What is the reason for using mercury, neon, or argon in Franck and Hertz experiment?

Question

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?

Answer 1

As the citation said, the experiment must be a monoatomic gas. Mercury is a metal that forms monoatomic gas, as do the noble gases neon and argon. Noble gases usually don't bind to each other(or different atoms) to form molecules in contrast to oxygen or nitrogen.

Answer 2

The reason for using these gases is that they are monoatomic. These gases are in the group of noble gases, so they provide an environment with the least reaction for testing, but what is the reason for using mercury? Mercury, although a liquid metal, but in the gas phase behaves like the same monoatomic gases.