Plum pudding model: Difference between revisions

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Thomson probed the structure of atoms through [[beta particle]] scattering, whereas his former student [[Ernest Rutherford]] was interested in [[alpha particle]] scattering. Beta particles are electrons emitted by radioactive decay, whereas alpha particles are essentially helium atoms, also emitted in process of decay. Alpha particles have considerably more momentum than beta particles and Rutherford found that matter scatters alpha particles in ways that Thomson's plum pudding model could not predict.

Between 1908 and 1913, [[Ernest Rutherford]], [[Hans Geiger]], and [[Ernest Marsden]] collaborated on a series of experiments in which they bombarded thin metal foils with a beam of alpha particles and measured the intensity versus scattering angle of the particles. They found that the metal foil could scatter alpha particles by more than 90&deg;°.<ref name="BelyaevRoss2021">{{Cite book |last1=Belyaev |first1=Alexander |url=https://link.springer.com/10.1007/978-3-030-80116-8 |title=The Basics of Nuclear and Particle Physics |last2=Ross |first2=Douglas |date=2021 |publisher=Springer International Publishing |isbn=978-3-030-80115-1 |series=Undergraduate Texts in Physics |___location=Cham |language=en |doi=10.1007/978-3-030-80116-8|bibcode=2021bnpp.book.....B }}</ref>{{rp|4}} This should not have been possible according to the Thomson model: the scattering into large angles should have been negligible. The odds of a beta particle being scattered by more than 90&deg;° under such circumstances is astronomically small, and since alpha particles typically have much more momentum than beta particles, their deflection should be smaller still.<ref>Rutherford (1911): "This scattering is far more marked for the β than for the α particle on account of the much smaller momentum and energy of the former particle."</ref> The Thomson models simply could not produce electrostatic forces of sufficient strength to cause such large deflection. The charges in the Thomson model were too diffuse. This led Rutherford to discard the Thomson for a new model where the positive charge of the atom is concentrated in a tiny nucleus.

Rutherford went on to make more compelling discoveries. In Thomson's model, the positive charge sphere was just an abstract component, but Rutherford found something concrete to attribute the positive charge to: particles he dubbed "[[proton]]s". Whereas Thomson believed that the electron count was roughly correlated to the atomic weight, Rutherford showed that (in a neutral atom) it is exactly equal to the atomic number.