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3 December, 18:17

In 1911, Ernest Rutherford tested the atomic model existing at the time by shooting a beam of alpha particles (42He, helium nuclei) at a very thin sheet of gold foil. He found that while most particles went straight through the foil undeflected, a very few were deflected at great angles as they passed through the foil. Why was this discovery a reason to change the atomic model

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  1. 3 December, 20:48
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    At the time of Rutherford's experiment, the accepted model for the atom was the Thomson plum-pudding model of the atom, in which the atom consists of a "sphere" of positive charge distributed all over the sphere, with tiny negative particles (the electrons) inside this sphere.

    In his experiment, Rutherford shot alpha particles towards a very thin sheet of gold foil. He observed the following things:

    1 - Most of the alpha particles went undeflected, but

    2 - Some of them were scattered at very large angles

    3 - A few of them were even reflected back to their original directions

    Observations 2) and 3) were incompatible with Thomson model of the atom: in fact, if this model was true, all the alpha particle should have gone undeflected, or scattered at very small angles. Instead, due to observations 2) and 3), it was clear that:

    - The positive charge of the atom was all concentred in a tiny nucleus

    - Most of the mass of the atom was also concentrated in the nucleus

    So, Rutherford experiment lead to a change in the atomic model of the atom, as it was clear that the plum-pudding model was no longer adequate to describe the results of Rutherford's experiment.
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