Rutherford's alpha-particle scattering experiment was responsible for the discovery of
- (a)Electron
- (b)Proton
- (c)Nucleus
- (d)Helium
Correct — C, Nucleus. In the experiment carried out under Rutherford's direction, a thin gold foil was bombarded with fast alpha particles and a fluorescent screen was used to see where they went. Almost all of them passed straight through with little or no deviation, a small number were turned through large angles, and a very few came back towards the source. The first observation showed that an atom is mostly empty space. The second and third could not be explained at all by the then-accepted picture of positive charge spread evenly through the atom, because such a diffuse charge could never turn a heavy, fast alpha particle around. Rutherford concluded that the positive charge and very nearly all the mass of an atom are packed into a minute central region, which he called the nucleus, with the electrons occupying the space outside it.
- (a)Electron — The electron was discovered years earlier by J. J. Thomson from his work on cathode rays, and it was Thomson's model of the atom that this scattering experiment overturned. Nothing in the alpha-particle results identified the electron.
- (b)Proton — Rutherford is indeed credited with the proton, but from a different experiment altogether — bombarding nitrogen with alpha particles and detecting hydrogen nuclei among the products. The gold-foil scattering told him where the positive charge sits, not what particle carries it.
- (d)Helium — Helium had been identified long before, and the fact that the alpha particle is itself a helium nucleus was established by a separate experiment in which alpha particles collected in a tube were shown to give helium's spectrum. In the scattering experiment the alpha particle is the tool, not the discovery.
Rutherford's model replaced the earlier picture of an atom as a sphere of positive charge with electrons embedded in it. It holds that an atom has a tiny, dense, positively charged nucleus carrying nearly all of the mass, and that the electrons move in the large empty space around it. The nuclear model explained the scattering results but left an open question, since an electron moving in a curved path should radiate energy and spiral inward, which is what Bohr's model later addressed.
Keep three discoveries and their experiments apart, because the examiner mixes them freely. Cathode rays gave the electron to Thomson; alpha-particle scattering off gold foil gave the nucleus to Rutherford; and bombardment of beryllium gave the neutron to Chadwick. It is also worth knowing what the scattering experiment could not show. It gave no information about how the electrons are arranged or about fixed energy levels, so any statement about electrons moving in orbits of fixed energy belongs to Bohr and not to Rutherford — NDA has set exactly that trap.
- Most alpha particles passed straight through the gold foil, showing that an atom is mostly empty space.
- A very small fraction were deflected through large angles, some almost straight back, pointing to a small, dense, positively charged centre.
- Rutherford concluded that nearly all the mass and all the positive charge of an atom are concentrated in the nucleus.
- The electron was discovered earlier by J. J. Thomson from cathode rays, and the neutron later by James Chadwick.
- Fixed energy levels for electrons came from Bohr's model, not from the scattering experiment.
- Crediting the scattering experiment with the discovery of the proton or the electron.
- Attributing fixed electron orbits to Rutherford; that conclusion belongs to Bohr.
- Forgetting that the alpha particle used as a probe is itself a helium nucleus.
NDA asks who discovered what and by which experiment, or which conclusion cannot be drawn from the gold-foil results.
The 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
Identifies the projectile used in the scattering experiment — the alpha particle is a helium nucleus, which is why helium appears as a distractor in this NDA question.
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
Answer(c) Atomic Nucleus
The same question repeated almost word for word four years later, with the same key — a clear signal of how reliably this item returns.
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.
Marks the boundary of what the experiment proved, which is the distinction that keeps the proton and the electron out of the answer here.
- practice — not a real PYQ
In Rutherford's alpha-particle scattering experiment, the observation that most of the particles passed straight through the gold foil showed that
- (a)the atom is mostly empty space
- (b)the atom has a positively charged nucleus
- (c)electrons revolve in fixed orbits
- (d)the alpha particle is a helium nucleus
Answer(a) the atom is mostly empty space — undeviated passage of the great majority of the particles is what established this.
- practice — not a real PYQ
The neutron was discovered by
- (a)J. J. Thomson
- (b)Ernest Rutherford
- (c)James Chadwick
- (d)Niels Bohr
Answer(c) James Chadwick — he identified the uncharged nuclear particle, while Thomson found the electron and Rutherford the nucleus.