The chemical formula for cyanogen chloride is NCCl. The chemical formula for hydrogen cyanide is HCN.

Cyanogen chloride differs from hydrogen cyanide in chemical structure and formula.

Hydrogen cyanide is HCN, composed of the elements hydrogen, carbon, and nitrogen.

Cyanogen chloride is ClCN, composed of the elements chlorine, carbon, and nitrogen.

From the formulas and structures it is possible to infer that cyanogen chloride is denser as a gas, and less likely to produce cyanide ions (CN-) than hydrogen cyanide.

Arsine Radon Carbon Monoxide Boron trichloride Chlorine pentafluoride Chlorine trifluoride Cyanogen Cyanogen chloride Diborane Dinitrogen tetroxide Fluorine Germane Hydrogen selenide Nitric oxide Nitrogen dioxide Nitrogen trioxide Nitrosyl chloride Oxygen difluoride Phosgene Phosphine Phosphorus pentafluoride Selenium hexafluoride Stibine Sulfur tetrafluoride Tellurium Hexafluoride Tetraethyldithiopyrophosphate

Cyanogen is NCCN and hydrogen cyanide is HCN. I have no idea what "hydrogen cyanogen" might mean unless someone's getting the two of those confused.

Methionine