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Around 1906-1914 several classical experiments on scattering of $\alpha$-particles on gold and platinum foils have been performed by Rutherford, Geiger, and Marsden. In standard literature on the subject it is tacitly assumed that the atoms of gold and platinum remained at rest after the interaction with $\alpha$-particles.

My question is why it is the case, particularly what was known at the time of experiments? Was it known at the time that atoms of gold (or platinum) interact with each other and form a crystal lattice?

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First, while your textbook may assume that the target atoms remained at rest (it makes the math easier), Rutherford, Geiger, and Marsden knew how to do the full kinematics problem and did not assume that. For gold and platinum it makes almost no difference in the analysis since those elements are ~50x more massive than an $\alpha$ particle (He ion).

Second, crystallography had a long history before then, and x-ray diffraction was demonstrated in 1912 (see Wikipedia on that), so, yes, scientists such as Rutherford and his group would have been reasonably sure that their targets were polycrystalline at the least. Still, Rutherford scattering results do not depend on the target being crystalline - crystals, amorphous, liquid, or gas targets will show similar scattering of the $\alpha$ particles.

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  • $\begingroup$ Many thanks. Regarding you last sentence about independence of the results of experiment on the crystal structure: is it again due to heavy weight of nuclei in comparison to the weight of $\alpha$-particles? What will happen if the gas is say atomic hydrogen? (I know, this is another question...) $\endgroup$
    – MKO
    Sep 1 '20 at 14:39
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    $\begingroup$ Well, kinematically, an $\alpha$ particle won't backscatter off hydrogen. Gas phase scattering was done quite a bit, for example Blackett in 1925 to identify the 14N(a,p)17O nuclear reaction (looking at 25000 pictures with 415000 alpha tracks to find 8 events). $\endgroup$
    – Jon Custer
    Sep 1 '20 at 14:49

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