Rich S. wrote:
...
It's been a long time since I've studied any basic chemistry and I didn't
know that much about it then. If different gaseous elements tended to
separate out, wouldn't Nitrogen separate from Oxygen in the atmosphere?

Oxygen and Nitrogen are close to having the same MW which minimizes
stratification effects. But even relativley heavy molcules like
styrene will not stratify measurably over a height of a meter or
so, given sufficient time to diffuse through the volume.

Diffusion rate rather than desity differences dominate uneven
distributions of gasses in most situations. IIRC diffusion rates
are inversely proportionate to the square of MW but bouyancy
is directly proportionate.

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FF