Since Hydrogen is lighter than oxygen and nitrogen, is the upper atmosphere of the Earth mostly hydrogen?
These planned space planes that I’ve heard about that skip along on the edge of space like a pebble on water and that scoop up oxygen to burn, how can they if it is mostly hydrogen?
When someone finally perfects fuel cells and hydrogen tanks on cars, can I erect a really tall pipe and just suck down all the hydrogen I need?
Nope. Hydrogen is really light, so almost all gaseous hydrogen in the atmosphere just drifts off into space. Gravity just doesn’t hold it down very well.
Lucky for us so much hydrogen got weighted down with oxygen and carbon atoms. Handy little atomic anchors to hold your hydrogen on the planet.
I think you’ll find very little free hydrogen in the atmosphere. (Not enough to form a significant layer?) Plus, hydrogen that reaches the upper atmosphere can drift off into space due to its low escape velocity.
Well, the Earth’s gravity doesn’t. But fortunately for us, the sun’s gravity does a nice job of holding hydrogen down. 
Wait a sec… Hydrogen has a lower escape velocity???
Are you telling me that a gram of hydrogen has a lower escape velocity than a gram of oxygen?
howardsims - this is a good link for a quick precis on the atmosphere’s structure - http://server5550.itd.nrl.navy.mil/projects/HAARP/ion1.html. It explains that the upper layer of atmosphre is mostly monatomic Oxygen or ionized O[sub]1[/sub] atoms & free electrons, of decreasing density as you move out from the planet’s surface, so they will be able to scoop Oxygen.
Re the escape velocity - if you had a gram of hydrogen in a (theroretically) massless container, and a gram of Oxygen in the same type of container, no. The issue rests on the mass of the individual molecules/atoms moving. Gas molecules move due to the available heat energy and individual Hydrogen molecules have less mass, so for the same amount of energy will move faster, Nitrogen & Oxygen atoms have more mass, so it would take more energy to accelerate them to the same speed. I probably didn’t explain it very clearly, but here’s some websites that are fairly clear:
http://www.physics.fsu.edu/courses/fall98/ast1002/section3/Chapter17.html (and search for hydrogen in the text)
http://utrao.as.utexas.edu/astf301/log/log14.html (and search for hydrogen in the text)
http://andes.as.arizona.edu/~mhamuy/atom_vel.html