Assertion (A): Rutherford concluded that most of the mass of an atom is concentrated in a small region at the centre called the nucleus.

Reason (R): According to Thomson's model, electrons are embedded in a uniformly distributed positive charge sphere.

Choose the correct option:

(a) Both (A) and (R) are true, and (R) is the correct explanation of (A).

(b) Both (A) and (R) are true, but (R) is not the correct explanation of (A).

(c) (A) is true but (R) is false.

(d) (A) is false but (R) is true. [Pg. No. 145]

(b) Both (A) and (R) are true, but (R) is not the correct explanation of (A).

Explanation: Rutherford concluded from his gold foil experiment that most of the mass and positive charge of an atom are concentrated in a very small central region called the nucleus, because a few $\alpha$-particles were strongly deflected or bounced back. Thomson's model is also true as a statement of his proposal, where electrons were embedded in a uniformly distributed positive sphere, but this model does not explain Rutherford's conclusion. In fact, Rutherford's observations disproved Thomson's model.