Question

What are the postulates of JJ Thomson? What are his drawbacks?

Answer 1

Hint: J.J Thomson is also known for the discovery of electrons. You must have heard about the cathode ray tube experiment; this was the time when J.J Thomson realized that atoms consisted of some negatively charged particles which he called corpuscles or electrons.

Complete step by step solution:

Postulates of Thomson’s atomic model:
According to Thomson atoms were positively charged spheres and the electrons were embedded in it.
The negative and positive charges were equal in magnitude, so the overall atom was electrically neutral.
The positive charge on the atom was uniformly distributed.
Thomson’s atomic model was also compared to the watermelon or plum pudding, where the electron in the sphere of positive charge was compared to the plums in the pudding. Similarly, in watermelon, the seeds were compared to the electrons while the red part of the melon was compared to the sphere of positive charge.

Limitations/Drawbacks of Thomson’s atomic model:
The model did not mention the presence of nucleus in the atom.
The model said that the sphere is positively charged and it holds the electron, which is not possible. So, the model failed to explain the stability of an atom.
It failed to explain Rutherford’s scattering experiment, and was unable to explain the scattering of alpha particles when projected on gold foils.

Note: Thomson studied electrical and magnetic properties of cathode rays. He also calculated the mass to charge ratio of electrons. Using this ratio, he concluded that the size of these particles was even smaller than hydrogen ions.