any of a group of colourless, volatile, chemically reactive, lacrimatory (tear-producing), highly poisonous compounds, the molecules of which contain the cyano group (-CN) linked to one of the halogen elements (fluorine, chlorine, bromine, or iodine).

Cyanogen fluoride, which is prepared by heating cyanuric fluoride, is a gas. Cyanogen chloride, made by the reaction of chlorine with hydrocyanic acid or its salts, is a liquid that has been suggested for use as a military poison gas. Cyanogen bromide is formed by the reaction of bromine with salts of hydrocyanic acid; it is a solid that has been used as a fumigant against insects and rodents and as a reagent for the study of the structure of proteins. Cyanogen iodide is made by treating a cyanide with iodine; at atmospheric pressure the solid vaporizes without melting at about 45° C (113° F); it has been used in taxidermy as a preservative.