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After listening to Khan Academy video lecture on Rutherford's gold foil experiment ( https://www.khanacademy.org/science/chemistry/electronic-structure-of-atoms/history-of-atomic-structure/v/rutherfords-gold-foil-experiment), I have this question:

If before start of the experiment, Rutherford was expecting alpha-particles to pass straight through positively charged soup, then at the end of the experiment, on what premises he concluded atom is mostly empty? Why he could not have concluded to have neutral soup filling the atomic volume, (other two propositions about small heavy nucleus at center with positive charge and orbiting electrons in the neutral soup still holding in his orbiting model)?

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  • $\begingroup$ Because you need some massive thing to bounce an alpha particle back at you. And it is small from the scattering cross section. $\endgroup$
    – Jon Custer
    Aug 26, 2018 at 23:43
  • $\begingroup$ @JonCuster As I mentioned, to cater for bouncing, he proposed nucleus at center, which I understood. What I did not understand, was the need for removing soup altogether from the plum pudding model. Earlier model had positively charged soup, and now that we are putting positive charge concentrated at center, we could as well replace positively charged soup with neutral soup, (all other properties of soup holding the same, like it would not bounce back alpha particles). So in the model, now we would have 3 things: nucleus, electrons and soup. $\endgroup$
    – user127249
    Aug 27, 2018 at 6:06
  • $\begingroup$ Well, they had identified the electron already, and the nucleus required for alpha scattering contained all the known mass, so what is there left? $\endgroup$
    – Jon Custer
    Aug 27, 2018 at 12:41

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I don't know that there's that much difference between a "negative soup" and the "electron cloud," which arose in the 1920s after the mathematical machinery to compute electron orbitals had been developed. But the electron cloud (unlike Thomson's positively-charged "plum pudding") contains only a tiny fraction of the atom's mass. Since de Broglie's wave-particle duality was also in the future and the electron had already been observed as a particle constituent of an atom, Rutherford interpreted his discovery of the nucleus to mean that there weren't any continuous materials making up an atom.

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