Why does the compressibility factor of gases like Carbon Dioxide and Nitrogen first decrease then increase with increase in pressure , but for Hydrogen and Helium it only increases with the increase in pressure ?

  • $\begingroup$ See my answer here, which addresses this question almost exactly. The only thing to add is the same reasoning I applied for $\ce{H2}$ also applies to helium. Note that for both molecules there is actually a minimum, but they are extremely tiny and not observable on the scale of the plots you're referring to. $\endgroup$ – jheindel Nov 9 '17 at 19:21