Re: What is the reason why hydrogen, oxygen, and nitrogen all end in -en?

Actually, they all end in -gen! The root word -gen- means "to produce or make" as in "generate." Hydro- means "water," so hydrogen is "that which makes water." Oxy- means "acid" and nitro- means "niter" (also known as saltpeter: potassium nitrate). Fluorine, Chlorine, Bromine, Iodine, and Astatine (all ending in -ine?!) are halogens: halo- means "salt." In each case, the element was first discovered as a necessary component of a common compound or reaction. The names reflect their discoveries as well as something about their chemical activities.

Brilliant observation!