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Thomson’s Model of an Atom

November 19, 2012

J.J. Thomson, F.R.S., Cavendish Professor of Experimental Physics, Cambridge, through his paper in the philosophical Magazine (March 1904) proposed a plausible model for the structure of atom. It is interesting to note that he viewed electrons as essential particles. He studiously persisted in referring to them as “negatively electrified corpuscles”. He envisaged them as moving in a “sphere of uniform positive electrification”. While this model of an atom proposed by Thomson has been thoroughly dismissed in the light of advances in the understanding of the structure of the atom and quantum theory, at that time it was indeed a revolutionary concept.

Thomson’s Model of an Atom – Main Features

He considered two possible effects when a number of negatively charged corpuscles in a sphere of uniform positive charge 1) how would they arrange themselves 2) What would they do to the structure of the atom. By constraining them to be in a singular plane, Thomson’s model of an atom caused the following effects to ensue: a) They would arrange themselves into a series of concentric rings. b) Each ring will be populated by appropriate number of corpuscles so as to impart maximum stability to the structure.

If they are not restricted to one plane, however, there will be number of nested shells. Analytical and geometrical complications dissuaded him from getting a general solution to the shell like structure. Hence he proceeded to propose placement of negative particles to be embedded in the solid positively charged sphere. Since this could be visualized as plum pudding in England and Raisin bread in USA, Thomson’s model of an atom was dubbed as respective culinary delicacy in that country. He tried unsuccessfully to make the model account for the spectral lines for some of the atoms.

Thomson’s Model of an Atom – its Disproof

The 1909 gold foil experiment was interpreted in 1911 by Rutherford to suggest a central nucleus with a concentrated powerful positive charge sufficient to hold around 100 electrons. HGJ Mosley suggested in 1913 that nuclear charge was close to the atomic number and Van den Broek refined this idea to state that atomic number is the nuclear charge. These resulted in Bohr’s model of atom resembling a solar system summarily replacing Thomson’s model of an atom. Although Thomson’s model of an atom could not withstand the test of time, it was indeed a harbinger of multilevel arrangement of electrons.

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