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The way Rutherford's classic gold foil experiment has been presented (including by Rutherford himself) doesn't make sense to me.

As many of you know, Rutherford famously described himself as being utterly shocked at the results brought to him by Geiger in 1909: "It was quite the most incredible event that has ever happened to me in my life. It was almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you." [Rutherford, Ernest; Ratcliffe, John A. (1938). Forty Years of Physics. In Needham, Joseph; Pagel, Walter. Background to Modern Science. Cambridge University Press.]

It makes no sense to me that he should have been so surprised. Rutherford wrote the following the year before, in 1908: "We can conclude with certainty from these experiments that the α particle after losing its charge is a helium atom. Other evidence indicates that the charge is twice the unit charge carried by the hydrogen atom set free in the electrolysis of water." [Ernest Rutherford & Thomas Royds, Philosophical Magazine 17, 281-286 (1909).] [Publication date was 1909, but authorship date was 1908.]

Thus, at the time of the gold-foil experiment, Rutherford believed (correctly) that his "15-inch shell[s]" (the α particles) were helium atoms that had lost negative charge. So essentially Rutherford, using particles that he knew to be highly concentrated atomic-scale matter derived from atoms (α particles), was shocked to find that atoms contained highly-concentrated atomic scale matter (?!).

I'm not saying he should have known specifically about the nuclear model of the atom prior to the experiment. I'm saying he shouldn't have been utterly shocked to find that atoms contain something that can strongly deflect α particles, given that he knew α particles themselves were simply atoms whose negative charge had been removed. [Plus he knew he was taking particles derived from very light atoms, and shooting them at very heavy atoms.]

To use Rutherford's own metaphor, it's as if he were saying: "I went to an armory and obtained some artillery shells. I then fired these at another armory, and was shocked to find, as a result, that armories contain artillery shells."

I.e., if Rutherford already knew this about helium ions, why wouldn't he expect other atoms would also have their positive charges concentrated as well? Was his thinking that neutral atoms were accurately described by a plum pudding model, yet once you removed the electrons the positive charge would collapse into a small space? If anything, given that he didn't know about nuclear binding, it would seem he should have expected the opposite—that removing the negative charges would cause the positive charges to expand and become even more diffuse.

Can someone familiar with the history of these experiments resolve this?

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    $\begingroup$ He knew alpha radiation is helium, he didn't know the particles were single helium atoms. However the elastic backscattering only makes sense if the charge in both target and projectile is concentrated in very small spaces. $\endgroup$ – Karl Sep 18 '18 at 10:07
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    $\begingroup$ In retrospect, most discoveries seem obvious. But that moment you realize that your viewpoint and mental model have to change is quite noticeable. $\endgroup$ – Jon Custer Sep 18 '18 at 13:25
  • $\begingroup$ @Karl You write: "He [Rutherford] knew alpha radiation is helium, he didn't know the particles were single helium atoms." However, Rutherford wrote the following in 1908, a few years prior to the gold foil experiment (published 1909): "We can conclude with certainty from these experiments that the α particle after losing its charge is a helium atom. Other evidence indicates that the charge is twice the unit charge carried by the hydrogen atom set free in the electrolysis of water." [Ernest Rutherford & Thomas Royds, Philosophical Magazine 17, 281-286 (1909).] $\endgroup$ – theorist Sep 18 '18 at 14:43
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    $\begingroup$ Knowing that alpha particles are helium atoms does not mean very much. Individual atoms as individual particles had been known for quite some time. The question was what, exactly, were these particles? Knowing that an alpha is a helium atom does not imply that the nucleus is really tiny and strongly holds two protons and two neutrons (which weren't discovered until 1932) together. Knowing that an alpha rebounds intact from gold tells you something new - that there is a single, strongly held ball of mass for each atom (helium and gold). $\endgroup$ – Jon Custer Sep 18 '18 at 18:59
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    $\begingroup$ I don't think it is correct to say that Rutherford believed that the alpha was what we now think of as an atom with a single, effectively point-like nucleus. If the alpha had a smeared-out mass distribution, and other atoms (like gold) had a smeared-out mass distribution, then one would not expect one to bounce off of another in the way that was observed. $\endgroup$ – Jon Custer Sep 18 '18 at 22:31

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