This seems like the type of question which probably either has a stupidly trivial answer that will make me blush or a stupidly complex answer that will make me sexually aroused...

 

Why does air consist of 79.1% nitrogen, 28.9% oxygen , 0.1% argon and a few other things (or something like that) at sea level rather than there being a distinct level of mainly carbon dioxide with a layer of oxygen on top then a layer of nitrogen on that and a layer of argon on top with other lesser things between?

 

Is it just simply down to wind? If I fill a jar with air then leave it, could I later open it and remove different gases in a variation of fractional distillation?

The average kinetic energy of an object in thermal equilibrium is going to be of order kT (the multiplier is going to be 1/2 * # of degrees of freedom, so if we assume no rotation, it's 3/2). k is boltzmann's constant

 

For any of the molecules you list, the speed is going to be of order 100 m/s. Lots of molecules, lots of collisions. You'll always have a Maxwell-Boltzmann distribution of speeds. That's why they won't stratify.

 

Wind is a minor perturbation on top of all this

gasses do not layer out like immicible liquids, they are all miscible in each other. also, as swansont says the molecules are shifting about like crazy so they'll mix themselves.

 

you can temporarily create layers, such as a tank full of sulphur heaxafluoride which will hold the SF6 for a while but if left long enough even in a room with no breeze the gas would evenly distribute itself around the room.

Ok that seems to make sense thanks.