CDS (I) 2020 General Knowledge · Question 1 of 120

Correct — C, positively charged nucleus at the centre of an atom. Rutherford fired fast alpha particles — doubly charged helium ions of mass 4 u — at a gold foil about a thousand atoms thick, and expected them all to punch through a soft, evenly spread positive pudding with at most a slight wobble. Most did pass straight through. A few came off at small angles, and roughly one in twelve thousand bounced back the way it had come. Only one arrangement of matter explains that last group. To turn a heavy, fast, positively charged projectile around, the atom must carry a repelling charge that is both positive and packed into a tiny volume, and that volume must also hold nearly all the atom's mass. Rutherford put the radius of this centre at about a hundred thousand times smaller than the radius of the atom itself and named it the nucleus.

• (a)electron in an atom — The electron was already known — J. J. Thomson had found it in cathode rays more than a decade earlier, and his pudding model was built around electrons studded in positive matter. An alpha particle is also some 7,000 times heavier than an electron, so no electron could send one back; the foil experiment tells you nothing new about them.
• (b)proton in an atom — The experiment locates the positive charge; it does not identify the particle carrying it. NCERT credits the discovery of the proton to E. Goldstein's canal-ray work, and Rutherford only knocked protons out of nitrogen years after the gold-foil result.
• (d)isotopes of gold — Scattering depends on the nuclear charge, which is the same 79 for every gold nucleus. Nothing in the deflection pattern separates one mass number from another; isotopes were established later by mass spectrometry.

Before 1911 the accepted picture was J. J. Thomson's: a sphere of positive charge with electrons embedded in it like seeds in a watermelon. Rutherford's scattering result destroyed it and replaced it with the nuclear atom — a minute, dense, positively charged nucleus holding nearly all the mass, with electrons moving around it and almost all of the atom's volume empty.

The reasoning runs backwards from the surprise, not forwards from the theory. Ask what each observation forces on you: straight-through passage means mostly empty space, small-angle deflection means the positive charge occupies very little room, and rebound means that little room is where the mass sits too. Rutherford's own line for the rebound was that it was 'almost as incredible as if you fire a 15-inch shell at a piece of tissue paper and it comes back and hits you'. The trap in this item is that all four options are true statements about atoms — gold does have isotopes, atoms do have electrons and protons — so a candidate reading fast can pick any of them. The question asks what this particular experiment suggests, and only option (c) is a conclusion the scattering pattern actually forces.

• Alpha particles are doubly charged helium ions of mass 4 u, so they are heavy and fast enough to probe an atom's interior.
• The gold foil was about 1,000 atoms thick; gold was chosen because it can be beaten thinner than any other metal.
• About one alpha particle in 12,000 rebounded, which is the observation that demanded a concentrated positive centre.
• Rutherford calculated the nucleus to be roughly 100,000 times smaller in radius than the atom.
• The nuclear model could not explain why an orbiting electron does not radiate energy and spiral into the nucleus — that gap is what Bohr's postulates filled.

• Reading the experiment as the discovery of the proton or the neutron; it is the discovery of the nucleus.
• Remembering the result as 'most alpha particles were deflected' — most went straight through, and that is half the argument.
• Confusing Thomson's positively charged sphere with Rutherford's positively charged centre.

Either as a straight 'the gold-foil experiment led to the discovery of ___' item, or in the harder form that asks which conclusion does not follow from it.

• 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 led to which one of the following conclusions?

  • (a)The atom carries no positive charge
  • (b)Most of the space inside the atom is empty
  • (c)Electrons revolve in discrete orbits
  • (d)The nucleus contains neutrons

  Answer(b) Most of the space inside the atom is empty — undeflected passage is evidence about the volume of the atom, while the rare rebounds are the evidence about the concentrated nucleus.

• practice — not a real PYQ

  Which one of the following was the main drawback of Rutherford's nuclear model of the atom?

  • (a)It could not account for the mass of the atom
  • (b)It denied the existence of electrons
  • (c)It could not explain the stability of the atom
  • (d)It placed the positive charge outside the atom

  Answer(c) It could not explain the stability of the atom — an electron accelerating in a circular orbit should radiate energy and spiral into the nucleus.