Question

Answer the following questions, which helps you to better understand the difference between Thomson’s model and Rutherford’s model?
(a)Is the average deflection of $\alpha $ - particles by the thin gold foil predicted by Thomson's model much less, about the same, or much greater than that predicted by Rutherford's model?
(b)Is the probability of backward scattering (that is, scattering alpha – particles at angles greater than ${90^ \circ }$ ) predicted by Thomson’s model much less about the same or much greater than that predicted by Rutherford's model?
(c)Keeping the others fixed, it is experimentally found that for small thickness $t$ , the number of alpha – particles scattered at moderate angles is proportional to $t$ . What clue does this linear dependence on $t$ provide?
(d)In which model is it completely wrong to ignore multiple scattering for the calculation of average angle of scattering of alpha – particles by the thin foil?

Answer 1

Hint: Thomson’s model of atom assumed that electron is two thousand times lighter than the proton and believed that the atom is made up of thousands of electrons but this failed to explain the scattering of alpha particles by thin metal foils.


Complete Step by step answer: According to the Rutherford’s model, some of the alpha – particles were deflected by the gold sheet by very small angles and the volume occupied by the positively charged particles in the atom is very small when compared with the total volume of the atom.

(a)The average angle of the deflection of the alpha – particles by the thin gold foil predicted by Thomson's model is approximately similar to Rutherford’s model of atom.

(b)The probability of backward scattering (that is, scattering alpha – particles at angles greater than ${90^ \circ }$ ) predicted by Thomson’s model is much less than that of Rutherford's model.

(c) The main cause of scattering is the single collision. The chances of the single collision increase with the number of target atoms. As the number of target atoms increases with the increase in the thickness, the collision probability depends linearly on the thickness of the target.

(d) In Thomson’s model it will be wrong to ignore the multiple scattering for the calculation of average angle of scattering of alpha – particles by the thin foil because the single collision causes very little deflection in this model. Hence, the observed average scattering angle can be explained only by considering multiple scattering.

Note: Rutherford’s model observed that the major fraction of the alpha – particles bombarded towards the gold sheet passed through it without any deflection.
Thomson’s model observed that an atom consists of the positively charged sphere with electrons embedded in it.