# Why do Hydrogen and Helium have molar volumes higher than an ideal gas?

In my chemistry textbook, there is a table of the molar volumes for different gases. Most of them are below 22.42 l/mol, the ideal gas molar volume, but I noticed that hydrogen and helium do not. Hydrogen is 22.433 l/mol and helium is 22.434 l/mol. 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?

• 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. Aug 16, 2016 at 20:24
• 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. Aug 16, 2016 at 21:06
• Are these tiny differences between hydrogen and helium really significant? Aug 17, 2016 at 13:20
• @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] Aug 18, 2016 at 20:17
• The ideal gas approximation has nothing to do with mass. Ideal gases just have neither molecular volume nor intermolecular interactions! Sep 19, 2021 at 15:53

$$\begin{array}{cr} \text{Gas} & T_\mathrm{crit}/\pu{K} & P_\mathrm{crit}/\pu{bar} & T_\mathrm{r} & P_\mathrm{r}\\ \hline \text{H}_{\text{2}} & 32.9 & 12.9 & 8.3 & 0.08 \\ \text{He} & 5.2 & 2.3 & 52.5 & 0.44 \\ \text{N}_\text{2} & 126 & 33.9 & 2.2 & 0.03 \end{array}$$