In the book of Modern Physics Experiment by Melissinos, it is stated at page 13 that
Another important point is that in principle the experiment must be performed with a monatomic gas; since if a molecular vapor is bombarded. it is possible for the electrons to transfer energy to the molecular energy levels which form almost a continuum. Some of the preferred elements for the Frank-Hertz experiment are Mercury, Neon, and Argon.
What is the reason for using (as a preferable atoms) Mercury, Neon or Argon in the Franck-Hertz experiment? What are the advantages of these atoms?