text: Different gases will have different mean free paths for molecules and electrons.  This is because different molecules have ionization cross sections, that is, different effective diameters.  Noble gases like helium and argon are monatomic harder to ionize and tend to have smaller effective diameters.  This gives them greater mean free paths.
  Ionization potentials differ between molecules, as well as the speed that they recapture electrons after they have been knocked out of orbit.  All three effects change the number of collisions needed to cause an exponential growth in free electrons.  These free electrons are necessary to cause an arc.