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Plum pudding model

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A hypothetical atom with seven electrons, arranged in a pentagonal dipyramid, as imagined by Thomson in 1905.

The plum pudding model is an obsolete scientific model of the atom. It was first proposed by J. J. Thomson in 1904[1] following his discovery of the electron in 1897 and subsequently rendered obsolete by Ernest Rutherford's discovery of the atomic nucleus in 1911. It was the first model to describe an internal structure for the atom. The model tried to account for two properties of atoms then known: that there are electrons and that atoms have no net electric charge. Logically there had to be a commensurate quantity of positive charge to balance out the negative charge of the electrons, but having no clue as to the source of this positive charge, Thomson tentatively proposed it was everywhere in the atom, the atom being in the shape of a sphere. Following from this, Thomson imagined that the balance of electrostatic forces in the atom would distribute the electrons in a more or less even manner throughout this hypothetical sphere.[2]

Thomson was not able to develop a complete model that could predict any other known properties of the atom such as emission spectra or valencies. In modern textbooks the plum pudding model is only briefly mentioned to explain how the atomic nucleus was discovered.

Thomson's model is popularly referred to as the "plum pudding model" with the notion that the electrons are distributed with similar density as raisins in a plum pudding. Neither Thomson nor his colleagues ever used this analogy.[3] It seems to have been a conceit of popular science writers to make the model accessible to the layman. The analogy is perhaps misleading because Thomson likened the sphere to a liquid rather than a solid since he thought the electrons moved around in it.[4]

Background[edit]

Main article: 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[5]: 175  was vortex theory of the atom, proposed by William Thomson (later Lord Kelvin) in 1867.[6] By 1890, J.J. Thomson had his own version called the "nebular atom" hypothesis, in which atoms were composed of immaterial vortices and suggested there were similarities between the arrangement of vortices and periodic regularity found among the chemical elements.[7]

Thomson's discovery of the electron in 1897 changed his views. While Thomson called them "corpuscles" (particles), but they were more commonly called "electrons", the name G. J. Stoney had coined for the "fundamental unit quantity of electricity" in 1891.[8] However even late in 1899, few scientists believed in subatomic particles.[9]: I:365 

Another emerging scientific theme of the 19th century was the discovery and study of radioactivity. Thomson discovered the electron by studying cathode rays and in 1900 Henri Becquerel determined that the highly penetrating radiation from uranium, now called beta particles, had the same charge/mass ratio as cathode rays.[9]: II:3  These beta particles were believed to be electrons traveling at much high speeds. These beta particles would be used by Thomson to probe atoms to find evidence for his atomic theory. The other form of radiation critical to this era of atomic models was alpha particles. Heavier and slower than beta particles, these would be the key tool used by Rutherford to find evidence against Thomson's model.

In addition to the emerging atomic theory, the electron, and radiation, the last element of history was the many studies of atomic spectra published near the end of the 19th century. Part of the attraction of the vortex model was its possible role in describing the spectral data as vibrational responses to electromagnetic radiation.[5]: 177  Neither Thomson's model or its successor, Rutherford's model made progress towards understanding atomic spectra. That would have to wait until Niels Bohr built the first quantum-based atom model.

Development[edit]

From 1897 through 1913, Thomson proposed a series of increasingly detailed polyelectron models for the atom.[5]: 178  His first versions were qualitative culminating in his 1906 paper and follow on summaries. Thomson's model changed over the course of its initial publication, finally becoming a model with much more mobility containing electrons revolving in the dense field of positive charge rather than a static structure. Thomson attempted unsuccessfully to reshape his model to account for some of the major spectral lines experimentally known for several elements.[10]

1897[edit]

In a paper titled Cathode Rays, 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. Thomson believed 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 in fact 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 actually divisible, making reference to William Prout who in 1815 noted that the atomic weights of various elements were multiples of hydrogen's atomic weight and hypothesized that all atoms were hydrogen atoms fused together. Prout's hypothesis was dismissed when it was found that some elements seemed to have a non-integer atomic weight, e.g. chlorine has an atomic weight of about 35.5, and such discrepancies wouldn't be explained until the discovery of isotopes and nuclear structure in the early 20th century.

