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I know air is made up of roughly 78% nitrogen, 20% oxygen, etc.. but I would like to know what percentage of air is just space and has no atoms at all.

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    $\begingroup$ You can try to estimate the "length" of empty space among molecules by a quantity called mean free path. Look at the table there, en.wikipedia.org/wiki/Mean_free_path#Kinetic_theory_of_gases On the other hand this is a philosophical question as. Is the space between atoms and molecule empty? Is the space really empty? $\endgroup$
    – AChem
    Dec 29, 2021 at 2:03
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    $\begingroup$ I once watched a VSauce video where he mentioned that space has only 10 atoms per cubic cm and interstellar space has 1 atom per cubic cm. In comparison, the best vacuum achieved in laboratory is ultra high vacuum (UHV) where there are 100 atoms per cubic cm. $\endgroup$ Dec 29, 2021 at 3:48
  • $\begingroup$ I see the clash of 2 meanings of air. Air(1) as a particular mixture of gaseous components, surrounding the Earth. Air(2) as the space occupied by Air(1). Air(2) = vacuum + Air(1). Comparing densities of liquid and gaseous air is trivial. $\endgroup$
    – Poutnik
    Dec 29, 2021 at 8:56
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    $\begingroup$ I don't think this is a homework question. It is a naive question but not entirely trivial or inappropriate here. Not sure it should be closed. $\endgroup$
    – matt_black
    Dec 29, 2021 at 18:26
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    $\begingroup$ I agree, this is not a homework question. $\endgroup$
    – AChem
    Dec 30, 2021 at 3:46

2 Answers 2

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One way to answer this is to compare nitrogen gas at standard pressure and temperature to liquid nitrogen at 1 atm. The density ratio is about 1500. In the liquid, space is nearly filled by the N2 molecules, so it follows that the molecules in the gas only occupy 1/1500 of the total volume. Also, since the cube root of 1500 is about 12, it must be true that the typical distance between molecules is about 12 times the mean molecular diameter.

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  • $\begingroup$ Good answer and a cool user name! $\endgroup$
    – AJKOER
    Jan 1, 2022 at 17:26
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Air is mostly molecules not atoms and most of the space in a gas is empty

Most of the gaseous components of air are not individual atoms but molecules (oxygen, nitrogen are diatomic, carbon dioxide and water vapour are triatomic). The only common single atom is argon (about 1% of the atmosphere by mole ratio).

But this is mostly irrelevant to the underlying question. Gases occupy far more space than the molecular size of the components of the gas. To a good approximation, all gases occupy just over 22 L of space at standard temperature and pressure whatever the molecule is. This is far larger (by a factor of 1000-2000 times the volume taken up by the molecules or atoms making up the gas if we use the normal size of the molecules (as 10ppb points out in their answer).

So most of the space in a gas is not "occupied" by anything at a given snapshot in time. But, remember, gas molecules move around quickly which is why they "fill" a space so a view at one moment in time is a misleading way to think about the issue.

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    $\begingroup$ Molecules are made up of atoms, aren't they? $\endgroup$
    – Barmar
    Dec 29, 2021 at 15:05
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    $\begingroup$ @Barmar Yes, but when working out the volume occupied by a gas, you count molecules not atoms. The distinction matters. $\endgroup$
    – matt_black
    Dec 29, 2021 at 18:23

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