The noble gas which has the smallest density at standard temperature and pressure would be helium. Looking at the periodic table of elements, among the noble gases, it is helium that has the lightest molecular mass. At stp, helium exist as a gas so it would occupy a larger volume. Given a small mass and large volume, you will have a small density. Helium is colorless, non-toxic, inert and a monoatomic gas. It has the lowest boiling point compared to all the elements. It is the second lightest element after hydrogen. Also, it is one of the most abundant element that can be observed in the universe.
