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.