From the study of discharge of electricity through gases, it became clear that an atom consists of positive and negative charges J.J. Thomson tried to explain the arrangement of positive charge and the electrons inside the atom. According to him, an atom is a sphere of positive charge having a radius of the order of 10 angstrom. The positive charge is uniformly distributed over the entire sphere and the electrons are embedded in the sphere of positive charge. The total positive charge inside the atom is equal to the total negative charge carried by the electrons, so that every atom is electrically neutral.According to Thomson, if there is a single electron in the atom( hydrogen atom), the electron must be situated at the center of the positive sphere. For an atom with two electrons (helium atom), the electrons should be situated symmetrically with respect to the center of the sphere i.e opposite sides of the center at a distance of 2r, where r is the radius of the positive sphere. In a three electron system of the atom, the electrons should be at the corners of a symmetrically placed equilateral triangle, the side of which was equal to the radius of the sphere. In general, the electrons of an atom are located in a symmetrical pattern with respect to the center of the sphere. It was suggested that spectral radiations are due to the simple harmonic motion of these electrons on both sides of their mean positions. Moreover, the stability of the atom was very well explained on the basis of this model.
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