Editing History of atomic theory (section)

==Discovery of the electron==
{{Main|Electron|Plum pudding model}}
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 | image1 = Thomson_atom_electron_arrangements.jpg
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 | caption1 = A 1905 diagram by [[J. J. Thomson]] illustrating his hypothesized arrangements of electrons in an atom, ranging from one to eight electrons.
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 | caption2 = The arrangement of seven electrons in a pentagonal dipyramid.
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Atoms were thought to be the smallest possible division of matter until 1899 when [[J. J. Thomson]] discovered the [[electron]] through his work on [[cathode ray]]s.<ref name="PaisInwardBound"/>{{rp|86}}<ref name="Whittaker">{{Cite book |last=Whittaker |first=Edmund T. |title=A history of the theories of aether & electricity. 1: The classical theories |date=1989 |publisher=Dover Publ |isbn=978-0-486-26126-3 |edition=Repr |location=New York}}</ref>{{rp|364}}

A [[Crookes tube]] is a sealed glass container in which two [[electrode]]s are separated by a vacuum. When a [[voltage]] is applied across the electrodes, cathode rays are generated, creating a glowing patch where they strike the glass at the opposite end of the tube. Through experimentation, Thomson discovered that the rays could be deflected by [[electric field]]s and [[magnetic field]]s, which meant that these rays were not a form of light but were composed of very light charged particles, and their charge was negative. Thomson called these particles "corpuscles". He measured their mass-to-charge ratio to be several orders of magnitude smaller than that of the hydrogen atom, the smallest atom. This ratio was the same regardless of what the electrodes were made of and what the trace gas in the tube was.<ref name="thomson">{{cite journal|author=J. J. Thomson |url=https://web.mit.edu/8.13/8.13c/references-fall/relativisticdynamics/thomson-cathode-rays-1897.pdf |title=Cathode rays|journal=Philosophical Magazine|volume=44|pages=293–316|year=1897 |doi=10.1080/14786449708621070|issue=269}}<br />"From these determinations we see that the value of m/e is independent of the nature of the gas, and that its value 10<sup>−7</sup> is very small compared with the value 10<sup>−4</sup>, which is the smallest value of this quantity previously known, and which is the value for the hydrogen ion in electrolysis."</ref>

In contrast to those corpuscles, positive ions created by electrolysis or X-ray radiation had mass-to-charge ratios that varied depending on the material of the electrodes and the type of gas in the reaction chamber, indicating they were different kinds of particles.<ref name="Whittaker"/>{{rp|363}}

In 1898, Thomson measured the charge on ions to be roughly 6 × 10<sup>−10</sup> [[electrostatic units]] (2 × 10<sup>−19</sup> Coulombs).<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|85}}<ref>{{cite journal |author=J. J. Thomson |date=1898 |title=On the Charge of Electricity carried by the Ions produced by Röntgen Rays |journal=The London, Edinburgh and Dublin Philosophical Magazine and Journal of Science |volume=46 |issue=283 |series=5 |pages=528–545 |doi=10.1080/14786449808621229 |url=https://archive.org/details/londonedinburgh5461898lon/page/528/mode/2up}}</ref> In 1899, he 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"/>{{rp|86}} By this combination he showed that electron's mass was 0.0014 times that of hydrogen ions.<ref>{{cite journal |author=J. J. Thomson |year=1899 |title=On the Masses of the Ions in Gases at Low Pressures. |journal=Philosophical Magazine |series=5 |volume=48 |number=295 |pages=547–567 |url=https://www.chemteam.info/Chem-History/Thomson-1899.html}}<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> These "corpuscles" were so light yet carried so much charge that Thomson concluded they must be the basic particles of electricity, and for that reason other scientists decided that these "corpuscles" should instead be called [[electron]]s following an 1894 suggestion by [[George Johnstone Stoney]] for naming the basic unit of electrical charge.<ref>{{Cite book |last1=Olenick |first1=Richard P. |title=Beyond the Mechanical Universe: From Electricity to Modern Physics |title-link=The Mechanical Universe |last2=Apostol |first2=Tom M. |last3=Goodstein |first3=David L. |date=1986-12-26 |publisher=Cambridge University Press |isbn=978-0-521-30430-6 |pages=435 |language=en}}</ref>

