Question

Obviously, a consolidated mass of helium (such as a He balloon) will rise to hundreds of thousands of feet of altitude. However, in the actual atmosphere, all of the He is at very low concentration. Is there a layer rich in He at the top of the atmosphere, or is the mixing due to convection and diffusion strong enough to keep things well mixed? Or does an individual molecule of He not really have a "density"??? I'd tend to think that the individual molecule does have a density, but the mixing prevents a layer of He from being formed at the top of the atmosphere.

Answer 1

I doubt there would be much of a difference, but then again Helium is lighter than nitrogen or Oxygen so it would be less affected by gravity, but it comes in such low concentrations in the atmosphere of any altitude that it plays very little effect.

The average vacuum around the He molecule that is closer to the He than any other air molecule makes up the the molecule's "volume" microscopically, and its actual mass, its mass.

But intuitively, the density of 1 mol of atmosphere compressed into a fixed volume should be less for high altitude atmospheres, yes.

Answer 2

Any He near the top of the atmosphere would easily escape into space.
There is very little He on Earth, It is non-reactive, and is difficult to keep in any particular place.

Answer 3

Yes, most of the Earth's loss in mass comes from Hydrogen doing that.