# Which is the lightest gas amongst CO, CO_2, "chlorine", CS, Xe?

The one trick in this question is the molecular weight of chlorine gas. Save for the Noble Gases, ALL the elemental gases are bimolecular: ${H}_{2}$, ${N}_{2}$, ${O}_{2}$, ${F}_{2}$, $C {l}_{2}$ (in fact all the halogens are bimolecular in the elemental state, gas, solid, or liquid).