If hydrogen is the lightest gas known, would it rise making the outermost part of the atmosphere mostly hydrogen?
Images to associate with your answer would be much appreciated.
I had a similar question, and then I learned that at the molecular level it is diffusion what dominates, that means that despite Hydrogen is lighter, it won't rise to the top of the atmosphere. Flotation as we visualize it doesn't really work at the molecular level (because it is overcome by chemical diffusion). Therefore, the Hydrogen in the atmosphere is evenly distributed trough it.
To clarify it further, a parcel of hydrogen would indeed rise, but as soon as it gets diluted in the surrounding air, the buoyancy of individual molecules/atoms would become negligible in comparison to the chemical diffusion that will take the atoms in any direction, not just upwards, leading to further dilution of the original molecules within the parcel throughout the atmosphere.
As for why that's the case, let's me explain how I visualize it: Imagine you put two a thousand million ping-pong balls in the middle of the Pacific ocean. Half of them with a piece of Iron inside. If you put a magnet in the coast of Mexico, you would expect all the balls with the piece of iron to end up in Mexico. However, the forces of the wind and waves hitting the balls in random ways is so much more powerful than the magnetic attraction in this case, that most likely the balls will end up all over the Pacific. In the same way, all molecules in the air are bouncing against each other with such a strength, that the fact that one is lighter than the other doesn't really play a role in the trajectory a molecule will follow on its journey trough the atmosphere.
In any case there is not much Hydrogen in the atmosphere, that only contains a 0.00005 % of molecular Hydrogen (H2).