This is a very naive question. But I think, it's an important point that has been unattempted in textbooks. The question is: How far should one trust the Bohr-Sommerfeld model or the atomic shell theory for all elements in the periodic table? This question generally comes in mind, since we know that the Bohr's model was a kind of hypothesis or ansatz to explain the hydrogen atom spectra. And Sommerfeld modified the quantization condition in a spirit of generalization, using the analogy to the planetary motion under the central force due the sun. These all can be put together as the initial development of quantum mechanics and often regarded as theories of Old Quantum Mechanics. Now we know that the H-atom can be exactly solved from the Schroedinger's equation. But what about atoms with higher atomic numbers (may be we can only expect hydrogen-like wavefunctions for alkali atoms)? Should Bohr-Sommerfeld model (apart from the relativistic correction) still be working ? Then why do people say that in cuprates copper has d9 electronic state? Doesn't that sound imprecise? Thanks.