Editing Rutherford scattering experiments (section)

===Scattering theory and the new atomic model===
[[File:Geiger-Marsden experiment expectation and result.svg|upright=1.5|thumb|Left: Had Thomson's model been correct, all the alpha particles should have passed through the foil with minimal scattering.<br/>Right: What Geiger and Marsden observed was that a small fraction of the alpha particles experienced strong deflection.]]
In a 1909 experiment, Geiger and Marsden discovered that the metal foils could scatter some alpha particles in all directions, sometimes more than 90°.<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 have been impossible according to Thomson's model.<ref name="BelyaevRoss2021"/>{{rp|4}} According to Thomson's model, all the alpha particles should have gone straight through.

In Thomson's model of the atom, the sphere of positive charge that fills the atom and encapsulates the electrons is permeable; the electrons could move around in it, after all. Therefore, an alpha particle should be able to pass through this sphere if the electrostatic forces within permit it. Thomson himself did not study how an alpha particle might be scattered in such a collision with an atom, but he did study [[beta particle]] scattering.<ref name=Heilbron1968/>{{rp|277}} He calculated that a beta particle would only experience very small deflection when passing through an atom,<ref name="ThomsonScattering1910"/> and even after passing through many atoms in a row, the total deflection should still be less than 1°.<ref>Beiser (1968). ''Perspectives of Modern Physics'', p. 109</ref> Alpha particles typically have much more momentum than beta particles and therefore should likewise experience only the slightest deflection.<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 extreme scattering observed forced Rutherford to revise the model of the atom.<ref name=Baily2013/>{{rp|25}}  The issue in Thomson's model was that the charges were too diffuse to produce a sufficiently strong electrostatic force to cause such repulsion. Therefore they had to be more concentrated. In Rutherford's new model, the positive charge does not fill the entire volume of the atom but instead constitutes a tiny nucleus at least 10,000 times smaller than the atom as a whole. All that positive charge concentrated in a much smaller volume produces a much stronger electric field near its surface. The nucleus also carried most of the atom's mass. This meant that it could deflect alpha particles by up to 180° depending on how close they pass. The electrons surround this nucleus, spread throughout the atom's volume. Because their negative charge is diffuse and their combined mass is low, they have a negligible effect on the alpha particle.<ref name=LeoneRobotti2018/>

To verify his model, Rutherford developed a scientific model to predict the intensity of alpha particles at the different angles they scattered coming out of the gold foil, assuming all of the positive charge was concentrated at the centre of the atom. This model was validated in an experiment performed in 1913. His model explained both the beta scattering results of Thomson and the alpha scattering results of Geiger and Marsden.<ref name=Heilbron1968/>{{rp|285}}