[[Image:Plum_pudding_atom.svg|right|thumb|250x|A schematic representation of the plum pudding model of the atom. In Thomson's mathematical model the "corpuscles" (or modern electrons) were arranged non-randomly, in rotating rings.]]
The '''plum pudding model''' of the [[atom]] was proposed by [[J. J. Thomson]], who discovered the [[electron]] in 1897. The plum pudding model was proposed in 1904 before the discovery of the atomic nucleus. In this model, the atom is composed of electrons (which Thomson still called "corpuscles," though [[G.J. Stoney]] had proposed that atoms of electricity be called ''electrons'' in 1894), surrounded by a soup of positive charge to balance the electron's negative charge, like [[plum]]s surrounded by [[pudding]]. The electrons (as we know them today) were thought to be positioned throughout the atom, but with many structures possible for positioning multiple electrons, particularly rotating rings of electrons (see below). Instead of a soup, the atom was also sometimes said to have had a cloud of positive charge.

The model was disproved by the 1909 [[Geiger-Marsden experiment|gold foil experiment]], which was interpreted by [[Ernest Rutherford]] in 1911 to imply a very small nucleus of the atom containing a very high positive charge (enough to balance about 100 electrons in gold), thus leading to the [[Rutherford model]] of the atom, and finally (after [[Henry Moseley]]'s work showed in 1913 that the nuclear charge was very close to the atomic number) to the [[Antonius Van den Broek]] suggestion that atomic number '''is''' nuclear charge. Eventually, by 1913, this work had culminated in the solar-system-like (but quantum-limited) [[Bohr model]] of the atom, in which a nucleus containing an atomic number of positive charge is surrounded by an equal number of electrons in orbital shells.

Thomson's model was compared (though not by Thomson) to a British treat called [[plum pudding]], hence the name. It has also been called the '''chocolate chip cookie model'' or "Blueberry Muffin Model"', but only by those who have not read Thomson's original paper (''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''), published in the ''Philosophical Magazine'' (the leading British science journal of the day). For an excerpt see [http://dbhs.wvusd.k12.ca.us/webdocs/Chem-History/Thomson-Structure-Atom.html].

A little-known (or now forgotten) fact about the original Thomson "plum pudding" model is that it was dynamic, not static. The electrons were free to rotate within the blob or cloud of positive substance. These orbits were stabilized in the model by the fact that when an electron moved farther from the center of the positive cloud, it felt a larger net positive inward force, because there was more material of opposite charge, inside its orbit (A particle like a small black hole would feel the same restorative force if it penetrated the body of the Earth; such a particle would feel only the gravity of the Earth inside its radius). In Thomson's model, electrons were free to rotate in rings which were further stabilized by interactions between the electrons, and spectra were to be accounted for by energy differences of different ring orbits. Thomson attempted to make his model account for some of the major spectral lines known for some elements, but was not notably successful at this. Still, Thomson's model (along with a similar Saturnian ring model for atomic electrons, put forward also in 1904 by [[Hantaro Nagaoka|Nagaoka]] after the Maxwell model of Saturn's rings), were earlier harbingers of the later and more successful solar-system-like [[Bohr model]] of the atom.

==See also==
* [[1904 in science]]

==External links==
*[http://dbhs.wvusd.k12.ca.us/webdocs/Chem-History/Thomson-Structure-Atom.html "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" — J.J. Thomson's 1904 paper proposing the plum pudding model.

==References==

*[http://dbhs.wvusd.k12.ca.us/webdocs/Chem-History/Stoney-1894.html Of the "Electron", or Atom of Electricity] - by G. J. Stoney, Philosophical Magazine, Series 5, Volume 38, p. 418-420, October 1894

[[Category:Obsolete scientific theories]]
[[Category:History of physics]]

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