Is there strong supporting evidence of discrete electron shells or orbitals surrounding atomic nuclei? I realize the math works out and we have energy frequencies emitted, perhaps even atomic diameter measurements. But these still seem indirect and allow for other possible explanations. Is there direct experimental evidence for these? The orbitals concept is always shown almost as fact, not theory, so wondering what experiments must support such strong conclusions and don't mention any other alternatives?
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asked May 30, 2015 at 20:18
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$\begingroup$ Bohr's shell model is an old and discarded atomic model and one shouldn't try to interpret too much into the occasional and historically motivated references to it in chemistry and e.g. for the description of x-ray emission spectra. The Schroedinger equation together with a few kludges like electron spin is a pretty handy approximate explanation for the structure of the periodic table and it should be the lowest level of explanation that one should have in mind these days while talking about atomic structure. $\endgroup$– CuriousOneMay 30, 2015 at 20:31
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3$\begingroup$ You should not expect "strong supporting evidence" for the Bohr model, since the Bohr model is false. The model of atomic orbitals, however, yields excellent predictions and has not been falsified. The question "Does this allow for other possible explanations" is, in science, trivially answered with "Yes" unless you ask about a specific alternative eplanation, so I'm not quite sure what you are asking for. $\endgroup$– ACuriousMind ♦May 30, 2015 at 20:32
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$\begingroup$ @ACuriousMind: Thank you. Is there evidence for "atomic orbitals" at descrete interval distances from the atomic nucleus? You say it hasn't been falsified and it makes predictions, but didn't cite evidence for this, specifically. Thank you. $\endgroup$– Brad Cooper - Purpose NationMay 30, 2015 at 20:43
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$\begingroup$ Well, the discrete spectral lines of atoms are quite direct evidence, for example. Hyperfine measurements, ionization energies, that sort of thing. $\endgroup$– ACuriousMind ♦May 30, 2015 at 20:52
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$\begingroup$ Yes, as mentioned, aware of spectral lines, but why does that only mean descrete orbitals? And could you provide a little more info about hyperfine and ionization energies? $\endgroup$– Brad Cooper - Purpose NationMay 30, 2015 at 20:57
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1 Answer
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Historically, the first strong evidence for the existence of discrete orbitals was the Franck-Hertz experiment. They threw energetic electrons to a gas tube (in their original experiment, they used mercury), and they found very sudden drops in the transmitted electron flux at concrete energies. Those events occurred when electrons inelastically collided with mercury atoms, and the mercury atoms only could absorb the energy in the collision at certain discrete energies. They also found that light with the right frequency was emitted after the collisions.
Since then, a really huge amount of experiments have been made that are even capable to manipulate atomic energy levels (for example, qubits based on the Jaynes-Cummings hamiltonian)
EDIT: To address the part of the discrete distances, I think that it's enough to invoke Coulomb's law. But I'll include a example of its consequences: the paramagnetism of rare-earth ions.
Paramagnetism is caused by the angular moment of unpaired electrons. Angular moment has two origins: rotation (i.e., orbital angular momentum) and intrinsic (i.e. spin). In many elements, like transition metals, unpaired electrons are located in the outer shells. In a crystal, these electrons are near the neighbor atoms, so they "feel" their electric field. As a consequence, their orbital angular momentum is affected, and can even be averaged to zero (it is said to be 'quenched'). Thus, for most metals, the magnetic moment comes only from spin. But in the lanthanides, the unpaired electrons are located in the 4f orbital, that it is very close to the nucleus, and the 5s and 5p orbitals are filled and they are farther (of course, I'm talking about the areas where the probability is larger, electrons are not deterministically located). 
Those outer orbitals 'shield' the 4f electrons from the crystalline electric field, so they behave as if they were free electrons: their magnetic moments come from both orbital and spin angular momentum, and these elements have larger paramagnetic responses. That wouldn't happen if the 4f and 5s 5d electrons weren't located at different distances from the nucleus.
answered May 30, 2015 at 21:07
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$\begingroup$ @bosoneado: thanks, but still unsure how they go from electrons with descrete energies to decrete orbitals? Seems like they jumped to the newtonian analog without considering other alternatives? $\endgroup$ May 30, 2015 at 22:30
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2$\begingroup$ @PurposeNation I don't understand what do you mean by 'the newtonian analog'. The Frank-Hertz experiment shows that the atoms can't absorb arbitrary energies, they only absorb in discrete amounts. If an atom can only gain or lose energy in discrete steps, then its energy can only take discrete values. $\endgroup$ May 30, 2015 at 22:41
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1$\begingroup$ @PurposeNation OK, I didn't understood your question correctly. I'll edit my answer to give a (maybe indirect) evidence for the spatial quantizantion of the orbits. But you should realize that the Coulomb law still holds, and the discretness in energies directly implies discretness in orbits. $\endgroup$ May 30, 2015 at 23:03
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1$\begingroup$ @PurposeNation: The "distances" of electrons in orbitals are not discrete, only the expectation values differ for 1s, 1p, 2s etc. . I think you are laboring under very strained assumptions about the meaning of "orbital" here. $\endgroup$ May 30, 2015 at 23:16
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1$\begingroup$ It is important to note that electrons are in orbitals, not orbits. And even high $n$ s-orbitsls have non-zero amplitude at zero radius. There is no point in defending something as incomplete as the Bohr model. It was important to show the way but us now of only pedagogical value. $\endgroup$ May 31, 2015 at 1:36