The lightest gas on earth is Hydrogen.
Hydrogen is the lightest gas on earth and also the most explosive.
As well as being light and explosive the hydrogen gas is also very abundant on earth.
Also the atomic weight of Radon is 222 atomic mass units making it the heaviest known gas.
The gas Radon is 220 times heavier than the lightest gas, Hydrogen.
Hydrogen Gas also has 7 percent the density of air does.
Hydrogen is also the chemical element with the symbol H and atomic number 1.
With a standard atomic weight of 1.008, hydrogen is the lightest element in the periodic table.
Hydrogen is also the most abundant chemical substance in the universe, constituting roughly 75% of all baryonic mass.