Given below are two statements: one is labelled as Assertion (A) and the other is labelled as Reason (R).
Assertion (A): The force of repulsion between the atomic nucleus and an α-particle varies with distance according to the inverse square law.
Reason (R): The α-particle scattering experiment was performed by Geiger and Marsden at Rutherford’s suggestion.
Choose the correct answer from the options given below.
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Both A and R are correct and R is NOT the correct explanation of A.
Assertion (A) The force of repulsion between the atomic nucleus and an α-particle varies with distance according to the inverse square law. — True. The α-particle (charge \( 2e \)) and the nucleus (charge \( Ze \)) repel each other by Coulomb’s law. So the force is proportional to \( 1/r^{2} \), where \( r \) is the distance between them.
Reason (R) The α-particle scattering experiment was performed by Geiger and Marsden at Rutherford’s suggestion. — True. In 1911, at the suggestion of Rutherford, H. Geiger and E. Marsden directed a beam of α-particles at a thin gold foil and studied how the particles were scattered.
Why R does not explain A?
R is a historical fact about who carried out the experiment. It says nothing about how the force depends on distance.
The inverse square dependence comes from the Coulomb force between two positive charges, the α-particle and the nucleus, \( F=(1/4\pi\varepsilon_0)(2e)(Ze)/r^{2} \).
NCERT (2026-27):
Alpha-particle Scattering and Rutherford’s Nuclear Model of Atom – “… Under these assumptions, the trajectory of an alpha-particle can be computed employing Newton’s second law of motion and the Coulomb’s law for electrostatic force of repulsion between the alpha-particle and the positively charged nucleus. The magnitude of this force is
\( \displaystyle F=\frac{1}{4\pi\varepsilon_0}\,\frac{(2e)(Ze)}{r^{2}} \)\( \qquad (12.1) \)
where r is the distance between the α-particle and the nucleus.”
Alpha-particle Scattering and Rutherford’s Nuclear Model of Atom – “At the suggestion of Ernst Rutherford, in 1911, H. Geiger and E. Marsden performed some experiments.”