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At the time when Rutherford's gold foil experiment was performed, Thomson's plum pudding model was believed to be true (at least by Rutherford himself and his students).

With this model in mind Rutherford predicted that most of the alpha particles will be deflected by at most a fraction of a degree (sourced by this Wikipedia page), but why?

In my opinion, since according to the plum pudding model the mass of an atom was assumed to be uniformly distributed and the atomic mass of gold is nearly 50 times larger than the mass of an alpha particle, and gold is solid, therefore much less intermolecular space will be present, so, most of the alpha particles should rebound or get deflected by a large angle.

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    $\begingroup$ Perhaps see this answer to get started: chemistry.stackexchange.com/a/10392/79678. Search for “Rutherford” here for more information. $\endgroup$
    – Ed V
    Commented Jul 21, 2020 at 12:36
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    $\begingroup$ Strongly related (almost duplicate): chemistry.stackexchange.com/questions/106819/… $\endgroup$ Commented Jul 21, 2020 at 12:48
  • $\begingroup$ en.wikipedia.org/wiki/Rutherford_scattering $\endgroup$
    – Buck Thorn
    Commented Jul 21, 2020 at 13:21
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    $\begingroup$ What is the alpha particle bouncing off of? What are the kinematics of that situation? Without using backspin, you don't get backscattering in pool/snooker because the balls are all the same mass. $\endgroup$
    – Jon Custer
    Commented Jul 21, 2020 at 13:51
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    $\begingroup$ Further, given that Thompson's model was proposed after $\alpha$ backscattering was observed, it is clear that the assumption in the first sentence is not valid. $\endgroup$
    – Jon Custer
    Commented Jul 21, 2020 at 14:35

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The nucleus has a radius roughly 10⁴ times smaller than the size of the atom itself (imagine a sports ball in a stadium). That would mean that its volume were 10¹² smaller than the volume of an atom. Sure the gold nucleus is ~30 times as charged and is ~50 times heavier. But dilute that charge and mass by a factor of a trillion, and suddenly those don't seem so significant anymore.

That's the worst thing about those schematic diagrams that you see for the Rutherford experiment. They blow up the size of the nucleus to prove a point, but those diagrams are definitely "Not to Scale™".

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  • $\begingroup$ I know that but I asked what led Rutherford think that Alpha particle should pass through the gold foil keeping in mind plum pudding model $\endgroup$
    – Tushar
    Commented Jul 21, 2020 at 13:14
  • $\begingroup$ he expectesdthis i.e. before conducting the experiment $\endgroup$
    – Tushar
    Commented Jul 21, 2020 at 13:15
  • $\begingroup$ That's what I'm saying to you. Rutherford is able to divide, so he can already estimate the density of matter and charge within the gold foil, and it's not that dense. $\endgroup$
    – Zhe
    Commented Jul 21, 2020 at 13:59
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    $\begingroup$ Why is a question with 3 upvotes closed? Surely there was a known difference between an alpha particle and gold and this had to be in his mind. Ostensibly one conducts experiments to answer a question, What was the question? The 16in projectile analogy is foolish; shooting a pingpong ball at a pillow filled with pingpong balls and having it bounce back is more appropriate and more exciting. $\endgroup$
    – jimchmst
    Commented Aug 29, 2023 at 21:00
  • $\begingroup$ @jimchmst I would expect that most ping pong ball collisions result in its bouncing back. That seems less than exciting. $\endgroup$
    – Zhe
    Commented Aug 31, 2023 at 17:52

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