BPSC 70th CCE Prelims General Studies · Question 29 of 143

Correct — C, J. J. Thomson. Of the four scientists named, Thomson came first, and what he offered was genuinely the first account of what an atom looks like on the inside. He had discovered the electron in 1897 — the work for which he received the Nobel Prize in Physics in 1906, awarded 'in recognition of the great merits of his theoretical and experimental investigations on the conduction of electricity by gases' — and having established that atoms contain negatively charged particles far lighter than the atoms themselves, he was obliged to explain how a neutral atom could hold them. His answer, published in 1904, is the plum pudding model, which Indian textbooks often render as the watermelon model: a sphere of positive charge spread uniformly through the atom, with the electrons embedded in it like plums in a pudding or seeds in a watermelon, the positive and negative charges exactly balancing so that the whole is neutral. It was the first atomic model to describe an internal structure at all, which is precisely the claim the stem is testing. The chronology disposes of the rest. Rutherford's nuclear model came in 1911, after alpha-particle scattering experiments carried out with Geiger and Marsden from 1908, and Bohr's quantised model in 1913. Goldstein's canal-ray work of 1886 is older than any of them but produced a discovery rather than a model. Note also the name deliberately kept off the list: John Dalton's atomic theory of the early nineteenth century is older still, but Dalton treated the atom as indivisible and described nothing inside it.

• (a)Rutherford — Rutherford — the model most people actually picture when they think of an atom, which is what makes this option attractive. Ernest Rutherford published the nuclear model in 1911, seven years after Thomson's, and he framed it explicitly as a replacement: the alpha-particle scattering carried out by Geiger and Marsden from 1908 showed that a small number of alpha particles bounced almost straight back off gold foil, which a diffuse cloud of positive charge could never have produced. Second in the sequence, not first.
• (b)E. Goldstein — E. Goldstein — the earliest name on this list by date, which is exactly why it is dangerous on a question that asks who was first. Eugen Goldstein discovered canal rays, also called anode rays, in 1886, and that work eventually led to the identification of the proton. But discovering a stream of positively charged particles is not the same as proposing a picture of how an atom is put together, and Goldstein advanced no model of atomic structure.
• (d)Neils Bohr — Neils Bohr — the last of the three model-builders on this list, in 1913, and the model most school courses finish with, so it is the one that comes to mind first when the word 'model' appears. Bohr kept Rutherford's nucleus and added the decisive quantum idea that electrons occupy fixed energy levels and radiate only when they jump between them, which repaired the instability of the Rutherford atom. Being the most familiar does not make it the earliest.

The models of the atom form a short chronological sequence, and almost every examination question on the topic is really a question about that order. John Dalton, early in the nineteenth century, proposed that matter is made of indivisible atoms, identical within an element and different between elements — a theory about atoms, with no interior. Thomson's discovery of the electron in 1897 destroyed indivisibility, and his 1904 plum pudding model was the first attempt to say what the interior contains: electrons embedded in a uniform sphere of positive charge. Rutherford's scattering experiments broke that picture in 1911 and replaced it with a nuclear atom, in which almost all the mass and all the positive charge sit in a nucleus occupying a tiny fraction of the volume while electrons move in the space around it. Bohr in 1913 kept the nucleus and solved the problem it created — a classically orbiting electron should radiate energy and spiral into the nucleus — by postulating fixed, quantised energy levels in which an electron does not radiate at all. Quantum mechanics later replaced Bohr's definite orbits with probability distributions, or orbitals, but the succession Dalton, Thomson, Rutherford, Bohr remains the spine of the topic.

Read the stem word by word: it asks who was the first to propose a model for the structure of an atom. Two of those requirements do real work. 'First' makes this a chronology question, and 'model for the structure' excludes both a bare discovery and a theory that denies there is any structure. Order the four names by date — Goldstein 1886, Thomson 1904, Rutherford 1911, Bohr 1913 — and then test each against the second requirement. Goldstein is earliest but offered a discovery of canal rays rather than a model, so he fails on structure. Thomson is next and passes on both counts. That is the answer, and it took no chemistry beyond the dates. Two traps are built into this design and both are worth naming in advance. The first is picking the earliest name on the list without checking whether that person proposed a model at all. The second is picking the most familiar model, which is Rutherford's or Bohr's, because familiarity feels like priority. If Dalton had been among the options the question would have been harder, and a candidate should be ready for a version that includes him: Dalton is earlier still, but his atom has no inside, so Thomson would remain the answer to a question about structure.

• J. J. Thomson discovered the electron in 1897 and was awarded the Nobel Prize in Physics in 1906 'in recognition of the great merits of his theoretical and experimental investigations on the conduction of electricity by gases'.
• He proposed the plum pudding model in 1904 — electrons embedded in a sphere of uniformly distributed positive charge — and it was the first atomic model to describe an internal structure.
• Rutherford published the nuclear model in 1911, on the strength of alpha-particle scattering experiments conducted with Geiger and Marsden from 1908; the alpha particle used as the probe is itself a helium nucleus.
• Bohr's model of 1913 retained Rutherford's nucleus and added quantised energy levels, resolving the objection that an orbiting electron should continuously radiate energy and collapse into the nucleus.
• E. Goldstein discovered canal rays, also called anode rays, in 1886, work that led towards the identification of the proton — a discovery of particles rather than a model of the atom.
• John Dalton's atomic theory of the early nineteenth century predates all of these but treats the atom as indivisible, so it describes atoms without describing anything inside one.

• Choosing the earliest name without checking what that scientist actually did. Goldstein's canal rays of 1886 predate Thomson but are a discovery, not a model of atomic structure.
• Choosing the most familiar model. Rutherford's and Bohr's are the pictures reproduced in every textbook, and both came after Thomson's.
• Treating Dalton's atomic theory as a model of atomic structure. Dalton's atom is indivisible and has no interior to describe, which is a different kind of claim altogether.

BPSC asks the history of science as a bare priority question — who was first — and fills the options with everyone associated with the topic, including at least one person who is chronologically earlier but did something other than what the stem describes, so the qualifying phrase in the stem has to be read as carefully as the names. UPSC rarely asks who proposed what; it asks the physics that the models were built to explain, such as what an alpha particle is or which principle governs the filling of orbitals, so the same chapter needs both a timeline of people and a working grasp of the experiments.

• practice — not a real PYQ

  Rutherford's alpha-particle scattering experiment led to the conclusion that

  • (a)electrons are embedded in a sphere of positive charge
  • (b)almost all the mass and the entire positive charge of an atom are concentrated in a very small nucleus
  • (c)electrons revolve only in fixed energy levels without radiating energy
  • (d)the atom is indivisible

  Answer(b) almost all the mass and the entire positive charge are concentrated in a very small nucleus. Option (a) describes Thomson's model, which the experiment disproved; option (c) is Bohr's addition of 1913; option (d) is Dalton's assumption.

• practice — not a real PYQ

  The electron was discovered by

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

  Answer(c) J. J. Thomson, in 1897, working on the conduction of electricity through gases — the discovery for which he won the 1906 Nobel Prize in Physics and which forced him to propose a model of what holds electrons inside a neutral atom. Chadwick discovered the neutron.