A<unk>  <unk>  is a<unk> —a chemical compound containing only carbon and hydrogen—that is composed of multiple<unk>  rings. The group is a major subset of the<unk><unk> s. The simplest of such chemicals are naphthalene, having two<unk>  rings, and the three-ring compounds anthracene and phenanthrene. The terms polyaromatic<unk>  or polynuclear<unk><unk>  are also used for this concept.<unk> s are uncharged, non-polar molecules, with distinctive properties due in part to the delocalized electrons in their<unk>  rings. Many of them are found in coal and in oil deposits, and are also produced by the combustion of organic matter—for example, in engines and incinerators or when biomass burns in forest fires.<unk> s are discussed as possible starting materials for abiotic syntheses of materials required by the earliest forms of life.
Polycyclic aromatic hydrocarbon
Alkylphenol
Organochlorine compound
Organobromine compound
Loline alkaloid
Polychlorinated naphthalene
Biogenic amine
Volatile organic compound