Question

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

Answer 1

Well, the Noble gases for one.

Explanation:

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 `Cl_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.)