I'm currently working on a paper dealing with aspects of Franck-Hertz experiment, and encountered some puzzling descriptions and symbols in an article published in the past, as follows: 1. The article includes a simplified energy level scheme of mercury. This scheme includes also 7th & 8th levels, despite the fact that the outermost level in Hg is n=6. Any reason or explanation to this 'odd' diagram ? 2. It's claimed in this article that when a triplet appears it means that the electronic configuration consists of 2 electrons in 6s6p orbitals "giving a total P state where the spins are coupled to a total spin with quantum number S=1". How this 6s6p configuration came to be so ? what's the meaning of the quoted phrase ? - I'm familiar with spin physics, but the quoted phrase is seemingly unclear. It'll great to elaborate on these. 3. d orbital isn't distributed equally probable along x, y & z axes. As it's shown in various textbooks, and Wikipedia isn't excluded, there is a kind of a spatial preference: x & y are populated first, and the 'leftovers' go to z. What's the reason for this spatial asymmetry ? Thanks to all participants, Bentzy cohen.