If hydrogen is the lightest gas known, would it rise making the outermost part of the atmosphere mostly hydrogen?

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    $\begingroup$ Hydrogen reacts with oxygen to water. Therefore, pure hydrogen does rise. But Helium does. Please have a look into the "Natural Occurrence" section of the Wikipedia article on Helium. $\endgroup$ – daniel.neumann Feb 22 '18 at 21:37

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).

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    $\begingroup$ @Imtherealsanic all molecules in the upper atmosphere are liable to be stripped away, and it takes less effort to strip away an Hydrogen molecule than a Nitrogen molecule. That's is part of the explanation of why Hydrogen is so rare on the Atmosphere, and so common elsewhere. $\endgroup$ – Camilo Rada Feb 22 '18 at 21:58
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    $\begingroup$ @Imtherealsanic Yes, but because that makes it lighter, so if a particle of the solar wind hit it, it will give it more speed, making it more likely that it will achieve escape speed and fly into the deep space. $\endgroup$ – Camilo Rada Feb 22 '18 at 22:08
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    $\begingroup$ @Imtherealsanic That's perhaps a separate question, but yes, it's much easier for hydrogen to escape into space because it has a faster velocity due to it's lower mass. Air molecule velocity is inversely proportional to mass, and the faster a molecule moves when it's in the top-upper atmosphere or exosphere, the greater chance that it will escape the planet. There's a ratio between velocity of the molecule and escape velocity that enables lighter molecules to escape. See en.wikipedia.org/wiki/Root-mean-square_speed and en.wikipedia.org/wiki/Atmospheric_escape $\endgroup$ – userLTK Feb 22 '18 at 22:08
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    $\begingroup$ @CamiloRada thank you. This will make the question easier to answer. :) $\endgroup$ – Eevee Feb 22 '18 at 22:12
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    $\begingroup$ @userLTK you too :) $\endgroup$ – Eevee Feb 22 '18 at 22:15

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