Question

Assertion (A): In Rutherford’s alpha particle scattering experiment, the presence of only few alpha particles at angle of scattering \( \pi \) led him to the discovery of the nucleus.
Reason (R): The size of the nucleus is approximately \( 10^{-5} \) times the size of an atom and therefore only few alpha particles are rebounded.

• A. Both Assertion (A) and Reason (R) are true, and Reason (R) is the correct explanation of Assertion (A).
• B. Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of Assertion (A).
• C. Assertion (A) is true, but Reason (R) is false.
• D. Both Assertion (A) and Reason (R) are false.

Hint:

In Rutherford’s alpha particle scattering experiment, the deflection of alpha particles at large angles provided evidence of the existence of a dense nucleus. The small size of the nucleus explains why only a few alpha particles were deflected significantly.

Answer 1

The Correct Option is A

In Rutherford’s alpha particle scattering experiment, a thin gold foil was bombarded with alpha particles, and it was observed how these particles scattered. The experiment led to the discovery of the atomic nucleus.

Assertion (A): The observation that only a few alpha particles were deflected at a large angle, close to \( \pi \), was significant. It indicated that a small, positively charged nucleus existed, which repelled those alpha particles. This inference was logical because if the positive charge were spread out over a larger volume, more particles would have been deflected at smaller angles instead of only a few at large angles.

Reason (R): The reason provided is that the nucleus is very small compared to the atom, approximately \( 10^{-5} \) times the size of the entire atom. Thus, most alpha particles passed through the atom without significant deflection, only occasionally being deflected by the dense, centrally located nucleus.

Since the presence of the nucleus causes such deflections, both the assertion and the reason are true. Moreover, the reason directly explains why the assertion is correct, as the small size of the nucleus accounts for the minimal number of large-angle scatterings.

Conclusion: Both Assertion (A) and Reason (R) are true, and Reason (R) is indeed the correct explanation of Assertion (A).