Alpha particle scattering experiment determines

The correct answer is Option (2) → the upper limit to the size of the nucleus.

In Rutherford’s alpha-particle scattering, an alpha particle heading straight at a nucleus stops at the distance of closest approach \( d \), where all its kinetic energy has become electrostatic potential energy.

    \( \displaystyle K = \frac{1}{4\pi\varepsilon_{0}}\,\frac{(2e)(Ze)}{d} \)

The alpha particle never reaches the nucleus itself, so the nucleus must be smaller than d: the experiment gives an upper limit to the size of the nucleus (about \( 10^{-14} \) m for gold).

NCERT (2026-27):
Alpha-Particle Trajectory – “… The fact that only a small fraction of the number of incident particles rebound back indicates that the number of \( \alpha \)-particles undergoing head on collision is small. This, in turn, implies that the mass and positive charge of the atom is concentrated in a small volume. Rutherford scattering therefore, is a powerful way to determine an upper limit to the size of the nucleus.”