In organic chemistry, the term for any compound or other material containing a diazo group, which basically consists of either bivalent :N=N: or bivalent =N=N (where N is the chemical element nitrogen, : denotes a single bond, and = denotes a double bond) bonded to one hydrocarbon group and one other element or group, such as C6H5N=NOH (benzenediazo hydroxide) and CH2=N=N (diazomethane).
The term diazo in platemaking and photography typically refers to a compound containing salts of diazonium, denoted by the general formula [Ar]N2X, where Ar represents an aryl group (any organic group created by the removal of a hydrogen atom from a hydrocarbon, such as phenyl, C6H5: , produced by the removal of a hydrogen atom from benzene, C6H6) and X represents a negatively-charged ion. One common diazonium salt is benzenediazonium chloride, C6H5N2Cl, which includes a diazo group, an aryl group (phenyl), and chlorine. The mixture of a diazonium salt with an azo dye, a dye made from organic compounds comprising an azo group (an organic molecule consisting of bivalent :N=N: joined to two hydrocarbon groups, such as azobenzene, C12H10N2) is sensitive to ultraviolet, violet, and blue light, and can be used effectively in light-sensitive coatings for photolithographic printing plates. An advantage of diazo coatings is that they are not affected by changes in temperature or humidity, which allows them to have a shelf-life in excess of that possessed by other types of plates. (See Diazo Plate and Plate: Offset Lithography.)