Who among the following discovered Proton ?

Correct — B, (b) E. Goldstein. Goldstein's discharge-tube work of 1886 is where positively charged particles first showed themselves. Working with a discharge tube fitted with a PERFORATED cathode, he found that besides the cathode rays streaming away from the cathode there was a second set of rays travelling in the opposite direction and passing through the perforations, or canals, in it — which is why he named them Kanalstrahlen, canal rays, also known as anode rays. Unlike cathode rays, whose properties are the same whatever gas is in the tube, the canal rays depended on the gas: their charge-to-mass ratio changed with it, and it reached its maximum when the tube contained HYDROGEN, the lightest gas. That lightest, indivisible positive particle, produced when a hydrogen atom loses its electron, is the proton, and Indian school chemistry — NCERT Class 9 onwards — teaches the discovery of the proton as the outcome of Goldstein's canal-ray experiment. The experiment also completed the picture of the atom as electrically neutral: the negative particle Thomson had found in the cathode rays had to be balanced by a positive one, and the canal rays supplied it.

Why the others are wrong

• (a)J.J. Thomson — J. J. Thomson discovered the ELECTRON, not the proton. In 1897 he showed that the cathode rays in a discharge tube were streams of negatively charged particles, measured their charge-to-mass ratio, and demonstrated that the ratio was the same whatever gas filled the tube and whatever the electrodes were made of — proving that this particle was a constituent of all matter and that the atom was therefore divisible. He went on to propose the plum-pudding model, in which electrons are embedded in a sphere of positive charge, and he received the Nobel Prize in Physics in 1906. His work is the direct counterpart of the canal-ray story: cathode rays gave the negative particle, anode rays the positive one.
• (c)E. Rutherford — The Commission's key does not take this option. It is worth knowing why it can look right, because Rutherford's claim on the proton is a real one: between 1917 and 1920 he bombarded nitrogen gas with alpha particles, detected hydrogen nuclei ejected from it, recognised that a hydrogen nucleus must be a fundamental building block of all nuclei, and gave it the NAME proton, from the Greek for 'first'. On that basis much of the international record credits Rutherford with the discovery, while the Indian textbook tradition credits Goldstein's earlier canal-ray work; the two attributions describe two different achievements in the same story. Rutherford's other contributions are examined at least as often and are not in dispute: the gold-foil alpha-scattering experiment of 1911 and the nuclear model of the atom that came from it, the classification of alpha and beta radiation, and the Nobel Prize in Chemistry of 1908.
• (d)J. Chadwick — James Chadwick discovered the NEUTRON, in 1932, and that is a straightforwardly different particle and a different date. He showed that a penetrating, highly energetic radiation emitted when beryllium was bombarded with alpha particles consisted of uncharged particles of mass close to that of the proton. The discovery mattered because it explained the gap between atomic number and atomic mass — an atom's mass is proton plus neutron while its charge is proton alone — and made isotopes intelligible as atoms differing only in neutron number. Chadwick received the Nobel Prize in Physics in 1935. The neutron is the last of the three particles a school syllabus deals with, and its discoverer is the standard fourth name in a question of this shape.

The structure of the atom was assembled from a sequence of experiments, and the examinable content is which experiment gave which particle or model. Dalton's atomic theory of the early nineteenth century treated the atom as indivisible. That fell in 1897, when J. J. Thomson identified the ELECTRON in cathode rays and proposed the plum-pudding model. Goldstein's canal-ray or anode-ray experiments of 1886 had already revealed positively charged rays travelling towards the cathode and passing through its perforations, and their charge-to-mass ratio, greatest for hydrogen, pointed to the positive particle later known as the PROTON. Rutherford's gold-foil experiment of 1911 then overturned the plum-pudding picture: most alpha particles passed straight through the foil, a few were deflected and a very few bounced back, showing that the atom's positive charge and nearly all of its mass are concentrated in a tiny central NUCLEUS about a hundred thousand times smaller than the atom, with electrons occupying the space around it. Rutherford's later bombardment of nitrogen with alpha particles, between 1917 and 1920, ejected hydrogen nuclei and gave the proton its name. Chadwick supplied the NEUTRON in 1932, which resolved why atomic mass exceeds the mass accounted for by protons alone and explained isotopes. Bohr's 1913 model, meanwhile, fixed the problem that a classical orbiting electron should radiate energy and spiral into the nucleus, by restricting electrons to discrete energy levels. Held together, that chain answers the whole family of questions a general ability paper asks about the atom.

Key facts

• E. Goldstein discovered canal rays, also called anode rays, in 1886 using a discharge tube with a perforated cathode.
• Canal rays are positively charged and travel in the direction opposite to cathode rays.
• The charge-to-mass ratio of canal rays depends on the gas in the tube and is greatest for hydrogen; the lightest such particle is the proton.
• Indian school chemistry, following the NCERT treatment of the structure of the atom, credits Goldstein with the discovery of the proton.
• Ernest Rutherford named the proton, from the Greek for 'first', after his 1917-1920 experiments ejecting hydrogen nuclei from nitrogen; much of the international record credits him with its discovery.
• J. J. Thomson discovered the electron in 1897 from cathode rays and proposed the plum-pudding model; Nobel Prize in Physics, 1906.
• James Chadwick discovered the neutron in 1932; Nobel Prize in Physics, 1935.
• Rutherford's 1911 gold-foil alpha-scattering experiment established the nuclear model of the atom; Nobel Prize in Chemistry, 1908.
• Niels Bohr's 1913 model introduced discrete energy levels to explain why orbiting electrons do not spiral into the nucleus.
• The proton carries a unit positive charge, the electron an equal negative charge, and the neutron none; proton and neutron have almost the same mass, roughly 1836 times the electron's.

Common traps

• Answering with the scientist who NAMED a particle when asked who discovered it, or the reverse. This item sits exactly on that fault line.
• Crediting Rutherford with the proton because his gold-foil experiment is the more famous piece of work. That experiment discovered the nucleus, not the proton.
• Mixing up Thomson (electron) and Chadwick (neutron), which is easy when all four options are surnames with initials.
• Assuming the international attribution and the Indian textbook attribution must agree. On this question they do not, and the Commission's key follows the textbook.
• Confusing cathode rays, which are electrons, with canal or anode rays, which are positive ions.

• practice — not a real PYQ

  The neutron was discovered in 1932 by

  • (a)J.J. Thomson
  • (b)E. Goldstein
  • (c)J. Chadwick
  • (d)Niels Bohr

  Answer(c) J. Chadwick

• practice — not a real PYQ

  Rutherford's alpha-particle scattering experiment on a thin gold foil led directly to the discovery of

  • (a)the electron
  • (b)the atomic nucleus
  • (c)the neutron
  • (d)discrete electron energy levels

  Answer(b) the atomic nucleus