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Bohr’s model of the atom predicted hydrogen’s spectral emissions lines —— a huge success.

What about Thomson’s plum pudding model? Did it make any successful predictions?

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    $\begingroup$ It "successfully" got a bunch of positive and negative charges to coexist together without flying apart. Remember that the strong force was not known at the time - nobody imagined that all the positive charge would sit in one small place with all the negative charge around it. And then Geiger & Marsden made people imagine that, and all the consequences of it. $\endgroup$
    – Jon Custer
    Sep 6, 2023 at 12:28
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    $\begingroup$ Predictions? No, explanatory power with respect to what was know before Rutherford's experiment, yes. $\endgroup$
    – Mauricio
    Sep 6, 2023 at 14:52
  • $\begingroup$ @JonCuster : "nobody imagined that all the positive charge would sit in one small place with all the negative charge around it." Nagaoka? $\endgroup$
    – akhmeteli
    Sep 7, 2023 at 15:15
  • $\begingroup$ @akhmeteli - and Lodge as well. Without the strong force to hold the protons together, and quantum mechanics to avoid radiation, it was tough to convince folks. Even Nagaoka's discussion of the 'Saturnian' model had lots of qualifiers around it. As he himself put it, "The rough calculation and rather unpolished exposition of various phenomena above sketched may serve as a hint to a more complete solution of atomic structure." (Phil. Mag. 1904). $\endgroup$
    – Jon Custer
    Sep 7, 2023 at 15:29
  • $\begingroup$ @JonCuster : "Without the strong force to hold the protons together, and quantum mechanics to avoid radiation, it was tough to convince folks." I am not sure this is relevant, as I am not sure Rutherford knew about the strong force in 1911, and I am pretty sure he did not know about "quantum mechanics to avoid radiation". And, by the way, the strong force does not hold protons together, it holds together a collection of protons and neutrons. $\endgroup$
    – akhmeteli
    Sep 7, 2023 at 15:42

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Yes, qualitative ones.

Thomson's model consists of a series of orbits, in each orbit a different number of electrons (less electrons for inner orbits, more for outer ones) all disposed in equally spaced intervals and traveling at high velocities. The different orbits need not be coplanar, and Thomson suggests that in fact, they will occupy different planes. All those orbital rings are encased in a sphere of positive charge such that the atom is neutral. Therefore Thomson's and Nagaoka's models are identical for the electron placement, but Nagaoka places the positive charge inside the inner orbit.

The important thing is that in Thomson's model the electron orbits are stable, therefore small perturbations of an electron give rise to an oscillating behavior with a characteristic frequency. As such each electron interacts with light (the source of perturbation) of only a given frequency (in first order perturbation theory) and Thomson's model can explain why atomic spectra are discrete. Contrast with Nagaoka's model that is not mechanically stable (although Nagaoka opens up the possibility that the instability in his model could explain $\beta$ particle emission).

In a follow-up paper (On the Number of Corpuscules in an Atom, Philosophical Magazine 6, 769-781 (1906)) Thomson uses his model to determine how many electrons each atom has. Through 3 different experiments, he concludes that the number of electrons is of the same order of the atomic weight and that the ratio of electrons and atomic weight is constant for all elements.

To summarize, Thomson's model is a mechanically stable atom that explains the spectral lines as a result of the interaction of electrons with light and that the number of electrons in an atom was of the same order as the atomic weight, so qualitative but not quantitative results. Rutherford subsequently will establish the number of electrons as half of the atomic weight (besides the nucleus containing the positive charges) and Bohr will give the correct formula for the spectral lines of Hydrogen.

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  • $\begingroup$ Well that is explanatory power but not really predictive one, unless we claim previous experimental results that motivated the plum pudding model count as predictions. $\endgroup$
    – Mauricio
    Sep 8, 2023 at 0:12
  • $\begingroup$ Thank you, very illuminating. $\endgroup$
    – littleO
    Sep 8, 2023 at 1:43
  • $\begingroup$ @Mauricio, maybe I'm splitting hairs here, but I tried to emphasize two things that were previously unknown. First, the atomic spectra are a result of the electron-light interaction, and Second, the rough number of electrons in a given atom. Should I have made it clearer that both facts were unknown at the time? Although I tend to agree that those are not clear predictions. I was going by the OP's reference of Bohr's explanation of the Balmer series. I will be happy to update the answer accordingly $\endgroup$ Sep 8, 2023 at 3:49
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Rutherford said:

In J. J. Thomson’s model of the atom the positive electricity was supposed (for mathematical reasons) to be distributed throughout a large sphere with the negative corpuscles moving inside it. This hypothesis has played a useful part in indicating possible lines of advance; but it does not fit in with more recent discoveries, which point to a concentrated positive nucleus.

(Discussion on the structure of atoms and molecules, in Report of the eighty-fourth meeting of the British Association for the advancement of science, Australia, July 28 – August 31, 1914 (John Murray, London, 1915), p. 293.

I am not sure what those "possible lines of advance" were.

EDIT (Sep 07, 2023): Heilbron (Physics Today 30, 4, 23 (1977)) considers Rutherford's nuclear model a product of Thomson's research program.

That the nuclear atom was an outgrowth of Thomson's research program appears plainly from the first page of Rutherford's first calculations on the "theory of the structure of atoms," reproduced in figure 2. Note the depiction of the scatterer as a tiny positive nucleus of charge ne, surrounded by a diffuse sphere of negative electricity of fixed radius.

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  • $\begingroup$ This does not seem to definitely answer the question. It is more like a hint. $\endgroup$
    – Mauricio
    Sep 7, 2023 at 13:02
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    $\begingroup$ "Lines of advance" is an idiom borrowed from military strategic planning used to describe the routes or directions that a military force plans to move or advance along during an operation. So in other words, the sentence could be read as, "This hypothesis has played a useful part in indicating possible [venues for further research and experimentation]..." Considering that WWI was kicking off at that time, it's possible the term was becoming more "entrenched" in the public lexicon. $\endgroup$
    – Andrew R.
    Sep 7, 2023 at 20:11
  • $\begingroup$ Some examples of the idiom in select papers: jstor.org/stable/2339921 ; academia.edu/7773917/… $\endgroup$
    – Andrew R.
    Sep 7, 2023 at 20:11
  • $\begingroup$ @AndrewR. : Thank you, I understand what the expression "lines of advance" means in general, but I don't know what specific "lines of advance" Rutherford had in mind, for example, it is possible that Thomson's model was useful for development of Rutherford's model, but I don't have any proof. $\endgroup$
    – akhmeteli
    Sep 7, 2023 at 20:28
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    $\begingroup$ @akhmeteli, just speculating here, but I interpret the quote (based on the phrasing I provided earlier) as Rutherford saying that Thompson's model/theory was "useful" in determining future plans for further experimentation, but other recent discoveries made Thompson's model obsolete before these plans were pursued (the modifier keyword "possible" being significant here). The specific language here doesn't seem to suggest that it was directly "useful" for Rutherford's model, but rather as if to say, "it was a good attempt, old sport." $\endgroup$
    – Andrew R.
    Sep 7, 2023 at 21:31

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