1898[edit]

In 1898, Thomson measured the positive charge on hydrogen ions to be roughly 6 × 10-10 electrostatic units (2 × 10-19 coulombs) and wrote that this was equal to an electron's negative charge.[11] 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.[5]: 86  By this combination he estimated that the electron's mass was 0.0014 times that of hydrogen ions (as a fraction: 1⁄714).[12]

1904[edit]

Thomson provided his first description of the atom in his 1904 paper On the Structure of the Atom. It was known for many years that atoms have no net electric charge. Thomson held that atoms must also contain some positive charge that cancels out the negative charge of their electrons.[13][14] Thomson published his proposed model in the March 1904 edition of the Philosophical Magazine, the leading British science journal of the day. In Thomson's view:

... the atoms of the elements consist of a number of negatively electrified corpuscles enclosed in a sphere of uniform positive electrification, ...[15]

Thomson believed that all the mass of the atom was carried by the electrons. This would mean that even a small atom would have to contain thousands of electrons, and the positive electrification the encapsulated them was without mass.[16]

Thomson's model was the first to assign a specific inner structure to an atom, though his original description did not include mathematical formulas.[3][17] Thomson based his atomic model on known experimental evidence of the day, and in fact, followed Lord Kelvin's lead again as Kelvin had proposed a positive sphere atom a year earlier.[18][19] Thomson's proposal, based on Kelvin's model of a positive volume charge, served to guide future experiments.

1905[edit]

A 1905 diagram by J. J. Thomson illustrating his hypothetical arrangements of electrons in an atom, ranging from one to eight electrons.
Thomson's practical experiment for exploring electron arrangements.

In a lecture delivered to the Royal Institution of Great Britain in 1905,[20] Thomson stated that calculating the arrangements of many electrons in a sphere of positive charge was too computationally difficult for him. However, he proposed a practical experiment to shed some light on their nature. It involved magnetized pins pushed into cork disks and set afloat in a basin of water. The magnetized pins were oriented such that they repelled each other. Above the center of the pool was suspended an electromagnet that attracted the pins towards the center. The equilibrium arrangement the pins took informed Thomson on what arrangements the electrons in an atom might take and he provided a brief table.

For instance, he observed that while five pins would arrange themselves in a stable pentagon around the center, six pins could not form a stable hexagon. Instead, one pin would move to the center and the other five would form a pentagon around the center pin, and this arrangement was stable. As he added more pins, they would arrange themselves in concentric rings around the center.

From this, Thomson believed the electrons arranged themselves in concentric shells, and the electrons could move about within these shells but did not move out of them unless electrons were added or subtracted from the atom.

1907[edit]

In 1907, Thomson published The Corpuscular Theory of Matter which further developed his ideas on the atom's structure and proposed further avenues of research.

In Chapter 6, he further elaborates his experiment using magnetized pins in water, providing an expanded table. For instance, if 59 pins were placed in the pool, they would arrange themselves in concentric rings of the order 20-16-13-8-2 (from outermost to innermost).

In Chapter 7, Thomson proposes a number of methods by which the number of electrons in an atom could be estimated, such as X-ray emission spectra, cathode ray absorption, or optical properties. Experiments by other scientists in this field had shown that atoms contain far fewer electrons than Thomson previously thought. Thomson now believed the number of electrons in an atom was a small multiple of its atomic weight,[21] perhaps 1 to 3 times.[22]

But this would mean that most of the atom's mass was not carried by its electrons. In this book he estimates that an electron has only 1⁄1700 the mass of a hydrogen atom (the correct figure is 1⁄1837). Thomson concluded that the rest of the atom's mass was carried by the positive electrification.[23] Thomson still did not know what substance constituted the positive electrification, though he noted that no scientist had yet found a positively-charged particle smaller than a hydrogen ion.[24]

