Why doesn't nitrogen float to the top?

[RocketManJames]

Here's what might be a dumb physics/chemistry question, but I tried to search for an answer and came up a bit short.

So, the other day (Valentine's) I saw a couple with a heart-shaped helium balloon. And, something kind of hit me. The helium balloon floats, because helium is lighter than air, which is mostly nitrogen and oxygen.

Now, nitrogren is lighter than oxygen, so why doesn't all the nitrogen float to the top and all the oxygen and other heavier gases like carbon dioxide sink lower down. If this was the case, wouldn't we all be sort of screwed with all the heavy CO2 at ground (breathing) level? And, even purish oxygen can't be safe.

So, based on this, I assume that the air is mixed up pretty good and the various gases that it is composed of aren't layered by their weight.

Why is this?

-RMJ

[purnell]

[ QUOTE ]
Now, nitrogren is lighter than oxygen, so why doesn't all the nitrogen float to the top and all the oxygen and other heavier gases like carbon dioxide sink lower down. If this was the case, wouldn't we all be sort of screwed with all the heavy CO2 at ground (breathing) level?

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In a closed space, with no air circulation or drafts, heavier gasses do settle to the floor. But outside air is thouroughly mixed becuse of local differences in temperature and pressure, aka the wind.

[SaulPaul]

also diffusion

[AWoodside]

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also diffusion

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Eh... the relationship between time and distance travelled by diffusion can be approximated by x^2 = 2Dt where D is a diffusion coefficient. D for air is about 0.25 (cm^2/s), I don't remember the exact number and it depends on a lot of factors but that's roughly right. So for an air molecule to diffuse across a 1 meter space, it would take about 20,000 seconds. Diffusion isn't enough when we're talking about large scale mixing, air currents and what not are the dominant mixing mechanisms.

[thylacine]

[ QUOTE ]
Here's what might be a dumb physics/chemistry question, but I tried to search for an answer and came up a bit short.

So, the other day (Valentine's) I saw a couple with a heart-shaped helium balloon. And, something kind of hit me. The helium balloon floats, because helium is lighter than air, which is mostly nitrogen and oxygen.

Now, nitrogren is lighter than oxygen, so why doesn't all the nitrogen float to the top and all the oxygen and other heavier gases like carbon dioxide sink lower down. If this was the case, wouldn't we all be sort of screwed with all the heavy CO2 at ground (breathing) level? And, even purish oxygen can't be safe.

So, based on this, I assume that the air is mixed up pretty good and the various gases that it is composed of aren't layered by their weight.

Why is this?

-RMJ

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The density of a gas (in the atmosphere) of molecular mass m, at height h is proportional to

e^{-Cmh}

for some constant C. In other words the density tapers off exponentially as a function of height h, but heavier gases taper off faster due to bigger m in the exponent. That's why the percentage of oxygen at the top of Everest is lower than it is at sea level.

BTWFWIW diffusion (in the Earth's gravitational potential well) definitely accounts for this.

[madnak]

There are also a lot of natural cycles that have an influence. The whole atmosphere was very different (no oxygen) before life started. Now organisms play a major role in the composition of the environment and in gas cycles.

[gull]

Carbon dioxide does sink. This is how volcanic gasses can kill.

[thylacine]

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Carbon dioxide does sink. This is how volcanic gasses can kill.

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True, but it eventually difuses throughout the atmosphere. What I said above refers to the eqilibrium state.