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In my Chemistry textbook, there is a table of the molar volumes for different gases. Most of them are below 22.42L, the ideal gas molar volume, but I noticed that Hydrogen and Helium do not. Hydrogen is 22.433L and Helium is 22.434L. Why is this? What makes these particles, with mass, have higher molar volumes than the basically mass-less particles of an ideal gas? And why does the molar volume increase when going from Hydrogen to Helium?

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    $\begingroup$ Mass is hardly relevant here. The deviation from ideal molar volume is an interplay of two factors, both of which are absent in ideal and present in real gases: (1) volume of molecules themselves and (2) attractive interactions between molecules. $\endgroup$ – Ivan Neretin Aug 16 '16 at 20:24
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    $\begingroup$ And of those two factors, the nonzero volume may outweigh the attractive force in the case of hydrogen and (to a greater extent) helium, giving a positive deviation from the ideal gas volume--whereas attractive forces dominate in the other gases, causing a negative deviation from ideal volume. $\endgroup$ – iad22agp Aug 16 '16 at 21:06
  • $\begingroup$ Are these tiny differences between hydrogen and helium really significant? $\endgroup$ – Chet Miller Aug 17 '16 at 13:20
  • $\begingroup$ @ChesterMiller The major differences that come about from the tiny differences in IMF's are in the boiling points (H2: -252.9 degC, He: -268.9) [values from wikipedia] $\endgroup$ – Dan Burden Aug 18 '16 at 20:17

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