Why does xenon exhibit more spectral lines than heavier elements like mercury?

[Goodver]

spectral lines

As far as I understand spectral lines represent allowed transitions between energy levels in a particular atom. And that the number of energy levels should increase with increase of a number of electrons, due to electron-electron and spin-angular-momentum couplings. Right? However it seems like this tendency is not quite so, since for instance Xenon has many spectral lines while mercury only few.

Why xenon has many spectral lines, but other elements having greater atomic number might have less?

I am on a master level.
Thank you.

 

[mfb]

Most electrons are bound too tighly to have transitions in the energy range of visible light. The number of electrons in the outer shell depends on the position in the period table (noble gases have many of them), and the specific position of lines and their intensity is a complicated thing.
It also depends on the state - single atoms have different lines than the same atoms in solids.