Which of the following was NOT established by Rutherford’s α-particle…

The correct answer is Option (2) → The number of electrons is roughly equal to the number of protons, which accounts for the neutrality of the atom.

The scattering experiment probed the atom with α-particles. These are turned aside by the positive charge and mass of the nucleus. The light electrons hardly affect them.

Option (1) The entire positive charge of the atom is concentrated at the centre. — Established. A large repulsive force is needed to turn an α-particle backwards. This is possible only if the positive charge is concentrated tightly at the centre of the atom.

Option (2) The number of electrons is roughly equal to the number of protons, which accounts for the neutrality of the atom. — Not established. The atomic electrons are very light and hardly affect the α-particles. So the scattering gives no count of the electrons. The neutrality of the atom was already known before the experiment.

Option (3) The nucleus is a dense mass at the centre of the atom. — Established. Only a small fraction of the α-particles rebound, so head-on collisions are rare. Hence, the mass and positive charge of the atom sit in a small volume at the centre.

Option (4) An α-particle passing very close to the nucleus is deflected through a large angle. — Established. This was seen in the experiment. An α-particle near a nucleus feels its intense electric field and is scattered through a large angle. About 1 in 8000 is deflected by more than \( 90^\circ \).

NCERT (2026-27):
Alpha-particle Scattering and Rutherford’s Nuclear Model of Atom – “… However, when α-particle happens to come near a nucleus, the intense electric field there scatters it through a large angle. The atomic electrons, being so light, do not appreciably affect the α-particles.”