In 1904, Thomson published a paper describing a new model of the atom.<ref>{{cite journal
 |author=J. J. Thomson
 |date=March 1904
 |title=On the Structure of the Atom: an Investigation of the Stability and Periods of Oscillation of a number of Corpuscles arranged at equal intervals around the Circumference of a Circle; with Application of the Results to the Theory of Atomic Structure
 |url=https://zenodo.org/record/1430726
 |journal=[[Philosophical Magazine]]
 |series=Sixth series
 |volume=7 |number=39 |pages=237–265
 |doi=10.1080/14786440409463107
 |archive-url=https://ghostarchive.org/archive/20221009/https://zenodo.org/record/1430726/files/article.pdf
 |archive-date=2022-10-09
 |url-status=live
}}</ref> Electrons reside within atoms, and they transplant themselves from one atom to the next in a chain in the action of an electrical current. When electrons do not flow, their negative charge logically must be balanced out by some source of positive charge within the atom so as to render the atom electrically neutral. Having no clue as to the source of this positive charge, Thomson tentatively proposed that the positive charge was everywhere in the atom, the atom being shaped like a sphere—this was the mathematically simplest model to fit the available evidence (or lack of it).<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> The balance of electrostatic forces would distribute the electrons throughout this sphere in a more or less even manner. Thomson further explained that [[ion]]s are atoms that have a surplus or shortage of electrons.<ref>J. J. Thomson (1907). ''On the Corpuscular Theory of Matter'', p. 26: "The simplest interpretation of these results is that the positive ions are the atoms or groups of atoms of various elements from which one or more corpuscles have been removed. That, in fact, the corpuscles are the vehicles by which electricity is carried from one body to another, a positively electrified body different from the same body when unelectrified in having lost some of its corpuscles while the negative electrified body is one with more corpuscles than the unelectrified one."</ref>

Thomson's model is popularly known as the [[plum pudding model]], based on the idea that the electrons are distributed throughout the sphere of positive charge with the same density as raisins in a [[plum pudding]]. Neither Thomson nor his colleagues ever used this analogy. It seems to have been a conceit of popular science writers.<ref>{{cite journal |author1=Giora Hon |author2=Bernard R. Goldstein |title=J. J. Thomson's plum-pudding atomic model: The making of a scientific myth |journal=Annalen der Physik |date=2013 |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}}</ref> The analogy suggests that the positive sphere is like a solid, but Thomson likened it to a liquid, as he proposed that the electrons moved around in it in patterns governed by the electrostatic forces.<ref>J. J. Thomson, in a letter to [[Oliver Lodge]] dated 11 April 1904, quoted in Davis & Falconer (1997):<br />
"With regard to positive electrification I have been in the habit of using the crude analogy of a liquid with a certain amount of cohesion, enough to keep it from flying to bits under its own repulsion. I have however always tried to keep the physical conception of the positive electricity in the background because I have always had hopes (not yet realised) of being able to do without positive electrification as a separate entity and to replace it by some property of the corpuscles."<br /></ref> Thus the positive electrification in Thomson's model was a temporary concept. Thomson's model was incomplete, it could not predict any of the known properties of the atom such as emission spectra or valencies.

In 1906, [[Robert A. Millikan]] and [[Harvey Fletcher]] performed the [[oil drop experiment]] in which they measured the charge of an electron to be about -1.6 × 10<sup>−19</sup>, a value now defined as [[elementary charge|-1 ''e'']]. Since the hydrogen ion and the electron were known to be indivisible and a hydrogen atom is neutral in charge, it followed that the positive charge in hydrogen was equal to this value, i.e. 1 ''e''.{{citation needed|date=October 2024}}

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