Question

Merit of Thomson atomic model

Answer 1

According to this model, an atom can be considered as a sphere of uniformly distributed positive charge in which there are electrons distributed symmetrically. The electrons must be held by the positive charges by electrostatic forces. The mutual repulsions between the electrons are balanced by the force of attraction towards the centre of the sphere. In an atom with a single electron like the hydrogen atom, the electron must be situated at the centre of the positive sphere. In an atom with two electrons, like the helium atom, the two electrons must be symmetrically situated on opposite sides of the centre at a distance equal to half the radius of the positive sphere. In the three-electron system, the electrons should be at the corners of a symmetrically placed equilateral triangle, the side of which is equal to the radius of the sphere. Proceeding in this manner, Thomson could explain the arrangement of electrons ranging from 1 to 100 inside the positive sphere. This model was also called 'the plum pudding model'. The electrons are like plums in a pudding (positive charge). The seeds in a watermelon fruit can be given as a parallel comparison

With this model he tried to explain the observed spectra of elements. According to him, the vibrations of electrons would give rise to electromagnetic energy radiated. He could explain with some success the process like chemical reaction, radioactivity etc.

This model had many drawbacks. The chief among them are:

(i) Since the weight of an electron is about a thousandth part of a hydrogen atom, it would mean that a single atom, especially of the heavier elements, would contain many thousand electrons. But J.J. Thomson himself found that the number of electrons in an atom cannot be greatly different from the atomic weight.

(ii) According to this model, hydrogen can give rise to only one spectral line, contrary to the observed fact of several lines.

(iii) This model could not explain the large angle scattering of alpha particles by thin metal foils
Thomson won Nobel prize for it

Answer 2

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