'Hydrogen and Helium are lost into space'.... I don't think so
Pretty much wot Goldenrivett said...
It's been a while so I can't remember the joys of the statistical mechanics and probabilities of all of this but as I recall it whilst the
average speed of the lighter gases such as Hydrogen and Helium may not be above escape velocity the distribution of velocities of the H/He atoms in the atmosphere means that at any one time
some of those will be travelling above escape velocity. If they can exit the atmosphere unhindered by impacting other gas molecules they are indeed lost in space...then at some time later some more H/He atoms end up being at the top of the velocity curve, and they leak away, etc etc.....there will always be a few around in the atmosphere, but they haven't been around long enough to have (by chance) to reach the magic velocity in the right place...when they do, they're gone..It's an ongoing process.
In fact all lighter gases are subject to the same risk, it's just that H and He, being lightweight, have the highest average speeds, and therefore are at most risk of being lost....