Editing Atom (section)

=== Discovery of the nucleus ===
[[File:Geiger-Marsden experiment expectation and result.svg|thumb|right|The [[Rutherford scattering experiments]]: The extreme scattering of some alpha particles suggested the existence of a nucleus of concentrated charge.]]
{{Main|Rutherford scattering experiments}}
The electrons in the atom logically had to be balanced out by a commensurate amount of positive charge, but Thomson had no idea where this positive charge came from, so he tentatively proposed that it was everywhere in the atom, the atom being in the shape of a sphere. This was the mathematically simplest hypothesis to fit the available evidence, or lack thereof. Following from this, Thomson imagined that the balance of electrostatic forces would distribute the electrons throughout the sphere in a more or less even manner.<ref>J. J. Thomson (1907). ''The Corpuscular Theory of Matter'', p. 103: "In default of exact knowledge of the nature of the way in which positive electricity occurs in the atom, we shall consider a case in which the positive electricity is distributed in the way most amenable to mathematical calculation, i.e., when it occurs as a sphere of uniform density, throughout which the corpuscles are distributed."</ref> Thomson's model is popularly known as the [[plum pudding model]], though neither Thomson nor his colleagues used this analogy.<ref name=HonGoldstein2013>{{cite journal |author1=Giora Hon |author2=Bernard R. Goldstein |date=6 September 2013 |title=J. J. Thomson's plum-pudding atomic model: The making of a scientific myth |journal=Annalen der Physik |volume=525 |issue=8–9 |pages=A129–A133 |doi= 10.1002/andp.201300732 |bibcode=2013AnP...525A.129H |url=https://onlinelibrary.wiley.com/doi/10.1002/andp.201300732 | issn=0003-3804}}</ref> Thomson's model was incomplete, it was unable to predict any other properties of the elements such as [[emission spectra]] and [[valency (chemistry)|valencies]]. It was soon rendered obsolete by the discovery of the [[atomic nucleus]].

Between 1908 and 1913, [[Ernest Rutherford]] and his colleagues [[Hans Geiger]] and [[Ernest Marsden]] performed a series of experiments in which they bombarded thin foils of metal with a beam of [[alpha particles]]. They did this to measure the scattering patterns of the alpha particles. They spotted a small number of alpha particles being deflected by angles greater than 90°. This shouldn't have been possible according to the Thomson model of the atom, whose charges were too diffuse to produce a sufficiently strong electric field. The deflections should have all been negligible. Rutherford proposed that the positive charge of the atom is concentrated in a tiny volume at the center of the atom and that the electrons surround this nucleus in a diffuse cloud. This nucleus carried almost all of the atom's mass. Only such an intense concentration of charge, anchored by its high mass, could produce an electric field that could deflect the alpha particles so strongly.<ref name=Heilbron2003p64-68>[[#refHeilbron2003|Heilbron (2003). ''Ernest Rutherford and the Explosion of Atoms'', pp. 64–68]]</ref>