Content deleted Content added
m Open access bot: url-access updated in citation with #oabot. |
|||
Line 33:
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/>
Line 45:
He then gives a detailed mechanical analysis of such a system, distributing the electrons uniformly around a ring. The attraction of the positive electrification is balanced by the mutual repulsion of the electrons. His analysis focuses on stability, looking for cases where small changes in position are countered by restoring forces.
After discussing his many formulae for stability he turned to analysing patterns in the number of electrons in various concentric rings of stable configurations. These regular patterns Thomson argued are analogous to the [[periodic law]] of chemistry behind the structure of the [[periodic table]]. This concept, that a model based on subatomic particles could account for chemical trends, encouraged interest in Thomson's model and influenced future work even if the details Thomson's electron assignments turned out to be incorrect.<ref>{{Cite journal |last=Kragh |first=Helge |date=2001 |title=The first subatomic explanations of the periodic system |url=http://link.springer.com/10.1023/A:1011448410646 |journal=Foundations of Chemistry |volume=3 |issue=2 |pages=129–143 |doi=10.1023/A:1011448410646|url-access=subscription }}</ref>{{rp|135}}
Thomson at this point believed that all the mass of the atom was carried by the electrons.<ref>{{harvnb|Thomson|1904}}: "We suppose that the mass of an atom is the sum of the masses of the corpuscles it contains, so that the atomic weight of an element is measured by the number of corpuscles in its atom."</ref> This would mean that even a small atom would have to contain thousands of electrons, and the positive electrification that encapsulated them was without mass.<ref>{{Cite journal |last=Baily |first=C. |date=January 2013 |title=Early atomic models – from mechanical to quantum (1904–1913) |url=http://link.springer.com/10.1140/epjh/e2012-30009-7 |journal=The European Physical Journal H |language=en |volume=38 |issue=1 |pages=1–38 |doi=10.1140/epjh/e2012-30009-7 |arxiv=1208.5262 |bibcode=2013EPJH...38....1B |issn=2102-6459}}</ref>
Line 66:
===1906 Estimating electrons per atom===
Before 1906 Thomson considered the atomic weight to be due to the mass of the electrons (which he continued to call "corpuscles"). Based on his own estimates of the electron mass, an atom would need tens of thousands electrons to account for the mass. In 1906 he used three different methods, X-ray scattering, beta ray absorption, or optical properties of gases, to estimate that "number of corpuscles is not greatly different from the atomic weight".<ref name=Thomson1906>{{Cite journal |last=Thomson |first=J.J. |date=June 1906 |title=LXX. On the number of corpuscles in an atom |url=https://www.tandfonline.com/doi/full/10.1080/14786440609463496 |journal=The London, Edinburgh, and Dublin Philosophical Magazine and Journal of Science |language=en |volume=11 |issue=66 |pages=769–781 |doi=10.1080/14786440609463496 |issn=1941-5982|url-access=subscription }}</ref><ref name=Heilbron1968>{{Cite journal |author=John L. Heilbron |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|q=one of the most important papers on atomic structure ever written}} This reduced the number of electrons to tens or at most a couple of hundred and that in turn meant that the positive sphere in Thomson's model contained most of the mass of the atom. This meant that Thomson's mechanical stability work from 1904 and the comparison to the periodic table were no longer valid.<ref name="PaisInwardBound" />{{rp|186}} Moreover, the alpha particle, so important to the next advance in atomic theory by Rutherford, would no longer be viewed as an atom containing thousands of electrons.<ref name=Heilbron1968/>{{rp|269}}
In 1907, Thomson published ''The Corpuscular Theory of Matter''{{sfn|Thomson|1907}} which reviewed his ideas on the atom's structure and proposed further avenues of research.
Line 187:
==Mathematical Thomson problem==
The [[Thomson problem]] in mathematics seeks the optimal distribution of equal point charges on the surface of a sphere. Unlike the original Thomson atomic model, the sphere in this purely mathematical model does not have a charge, and this causes all the point charges to move to the surface of the sphere by their mutual repulsion. There is still no general solution to Thomson's original problem of how electrons arrange themselves within a sphere of positive charge.<ref>{{Cite journal |last1=Levin |first1=Y. |last2=Arenzon |first2=J. J. |year=2003 |title=Why charges go to the Surface: A generalized Thomson Problem |journal=Europhys. Lett. |volume=63 |issue=3 |pages=415–418 |arxiv=cond-mat/0302524 |bibcode=2003EL.....63..415L |doi=10.1209/epl/i2003-00546-1 |s2cid=250764497}}</ref><ref>{{Cite journal |last=Roth |first=J. |date=2007-10-24 |title=Description of a highly symmetric polytope observed in Thomson's problem of charges on a hypersphere |url=https://link.aps.org/doi/10.1103/PhysRevE.76.047702 |journal=Physical Review E |language=en |volume=76 |issue=4 |pages=047702 |bibcode=2007PhRvE..76d7702R |doi=10.1103/PhysRevE.76.047702 |issn=1539-3755 |pmid=17995142 |quote=Although Thomson's model has been outdated for a long time by quantum mechanics, his problem of placing charges on a sphere is still noteworthy.|url-access=subscription }}</ref>
==Origin of the nickname==
| |||