Rutherford's α-particle scattering experiment on gold foil is responsible for the discovery of:

Correct — D, Atomic nucleus. In the experiment carried out by Geiger and Marsden under Rutherford's direction, a beam of alpha particles was fired at a very thin gold foil and the scattered particles detected on a fluorescent screen. Almost all of them passed straight through with little or no deflection, which meant the atom is mostly empty space. A small fraction were deflected through large angles, and a very few — about one in twenty thousand — came almost straight back. Rutherford's remark that it was as astonishing as a fifteen-inch shell bouncing off tissue paper captures why this mattered: a rebound like that can only happen if the alpha particle meets something both massive and concentrated. Spread the positive charge and mass thinly through the whole atom, as J. J. Thomson's model did, and nothing in the foil is heavy enough or localised enough to turn an alpha particle round. So the experiment established the nucleus — a tiny, dense, positively charged centre holding nearly all the atom's mass, with the electrons occupying the vast remaining volume. The proton was identified by Rutherford later, in work on nuclear disintegration, and the neutron by Chadwick in 1932.

• (a)Neutron — Discovered by James Chadwick in 1932, more than two decades later. The neutron is uncharged and so does not govern the electrostatic scattering of alpha particles at all.
• (b)Electron — Discovered by J. J. Thomson in 1897, from cathode-ray experiments. It was Thomson's model of the atom that the scattering experiment overturned.
• (c)Proton — The plausible near-miss. The experiment revealed a positively charged centre, but not yet the individual particle carrying that charge; Rutherford identified the proton in later work on nuclear disintegration.

Rutherford's nuclear model followed from the scattering results: nearly all the mass and all the positive charge sit in a nucleus whose radius is of the order of ten thousand times smaller than the atom's, with electrons somewhere in the space around it. The model explained the scattering beautifully and failed on something else — classical electromagnetism says an orbiting, and therefore accelerating, electron must radiate energy and spiral into the nucleus in a fraction of a second. That failure to account for the atom's stability is what Bohr's model was constructed to repair.

Questions on this experiment are asked in two directions, and it is worth being able to answer both. One asks what the experiment discovered — the nucleus, as here. The other asks what Rutherford's model could not explain — the stability of the atom, which is question 52 in this very paper. Keeping the two apart is the whole skill: the experiment succeeded at locating the mass and charge, and the model built on it failed on stability. The observation-to-conclusion chain is also worth holding as a chain rather than a fact, because papers sometimes ask for the intermediate step: most particles undeflected means the atom is mostly empty; a few deflected sharply means a concentrated charge; a very few reversed means that concentration is also massive. Each observation maps to one conclusion.

• Geiger and Marsden performed the scattering experiment under Rutherford's direction, firing alpha particles at thin gold foil.
• Most alpha particles passed through undeflected — the atom is mostly empty space.
• A very small fraction, roughly one in twenty thousand, were deflected through more than 90 degrees.
• Large-angle scattering requires a small, dense, positively charged centre — the nucleus.
• The nuclear radius is of the order of ten thousand times smaller than the atomic radius.
• The electron was found by J. J. Thomson in 1897 and the neutron by James Chadwick in 1932.

• Answering 'proton' because the experiment revealed positive charge — it revealed the nucleus, not the individual particle.
• Attributing the electron to Rutherford; that is Thomson, and his model is what Rutherford overturned.
• Confusing what the experiment discovered with what the resulting model could not explain.

Very frequently, and in both directions — what the experiment discovered, and what the nuclear model failed to explain. Both appear in this same paper, at questions 52 and 56.

• practice — not a real PYQ

  In Rutherford's scattering experiment, the observation that a very small fraction of alpha particles were deflected through large angles indicated that

  • (a)the atom is uniformly filled with positive charge
  • (b)the positive charge is concentrated in a very small volume
  • (c)electrons are embedded in a positive sphere
  • (d)the atom contains neutrons

  Answer(b) the positive charge is concentrated in a very small volume — a diffuse charge could not turn an alpha particle through a large angle.

• practice — not a real PYQ

  The neutron was discovered by

  • (a)Ernest Rutherford
  • (b)J. J. Thomson
  • (c)James Chadwick
  • (d)Niels Bohr

  Answer(c) James Chadwick — in 1932, more than twenty years after the alpha-scattering experiment.