Rutherford's new evidence[edit]

Main article: Geiger-Marsden experiments

Between 1908 and 1913, Ernest Rutherford, Hans Geiger, and Ernest Marsden collaborated on a series of experiments in which they bombarded metal foils with a beam of alpha particles and measured the intensity versus scattering angle of the particles. Gold was their preferred material because gold is very malleable and can therefore be made into an especially thin foil. The found that the gold foil could scatter alpha particles by more than 90 degrees. This should not have been possible according to the Thomson model. The scattering should have been negligible. The positive charge in the Thomson model is too diffuse to produce an electric field of sufficient strength to cause such scattering. Rutherford deduced that the positive charge of the atom, along with most of the atom's mass, was concentrated in a tiny nucleus at the center of the atom. Only such an intense concentration of charge and mass, could have scattered the alpha particle beam so dramatically.

Rutherford went on to show that the number of electrons in an atom is equal to the element's atomic number, and that the hydrogen atom is a singular particle which he called "the proton" and constitutes the positive charge of all atoms. It was later found that the protons carry only roughly half the mass of the atom, the rest being neutrons which have no charge.

Related scientific problems[edit]

As an important example of a scientific model, the plum pudding model has motivated and guided several related scientific problems.

Mathematical Thomson problem[edit]

A particularly useful mathematics problem related to the plum pudding model is the optimal distribution of equal point charges on a unit sphere, called the Thomson problem. The Thomson problem is a natural consequence of the plum pudding model in the absence of its uniform positive background charge.[25][26]

Origin of the nickname[edit]

Not Thomson's preferred analogy.

The first known writer to compare Thomson's model to a plum pudding was an anonymous reporter who wrote an article for the British pharmaceutical magazine The Chemist and Druggist in August 1906.

While the negative electricity is concentrated on the extremely small corpuscle, the positive electricity is distributed throughout a considerable volume. An atom would thus consist of minute specks, the negative corpuscles, swimming about in a sphere of positive electrification, like raisins in a parsimonious plum-pudding, units of negative electricity being attracted toward the centre, while at the same time repelling each other.[27]

The analogy was never used by Thomson nor his colleagues. It seems to have been a conceit of popular science writers to make the model easier to understand for the layman.[3]

References[edit]

