I'm sorry for being so vague here, but perhaps my post will ring a bell for someone who can fill in the details. A few weeks ago in a waiting room I was reading about a proposed rearrangement of the Periodic Table based on measurements of the ionization potentials (I think I got that right) of one of those high atomic number elements. You know, the ones that live for only a fraction of a second. It caught my interest. Then a few days later I saw an article about the same issue, this time not in a popular magazine but in a specialized one. I think it was probably Physics Today. Anyway, the point they made that I can't understand is this. They were saying that the inner electrons are moving so fast that their kinetic energy makes makes a significant contribution to the atom's mass. (They phrased it as the relativistic mass of the electrons, but for those of us who abhor that term I phrase it differently.) I get that. But then they said that the increased mass somehow changes the shielding effect that the inner electrons have on the outer electrons. I can't understand this. I can see how an increased charge on the inner electrons would effect the shielding they provide, but an increased mass! I don't get it. Anyway, the upshot is that the outer electrons have a different ionization potential, or something like that, as a result of this altered shielding. And for that reason it may be necessary for the chemists to change the way these atoms are grouped in the Periodic Table. Again, I apologize for the vagueness. Part of it is because I'm working from memory, but most of it is because I'm not a chemist.