Which is a monatomic gas at STP? chlorine, fluorine, helium, nitrogen?

Answer 1

Helium.

Answer 2

Check your periodic table. The atoms that form diatomic molecules are H, N, O, F, CL, Br .. they all have pretty high electronegativity (that means they all are trying pretty hard to suck in an extra electron so they are really anxious to combine, even with themselves!) But He is not trying to suck in an extra electron. so it isnt going to hook up with itself or anyone else. If you think about how the electrons are arranged on the outside of the atoms, that will tell you why.