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== Background ==
{{main | History of atomic theory}}
Throughout the 19th century evidence from chemistry and [[statistical mechanics]] accumulated that matter was composed of atoms. The structure of the atom was discussed, and by the end of the century the leading model<ref name="PaisInwardBound" />{{rp|175}} was the [[vortex theory of the atom]], proposed by [[Lord Kelvin|William Thomson]] (later Lord Kelvin) in 1867.<ref>{{Cite journal |last=Thomson |first=William |year=1869 |title=On Vortex Atoms |url=https://zenodo.org/record/2101269 |journal=[[Proceedings of the Royal Society of Edinburgh]] |volume=6 |pages=94–105 |doi=10.1017/S0370164600045430}}</ref> By 1890, J.J. Thomson had his own version called the "nebular atom" hypothesis, in which atoms were composed of immaterial vortices and suggested similarities between the arrangement of vortices and periodic regularity found among the chemical elements.<ref name="Kragh2002">{{Cite book |last=Kragh |first=Helge |title=Quantum Generations: A History of Physics in the Twentieth Century |date=2002 |publisher=[[Princeton University Press]] |isbn=978-0691095523 |edition=Reprint |pages=43–45}}</ref>
Thomson's discovery of the [[electron]] in 1897 changed his views. Thomson called them "corpuscles" ([[particle]]s), but they were more commonly called "electrons", the name [[George Johnstone Stoney|G. J. Stoney]] had coined for the "[[elementary charge|fundamental unit quantity of electricity]]" in 1891.<ref name="O'Hara1975">{{Cite journal |last=O'Hara |first=J. G. |date=March 1975 |title=George Johnstone Stoney, F.R.S., and the Concept of the Electron |journal=[[Notes and Records of the Royal Society of London]] |volume=29 |issue=2 |pages=265–276 |doi=10.1098/rsnr.1975.0018 |jstor=531468 |s2cid=145353314}}</ref> However even late in 1899, few scientists believed in subatomic particles.<ref name="Whittaker">{{Cite book |last=Whittaker |first=E. T. |title=A history of the theories of aether & electricity |date=1989 |publisher=Dover Publications |isbn=978-0-486-26126-3 |___location=New York}}</ref>{{rp|I:365}}
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In a paper titled ''Cathode Rays'',{{sfn|Thomson|1897}} Thomson demonstrated that [[cathode rays]] are not light but made of negatively charged particles which he called ''corpuscles''. He observed that cathode rays can be deflected by electric and magnetic fields, which does not happen with light rays. In a few paragraphs near the end of this long paper Thomson discusses the possibility that atoms were made of these ''corpuscles'', calling them ''primordial atoms''. Thomson believed that the intense electric field around the cathode caused the surrounding gas molecules to split up into their component ''corpuscles'', thereby generating cathode rays. Thomson thus showed evidence that atoms were divisible, though he did not attempt to describe their structure at this point.
Thomson notes that he was not the first scientist to propose that atoms are divisible, making reference to [[William Prout]] who in 1815 found that the atomic weights of various elements were multiples of hydrogen's atomic weight and hypothesised that all atoms were made of hydrogen atoms fused together.<ref name=Kragh2010>Helge Kragh (Oct. 2010). [https://css.au.dk/fileadmin/reposs/reposs-010.pdf Before Bohr: Theories of atomic structure 1850-1913]. RePoSS: Research Publications on Science Studies 10. Aarhus: Centre for Science Studies, University of Aarhus.</ref> [[Prout's hypothesis]] was dismissed by chemists when by the 1830s it was found that some elements seemed to have a non-integer atomic weight—e.g. [[chlorine]] has an atomic weight of about 35.45. But the idea continued to intrigue scientists. The discrepancies were eventually explained with the discovery of [[isotopes]] in 1912.
A few months after Thomson's paper appeared, [[George Francis FitzGerald|George FitzGerald]] suggested that the corpuscle identified by Thomson from cathode rays and proposed as parts of an atom was a "free electron", as described by physicist [[Joseph Larmor]] and [[Hendrik Lorentz]]. While Thomson did not adopt the terminology, the connection convinced other scientists that cathode rays were particles, an important step in their eventual acceptance of an atomic model based on sub-atomic particles.<ref>{{Cite journal |last=Falconer |first=Isobel |date=July 1987 |title=Corpuscles, Electrons and Cathode Rays: J.J. Thomson and the 'Discovery of the Electron' |url=https://www.cambridge.org/core/product/identifier/S0007087400023955/type/journal_article |journal=The British Journal for the History of Science |language=en |volume=20 |issue=3 |pages=241–276 |doi=10.1017/S0007087400023955 |issn=0007-0874|url-access=subscription }}</ref>
In 1899 Thomson reiterated his atomic model in a paper that showed that negative electricity created by ultraviolet light landing on a metal (known now as the [[photoelectric effect]]) has the same [[mass-to-charge ratio]] as cathode rays; then he applied his previous method for determining the charge on ions to the negative electric particles created by ultraviolet light.<ref name="PaisInwardBound">{{Cite book |last=Pais |first=Abraham |title=Inward bound: of matter and forces in the physical world |date=2002 |publisher=Clarendon Press [u.a.] |isbn=978-0-19-851997-3 |edition=Reprint |___location=Oxford}}</ref>{{rp|86}} He estimated that the electron's mass was 0.0014 times that of the hydrogen ion (as a fraction: {{sfrac|1|714}}).<ref name=Thomson1899>{{Cite journal |last=J. J. Thomson |year=1899 |title=On the Masses of the Ions in Gases at Low Pressures. |url=https://www.chemteam.info/Chem-History/Thomson-1899.html |journal=Philosophical Magazine |series=5 |volume=48 |pages=547–567 |number=295}}<br />"...the magnitude of this negative charge is about 6 × 10<sup>−10</sup> electrostatic units, and is equal to the positive charge carried by the hydrogen atom in the electrolysis of solutions. [...] In gases at low pressures these units of negative electric charge are always associated with carriers of a definite mass. This mass is exceedingly small, being only about 1.4 × 10<sup>−3</sup> of that of the hydrogen ion, the smallest mass hitherto recognized as capable of a separate existence. The production of negative electrification thus involves the splitting up of an atom, as from a collection of atoms something is detached whose mass is less than that of a single atom."</ref> In the conclusion of this paper he writes:<ref name=Kragh2010/>
{{blockquote|I regard the atom as containing a large number of smaller bodies which I shall call corpuscles; these corpuscles are equal to each other; the mass of a corpuscle is the mass of the negative ion in a gas at low pressure, i.e. about 3 × 10<sup>−26</sup> of a gramme. In the normal atom, this assemblage of corpuscles forms a system which is electrically neutral. The negative effect is balanced by something which causes the space through which the corpuscles are spread to act as if it had a charge of positive electricity equal in amount to the sum of the negative charges on the corpuscles.}}
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