecting bonds.
Heterocyclic aromatic rings.
The nitrogen (N)-containing aromatic rings can be separated into basic aromatic rings that are easily protonated, and form aromatic cations and salts (e.g., pyridinium), and non-basic aromatic rings.
In the oxygen- and sulfur-containing aromatic rings, one of the electron pairs of the heteroatoms contributes to the aromatic system (similar to the non-basic nitrogen-containing rings). The second lone pair extends in the plane of the ring (similar to the basic nitrogen-containing rings).
Criteria for aromaticity.
All of these molecules meet the following criteria:
In contrast, molecules with 4n pi electrons are antiaromatic.