"Air is a mixture of gases, primarily nitrogen and
oxygen. Oxygen is denser than both air and
nitrogen, at all temperatures and pressures, but
only slightly. Since they don't separate from
each other, we generally don't worry which is
lighter or heavier. The difference in the density
of nitrogen and oxygen gas comes from their
molecular weight, which is small (4 g/mol). You
can account for the density of air at normal
temperatures and pressures (68F, 1 atm) by adding
up the weighted percentages of nitrogen, oxygen
and argon. (0.1% is other gases like carbon
dioxide but we'll ignore it for this calculation.)
Air = 78.08% Nitrogen + 20.95% Oxygen + .93% Argon
0.7808 (1.2506) + 0.2095 (1.4290) + 0.0093 (1.7837) =
which is very close to the measured 1.205
kg/m3 density of air.
Note: Standard Temperature and Pressure
(0oC (273.15 K, 32oF) and 1
There is a common misconception that nitrogen
is denser than air because when liquid nitrogen
boils, the nitrogen gas pools on the floor. This
isn't because the nitrogen is denser but that the cold gas (N2) is denser than the warm gas (air) around it.
Our atmosphere is held to the earth's surface by gravity. The atmosphere is less dense at higher altitudes (which leads to lower air pressures) because the molecules feel less gravitational force. The thinner air has the same percentages of nitrogen and oxygen, but less of both. You generally don't notice that you are getting less oxygen per breath until you get 4000 ft or greater above sea level. At 5280 ft high, Denver has 17% less atmosphere than sea level so most visitors take a while to adjust.
You might think that heavier gasses (like
oxygen) would be concentrated closer to sea level
and lighter gasses concentrated in the upper
atmosphere because their weight/densities. This
would be true if there was no wind, but their
differences in weight are canceled out because our atmosphere is in constant motion (wind) due to local differences in temperature. There is
actually very little helium or hydrogen in our
atmosphere at all - these gases are so light that,
once released, they eventually drift to the top of
the atmosphere where they can escape Earth's
gravity off into space!
Here is an excellent summary of how air's
density affects weather:
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