Lightest known gas?
It is Hydrogen, it is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of1.00794 u (1.007825 u for Hydrogen-1), hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly composed of hydrogen in its plasma state. Naturally occurring elemental hydrogen is relatively rare on Earth.
aren't i clever! ;)