Matter & Energy

Matter is composed of atoms or groups of atoms called molecules. The arrangement of particles in a material depends on the physical state of the substance. In a solid, particles form a compact structure that resists flow. Particles in a liquid have more energy than those in a solid. They can flow past one another, but they remain close. Particles in a gas have the most energy. They move rapidly and are separated from one another by relatively large distances.

Aromatic Compounds

Aromatic Compounds, large group of organic chemical compounds that usually contain closed rings of carbon atoms. Some aromatic rings, however, may also contain an oxygen or a nitrogen atom. The term was originally restricted to the coal tar product benzene and its derivatives, but it now includes about half of all organic compounds, the remaining half being classified as aliphatic compounds.

Important aromatic compounds include all hormones and vitamins except vitamin C; virtually all flavorings, perfumes, and organic dyes, synthetic and natural; those alkaloids that are not alicyclic (aliphatic bases such as putrescine are sometimes incorrectly classified as alkaloids); some substances of military importance, such as TNT and tear gas; many drugs; and a wide variety of other substances.

Certain derivatives of aromatic compounds have special properties and special names. For example, the addition of the hydroxyl group to an aromatic hydrocarbon yields a phenol.

See also Hydrocarbons. For a general discussion of organic compounds, see Organic Chemistry.