# Why are all molecular compounds composed of atoms of two or more different elements?

We can certainly have atomic molecules, and the Noble Gases, room temperature gases, are CERTAINLY molecular species that obey state functions, as also are molecules such as dinitrogen, and dioxygen. However, COMPOUNDS, by definition, must contain AT LEAST TWO ELEMENTS that are chemically bound together...for instance, $N O$, $C O$, $C {O}_{2}$, $H C l$, ${H}_{2} O$...and all of these are molecular compounds, the which, if not gases at room temperature (what is the exception?), consist of a discrete molecular unit that is chemically bound together.