# Which element under normal conditions exists as monatomic (one atom) molecules?

All of the Noble gases are gaseous at room temperature, and these are demonstrably mono-atomic. The rest of the elemental gases are diatomic at room temperature; i.e. dinitrogen, dioxygen, ${F}_{2}$, and $C {l}_{2}$. Mercury vapour (which certainly exists even though the vapour pressure is low at RT) is treated as mono-atomic. (I once had to distill a large quantity of mercury; the distillation was fine but it took me about 2 days of work to clean the distillation setup.)