Question Analysis: Assertion-Reason type question.
- Step 1 (Analysis of Assertion): [0.5 Mark]
The Assertion states that energy levels in the Bohr model are discrete and quantised. According to Bohr's second postulate, the angular momentum of an electron is quantised: $$L = mvr = \frac{nh}{2\pi}$$ This leads to discrete energy levels given by: $$E_n = -\frac{me^4}{8\epsilon_0^2 n^2 h^2}$$ Thus, the Assertion is True.
- Step 2 (Analysis of Reason): [0.5 Mark]
The Reason states that the electrostatic force provides the centripetal force. This is the fundamental dynamical condition for circular motion in an atom: $$\frac{1}{4\pi\epsilon_0} \frac{Ze^2}{r^2} = \frac{mv^2}{r}$$ While this statement is physically correct, it describes the classical requirement for circular motion. It does not explain why the energy levels are quantised (which is a result of the wave-particle duality and the boundary condition of the electron wave). Thus, the Reason is True, but it is not the correct explanation of the Assertion.
- Step 3 (Final Concluded Answer): [0.0 Mark]
Correct Option: (b) Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of Assertion (A).
💡 EL10 Concept Insight: Think of the atom like a staircase: the electrostatic force is just the "gravity" holding you on the stairs, but the fact that you can only stand on specific steps (and not in between) is the "quantisation" rule that Bohr discovered.