Rutherford's alpha-particle (α) scattering experiment was responsible for the discovery of which one of the following?
- (a)Electron
- (b)Proton
- (c)Atomic Nucleus
- (d)Neutron
Correct — C, the Atomic Nucleus. In Rutherford's experiment a thin gold foil was bombarded with alpha particles; most passed straight through, but a few were deflected through large angles and some bounced almost straight back. Rutherford concluded that the atom's positive charge and nearly all its mass are concentrated in a tiny central core, the nucleus. The experiment did not discover the electron, proton or neutron, which were established by other work.
- (a)Electron — The electron was discovered earlier by J. J. Thomson through cathode-ray (discharge tube) experiments, not by alpha scattering.
- (b)Proton — The proton was identified in Rutherford's later experiments on nitrogen and by Goldstein's canal-ray work, not by the gold-foil scattering, which revealed the nucleus itself.
- (d)Neutron — The neutron was discovered by James Chadwick in 1932, well after the 1911 scattering experiment.
Rutherford's gold-foil (alpha-scattering) experiment of 1911 fired positively charged alpha particles at a very thin gold foil and recorded how they were deflected. The pattern of deflection led to the nuclear model of the atom, with a tiny dense positive nucleus at the centre.
The signature result is the rare large-angle deflection and back-scattering of alpha particles, which can only happen if a small, dense, positive centre exists. That inference names the nucleus, not any single subatomic particle.
- Rutherford in 1911 fired alpha particles at thin gold foil and watched their deflection.
- Most passed straight through, showing the atom is mostly empty space.
- A few large-angle deflections revealed a tiny, dense, positively charged nucleus.
- The result overturned Thomson's plum-pudding model of the atom.
- Crediting the experiment with discovering the proton or neutron.
- Confusing Rutherford's nuclear model with Thomson's plum-pudding model.
Asked what Rutherford's alpha-scattering experiment discovered; the atomic nucleus, not a specific particle.
UPSC_1996_GS1_Q1271996The alpha particle carries two positive charges. Its mass is very nearly equal to that of
- (a) two protons
- (b) an atom of helium
- (c) sum of masses of two positrons and two neutrons
- (d) two positrons as each positron carries a single positive charge
Answer(b) an atom of helium
Same experimental tool. The alpha particle that Rutherford scattered off gold foil is the nucleus of a helium atom (two protons and two neutrons), so knowing what the alpha particle is underlies both questions.
Which one of the following conclusions could not be derived from Rutherford's α-particle scattering experiment?
- (a) Most of the space in the atom is empty.
- (b) The radius of the atom is about 10⁵ times the radius of the nucleus.
- (c) Electrons move in a circular path of fixed energy called orbits.
- (d) Nearly all the mass of the atom resides in the nucleus.
Answer(c) Electrons move in a circular path of fixed energy called orbits.
The same experiment from the other side. That item asks which conclusion the scattering did not support (fixed orbits came from Bohr), reinforcing that the gold-foil result established the nucleus and empty space, which is exactly what this question tests.
- practice — not a real PYQ
In Rutherford's experiment, most alpha particles passing straight through the foil showed that
- (a)the atom is solid
- (b)the atom is mostly empty space
- (c)electrons are heavy
- (d)the nucleus is negative
Answer(b) the atom is mostly empty space.
- practice — not a real PYQ
The plum-pudding model of the atom was proposed by
- (a)Rutherford
- (b)J. J. Thomson
- (c)Chadwick
- (d)Bohr
Answer(b) J. J. Thomson — later replaced by Rutherford's nuclear model.