Why doesn’t the nitrogen in the air float above the oxygen in the atmosphere.
In: Physics
Never underestimate brownian motion. Molecules vibrate. Gas molecules chaotically ricochet around at high speed. An oxygen molecule at room temperature might be moving around 650 miles an hour. Sure, nitrogen molecules might way 12.5% less than nitrogen molecules on average, but all that chaotic motion keeps them well and mixed up.
Even if there weren’t air currents moving up and down, as there are not near as many in the upper atmosphere.
Turbulence in the atmosphere keeps the gasses mixed quite well. But above a certain altitude, the turbopause which is about 100km up, the densities of gas can have an effect on the composition of air at that point. So the air above 100km may be more nitrogen concentrated. The gases are also quite easily mixed because they don’t have such a strong attraction to one another. If you put oil in water, the oil almost immediately floats up because of how water molecules are more heavily attracted to each other. This doesn’t happen with nitrogen and oxygen. Mixing doesn’t happen very well above the turbopause due to lower pressures. So molecules have more space between them and less chance of colliding with one another allowing them to arrange themselves by their molecular weight.
Because the planet rotates. This causes huge amounts of sunlight to fall on parts of it. That gets, and keeps, the air all stirred up.
Gas is less dense than you think. If you could condense all the gas in your bedroom into a liquid, it would fit into a coffee mug. The space between the particles is enormous. Most of the particles are just bouncing off each other due to heat, barely influenced individually by gravity. In effect, buoyancy is negligent. The gas mixes into a fairly homogenous mixture.
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