  1. ^ "Plum Pudding Model". Universe Today. 27 August 2009. Retrieved 19 December 2015.
  2. ^ 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."
  3. ^ a b c Giora Hon; Bernard R. Goldstein (6 September 2013). "J. J. Thomson's plum-pudding atomic model: The making of a scientific myth". Annalen der Physik. 525 (8–9): A129–A133. Bibcode:2013AnP...525A.129H. doi:10.1002/andp.201300732.
  4. ^ J. J. Thomson, in a letter to Oliver Lodge dated 11 April 1904, quoted in Davis & Falconer (1997):
    "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."
  5. ^ a b c d Pais, Abraham (2002). Inward bound: of matter and forces in the physical world (Reprint ed.). Oxford: Clarendon Press [u.a.] ISBN 978-0-19-851997-3.
  6. ^ Thomson, William (1869). "On Vortex Atoms". Proceedings of the Royal Society of Edinburgh. 6: 94–105. doi:10.1017/S0370164600045430.
  7. ^ Kragh, Helge (2002). Quantum Generations: A History of Physics in the Twentieth Century (Reprint ed.). Princeton University Press. pp. 43–45. ISBN 978-0691095523.
  8. ^ O'Hara, J. G. (March 1975). "George Johnstone Stoney, F.R.S., and the Concept of the Electron". Notes and Records of the Royal Society of London. 29 (2): 265–276. doi:10.1098/rsnr.1975.0018. JSTOR 531468. S2CID 145353314.
  9. ^ a b Whittaker, E. T. (1989). A history of the theories of aether & electricity. New York: Dover Publications. ISBN 978-0-486-26126-3.
  10. ^ Models of the Atom, Michael Fowler, University of Virginia https://galileo.phys.virginia.edu/classes/252/more_atoms.html#Plum%20Pudding
  11. ^ J. J. Thomson (1898). "On the Charge of Electricity carried by the Ions produced by Röntgen Rays". The London, Edinburgh and Dublin Philosophical Magazine and Journal of Science. 5. 46 (283): 528–545. doi:10.1080/14786449808621229.
  12. ^ J. J. Thomson (1899). "On the Masses of the Ions in Gases at Low Pressures". Philosophical Magazine. 5. 48 (295): 547–567.
    "...the magnitude of this negative charge is about 6 × 10-10 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 × 10-3 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."
  13. ^ "Discovery of the electron and nucleus (article)". Khan Academy. Retrieved 9 February 2021.
  14. ^ Alviar-Agnew, Marissa; Agnew, Henry (4 April 2016). "4.3: The Nuclear Atom". Introductory Chemistry. LibreTexts. Retrieved 9 February 2021.
  15. ^ Thomson, J. J. (March 1904). "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". Philosophical Magazine. Sixth series. 7 (39): 237–265. doi:10.1080/14786440409463107. Archived (PDF) from the original on 2022-10-09.
  16. ^ J. J. Thomson (March 1904). "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". Philosophical Magazine. Sixth series. 7 (39): 237–265. doi:10.1080/14786440409463107. Archived (PDF) from the original on 2022-10-09.
    "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."
  17. ^ Thomson, J. J. (December 1899). "On the masses of the ions in gases at low pressures". The London, Edinburgh, and Dublin Philosophical Magazine and Journal of Science. 48 (295): 547–567. doi:10.1080/14786449908621447.
  18. ^ Models of the Atom, Michael Fowler, University of Virginia https://galileo.phys.virginia.edu/classes/252/more_atoms.html#Plum%20Pudding
  19. ^ Kumar, Manjit, Quantum Einstein, Bohr and the Great Debate, ISBN 978-0393339888, 2008.
  20. ^ J. J. Thomson (10 March 1905). The Structure of the Atom. Reprinted in Davis & Falconer (1997)
  21. ^ Thomson (1907). The Corpuscular Theory of Matter, p. 27: "the number of corpuscles in an atom of any element is proportional to the atomic weight of the element — it is a multiple, and not a large one, of the atomic weight of the element."
  22. ^ Thomson (1907). The Corpuscular Theory of Matter, pp. 145, 151
  23. ^ J. J. Thomson (1907). The Corpuscular Theory of Matter, p.162: "Since the mass of a corpuscle is only about one-seventeen-hundredth part of that of an atom of hydrogen, it follows that if there are only a few corpuscles in the hydrogen atom the mass of the atom must in the main be due to its other constituent — the positive electricity."
  24. ^ Thomson (1907). The Corpuscular Theory of Matter, pp. 23, 26
  25. ^ Levin, Y.; Arenzon, J. J. (2003). "Why charges go to the Surface: A generalized Thomson Problem". Europhys. Lett. 63 (3): 415–418. arXiv:cond-mat/0302524. Bibcode:2003EL.....63..415L. doi:10.1209/epl/i2003-00546-1. S2CID 250764497.
  26. ^ Roth, J. (2007-10-24). "Description of a highly symmetric polytope observed in Thomson's problem of charges on a hypersphere". Physical Review E. 76 (4): 047702. Bibcode:2007PhRvE..76d7702R. doi:10.1103/PhysRevE.76.047702. ISSN 1539-3755. PMID 17995142. 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.
  27. ^ "What is Matter?". The Chemist and Druggist. 69 (8): 329–330. 25 August 1906.

Bibliography[edit]

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