Editing History of atomic theory (section)

==Discovery of the nucleus==
{{Main|Rutherford scattering experiments}}
[[File:Geiger-Marsden experiment expectation and result.svg|right|400px|thumb|'''The [[Rutherford scattering experiments]]'''<br /> ''Left:'' Expected results: alpha particles passing through the plum pudding model of the atom with negligible deflection.<br /> ''Right:'' Observed results: a small portion of the particles were deflected by the concentrated positive charge of the nucleus.]]
Thomson's [[plum pudding model]] was challenged in 1911 by one of his former students, [[Ernest Rutherford]], who presented a new model to explain new experimental data. The new model proposed a concentrated center of charge and mass that was later dubbed the [[atomic nucleus]].<ref name=Heilbron1968>{{Cite journal |last=Heilbron |first=John L. |date=1968 |title=The Scattering of α and β Particles and Rutherford's Atom |url=https://www.jstor.org/stable/41133273 |journal=Archive for History of Exact Sciences |volume=4 |issue=4 |pages=247–307 |doi=10.1007/BF00411591 |jstor=41133273 |issn=0003-9519|url-access=subscription }}</ref>{{rp|296}}

[[Ernest Rutherford]] and his colleagues [[Hans Geiger]] and [[Ernest Marsden]] came to have doubts about the Thomson model after they encountered difficulties when they tried to build an instrument to measure the charge-to-mass ratio of [[alpha particles]] (these are positively-charged particles emitted by certain radioactive substances such as [[radium]]). The alpha particles were being scattered by the air in the detection chamber, which made the measurements unreliable. Thomson had encountered a similar problem in his work on cathode rays, which he solved by creating a near-perfect vacuum in his instruments. Rutherford didn't think he'd run into this same problem because alpha particles usually have much more momentum than electrons. According to Thomson's model of the atom, the positive charge in the atom is not concentrated enough to produce an electric field strong enough to deflect an alpha particle. Yet there was scattering, so Rutherford and his colleagues decided to investigate this scattering carefully.<ref name=Heilbron2003p64-68>[[#refHeilbron2003|Heilbron (2003). ''Ernest Rutherford and the Explosion of Atoms'', pp. 64–68]]</ref>

Between 1908 and 1913, Rutherford and his colleagues performed a series of experiments in which they bombarded thin foils of metal with a beam of alpha particles. They spotted alpha particles being deflected by angles greater than 90°. According to Thomson's model, all of the alpha particles should have passed through with negligible deflection. Rutherford deduced that the positive charge of the atom is not distributed throughout the atom's volume as Thomson believed, but is concentrated in a tiny nucleus at the center. This nucleus also carries most of the atom's mass. Only such an intense concentration of charge, anchored by its high mass, could produce an electric field strong enough to deflect the alpha particles as observed.<ref name=Heilbron2003p64-68/> Rutherford's model, being supported primarily by scattering data unfamiliar to many scientists, did not catch on until Niels Bohr joined Rutherford's lab and developed a new model for the electrons.<ref name=Heilbron1968/>{{rp|304}}

Rutherford model predicted that the scattering of alpha particles would be proportional to the square of the atomic charge.  Geiger and Marsden's based their analysis on setting the charge to half of the atomic weight of the foil's material (gold, aluminium, etc.). Amateur physicist [[Antonius van den Broek]] noted that there was a more precise relation between the charge and the element's numeric sequence in the order of atomic weights. The sequence number came be called the [[atomic number]] and it replaced atomic weight in organizing the [[periodic table]].<ref>{{cite journal |author=Eric Scerri |date=6 March 2017 |title=The Gulf between chemistry and philosophy of chemistry, then and now |journal=Structural Chemistry |volume=28 |issue=5 |pages=1599–1605 |doi=10.1007/s11224-017-0948-5}}</ref><ref>{{Cite journal |last=Van Der Broek |first=A. |date=1913-11-01 |title=Intra-atomic Charge |url=https://www.nature.com/articles/092372c0 |journal=Nature |language=en |volume=92 |issue=2300 |pages=372–373 |doi=10.1038/092372c0 |bibcode=1913Natur..92..372V |issn=0028-0836 |archive-url=https://archive.org/details/sim_nature-uk_1913-11-27_92_2300/page/372/mode/2up |archive-date=2021-07-06}}</ref>
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