Who was the first one to propose a model for the structure of an atom ?

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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.

Why the other options are wrong — (a) 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 — 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 — 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.

Concept: 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.

State PCS 2024 Q29 — Science & Technology PYQ with Answer

State PCS 2024 Q29 — Science & Technology PYQ with Answer