In Rutherford's gold foil experiment, if he at first thought the atom looked like the plum pudding (as Thomson had said), why did he think the alpha particles would not be repelled by the positive "pudding" and that they would go straight through? Is it because of the high energy of the emission of the alpha particles?
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1$\begingroup$ Yes high speed of the alpha particles and relatively small electric field at the foil/atoms due to spreading of charge in the plum pudding model. $\endgroup$– AlchimistaDec 18, 2018 at 13:42
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$\begingroup$ They already knew that alpha particles propagated through air, although the beam did spread out as one would expect from low-energy-transfer collisions (some did actually backscatter but were not detected in those experiments). To backscatter you need to obey the standard kinematic formulas - what you hit has to be heavier than you are, and in the plum pudding 'model' there were no compact high mass objects that would lead to backscattering. Forward scattering leading to broadening of the beam, as observed in air, was expected. $\endgroup$– Jon CusterDec 18, 2018 at 13:52
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Rutherford conducted his gold foil in order to prove Thomson's plum pudding model but the result was contradicting. He expected that most of the Alpha particle will show very minor deflections because the positive charge was evenly spread throughout the atom according to the plum pudding model.
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$\begingroup$ To my knowledge, this statement is not true. Thomson's model was nowhere nearly universally accepted. $\endgroup$– ZheJul 24, 2019 at 13:10