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When reading and learning about the gold foil experiment, the alpha particles mostly go straight through the gold foil and some get deflected or bounce back but it doesn't say anything about the direction of the alpha particle emitter. Did the alpha particle emitter stay in the same spot or get directed to different areas of the foil? How could they all hit the same spot and get deflected in so many different directions?

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    $\begingroup$ The source had area, and the foil had area. The alpha particles were collimated. This is just how scattering (and reaction cross sections) work. $\endgroup$
    – Jon Custer
    Jan 10 at 4:54
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    $\begingroup$ The original set-up had the alpha source some distance away so the particles must be were randomly hitting the gold foil not exactly at the same atom, if that is in the mind of OP. The alpha particles was "collimated" by hole in a lead chamber. Jon Custer can better explain about the divergence of the alpha particles. Do they diverge from the hole or just travel in straight lines? $\endgroup$
    – M. Farooq
    Jan 10 at 4:59
  • $\begingroup$ In this article on microbeams (ncbi.nlm.nih.gov/pmc/articles/PMC6092239) it clearly states that collimaters are used to focus not disperse as quoted " The most important aspect for a microbeam facility is the capability to confine the beam to a micrometer or even sub-micrometer diameter size. The two most popular approaches are collimation and magnetic focusing.". and "Collimators of various materials and thicknesses were tested to minimize the scattering and to achieve a better accuracy (Folkard et al. 1997)" Do alpha particles then differ from photons in this way? $\endgroup$
    – suse
    Jan 10 at 5:37
  • $\begingroup$ Rutherford performed his experiments 100 years ago, at that time these facilities did not exist. You should see his original paper. There is nothing in common in photons and alpha particles. Collimations means to make beams parallel not focus anything. $\endgroup$
    – M. Farooq
    Jan 10 at 5:52
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Even a small beam is very big compared to a gold atom

It seems you are making an assumption that, if the beam were perfectly focussed it would always show the same deflection (because geometry).

But whatever level of focus Rutherford could achieve with alpha particles, the scale is all wrong for this to be true. Even if the beam was only micrometers wide (very tight for the early technology of the day) it would be more than 10,000 times wider than a single gold atom. And, since the nucleus is what actually scattered the alpha particles, there is another factor of 10,000 or so in scale to consider since the nucleus is very much smaller than the atom.

So the question makes a false assumption. The focus of the macroscopic beam is irrelevant and can never, given reasonable technology, come close to focussing on single nucleus. The beam area in Rutherford's experiment will cover vast numbers of atoms and will hit them at random.

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They were hitting the same spot in the sense they landed on an area that was predicted by the spread of the beam. They weren't all traveling exactly the same path. This randomness allowed some of them to bounce from atom nuclei.

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