A ring compound, also called a cyclic compound, is one whose molecules contain atoms chemically bonded together to create a closed chain or circle. Generally, such molecules are organic, and so the ring configuration is composed of carbon atoms. Ring compounds most often contain five or six atoms to form pentagonal or hexagonal structures. Ring compounds may also be assembled to form double-ring structures. For example, 10 carbon atoms can be arranged into two conjoined hexagons, each hexagon sharing two carbon atoms with the other. Similarly, nine atoms can form a pentagon bonded to a hexagon, each ring sharing two atoms with the other ring. Many cyclic, or ring compounds exhibit the special properties of aromatic compounds. Aromatic compounds are volatile and have strong, characteristic odors. They are any of a large class of ring compounds that have alternating single and double bonds, including benzene and compounds that resemble benzene. Common aromatic compounds other than benzene include toluene, naphthalene, and anthracene, all of which are present in coal tar or creosote. Ring structures are generally very stable. Exceptions include three member ring compounds, like ozone. Ozone, consisting of three oxygen atoms bonded into a strained triangular ring, is highly unstable, and therefore very reactive.