CBSE Class 9 Answered

Now we prove that why thomson's model cannot be held true. 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.