Identify the correct statement in the context of Rutherford scattering of \( \alpha \)-particles by target nucleus.
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → The perpendicular distance of the initial velocity vector of the \( \alpha \)-particle from the centre of the nucleus is called impact parameter.
The impact parameter \( b \) measures how far off-centre an \( \alpha \)-particle is aimed at the nucleus. The smaller \( b \) is, the closer the particle passes to the nucleus, the stronger the Coulomb repulsion it feels and the larger its deflection; a head-on particle (\( b = 0 \)) bounces straight back.
Option (1) For large impact parameter values, \( \alpha \)-particle suffers large deviation. — Incorrect. A large impact parameter keeps the particle far from the nucleus, so the deviation is small.
Option (2) For small impact parameter values, the deviation in the trajectory of \( \alpha \)-particle is negligible. — Incorrect. A small impact parameter takes the particle close to the nucleus, so the deviation is large.
Option (4) Impact parameter depends on mass of the target nucleus. — Incorrect. It is fixed by how the particle is aimed, not by the mass of the target.
NCERT (2026-27):
Alpha-Particle Trajectory – “The trajectory traced by an α-particle depends on the impact parameter, b of collision. The impact parameter is the perpendicular distance of the initial velocity vector of the α-particle from the centre of the nucleus …. … It is seen that an α-particle close to the nucleus (small impact parameter) suffers large scattering. … For a large impact parameter, the α-particle goes nearly undeviated and has a small deflection (θ ≅ 0).”