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j.j thomson model of an atom explain in dettail with pic...for my project..

Asked by Dhruv Dewett(student) , on 29/10/13

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Posted by Mohit Gupta(MeritNation Expert)on 30/10/13

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Thomson 's Atomic Model

 [J. J. Thomson 's raisin bread model (plum pudding model)] J. J. Thomson considered that the structure of an atom is something like a raisin bread, so that his atomic model is sometimes called the raisin bread model. He assumed that the basic body of an atom is a spherical object containing N electrons confined in homogeneous jellylike but relatively massive positive charge distribution whose total charge cancels that of the N electrons. The schematic drawing of this model is shown in the following figure. Thomson 's model is sometimes dubbed a plum pudding model.

 [Thomson 's Model and Alpha Particle Scattering] As stated on the preceding page, Geiger and Marsden carried out an experiment in which the alpha rays were collided against a thin metal foil. Their results showed that almost all incident alpha particles penetrate the foil and go straight in forward direction but a few are scattered in very large angles. Is it possible to explain this result by using Thomson 's model? This question will be studied below, but before explaining it, let us present the answer in advance: The results of the alpha particle scattering cannot be explained by Thomson 's atomic model. Thomson 's raisin bread model (plum pudding model) therefore cannot be valid as an atomic model. The reason will be explained below. Since we need somewhat detailed mathematical expressions, it will be given on the other page, 2-4-A: Alpha Particle Scattering by Thomson 's model As seen on the other page (2-4-A), we can consider that the scattering angle of the alpha particle by Thomson 's model is at most 0.01 degrees. The thickness of the metal foil in the scattering experiment of the alpha rays is about 10-6 m. When assuming that the atoms are tightly packed in the metal, there are about 10000 atoms lining up in the direction of thickness, because the size of an atom is approximately 10-10m. (See the following figure.) When the alpha particle collides with these atoms in the metal foil 10000 times successively, the scattering angle of each individual collision in such a multiple scattering is less than 0.01 degrees as discussed above. The resultant scattering angle is obtained by an accumulation of these individual scatterings of 10000 times. One may expect that, even if the scattering angle of each individual scattering is very small like 0.01 degrees, we can have as large resultant angle as This is however unrealistic, because the direction of each individual scattering must be random, and an accumulation of random values would give nearly zero only. So that we never obtain such a large resultant scattering angle after the multiple scattering. Accordingly, such a large scattering angle as those obtained in Geiger and Marsden 's experiment cannot be reproduced by such a multiple scattering as stated above. Thus we can conclude that Thomson 's atomic model is not held.

With the growing evidence that the atom is composed of even smaller particles, attention was given to how the particles fit together. In the early 1900s J. J. Thomson reasoned that because electrons comprise only a very small fraction of the mass of an atom, they probably were responsible for an equally small fraction of the atom 's size. He proposed that the atom consisted of a uniform positive sphere of matter in which the electrons were embedded, as shown in Figure 2.8. This model became known as the "plum-pudding" model, after the name of a traditional English dessert. Thomson 's atomic model was very short-lived.

Figure 2.8  J. J. Thomson 's "plum-pudding" model of the atom. He pictured the small electrons to be embedded in the atom much like raisins in a pudding or like seeds in a watermelon. Ernest Rutherford proved this model wrong.

hope dis 1 will help!! : )

Posted by Sonal(student)on 3/1/12

ok i wil try 2 give u another anser!! : ))

Posted by Sonal(student)on 3/1/12

A schematic presentation 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 by J. J. Thomson, who discovered the electron in 1897, was proposed in 1904 before the discovery of the atomic nucleus in order to add the electron to the atomic model. In this model, the atom is composed of electrons (which Thomson still called "corpuscles", thoughG. J. Stoney had proposed that atoms of electricity be called electrons in 1894 [1] ) surrounded by a soup of positive charge to balance the electrons ' negative charges, like negatively-charged "plums" surrounded by positively-charged "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.

With this model, Thomson abandoned his earlier "nebular atom" hypothesis in which the atom was composed of immaterial vorticies. Now, at least part of the atom was to be composed of Thomson 's particulate negative corpuscles, although the rest of the positively-charged part of the atom remained somewhat nebulous and ill-defined.

The 1904 Thomson model was disproved by the 1909 gold foil experiment, which was interpreted by Ernest Rutherford in 1911 [2]   [3]  to imply a very small nucleus of the atom containing a very high positive charge (in the case of gold, enough to balance about 100 electrons), thus leading to the Rutherford modelof the atom. Although gold has an atomic number of 79, immediately after Rutherford 's paper appeared in 1911 Antonius Van den Broek made the intuitive suggestion that atomic number is nuclear charge. The matter required experiment to decide. Henry Moseley 's work showed experimentally in 1913 (seeMoseley 's law) that the effective nuclear charge was very close to the atomic number (Moseley found only one unit difference), and Moseley referenced only the papers of Van den Broek and Rutherford. This work culminated in the solar-system-like (but quantum-limited) Bohr model of the atom in the same year, in which a nucleus containing an atomic number of positive charge is surrounded by an equal number of electrons in orbital shells. Bohr had also inspired Moseley 's work.

Thomson 's model was compared (though not by Thomson) to a British dessert called plum pudding, hence the name. Thomson 's paper was published 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, ... [4]

In this model, 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 (see Gauss 's law). 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, also put forward in 1904 by Nagaoka after James Clerk Maxwell 's model of Saturn 's rings), were earlier harbingers of the later and more successful solar-system-like Bohr model of the atom.

hope dis 1 will help !!

Posted by Sonal(student)on 3/1/12