Which experimental discovery contradicted Thomson’s model of the atom and led to its rejection?
Alpha particle scattering showing deflection at large angles
J. J. Thomson's "plum pudding" model pictured the atom as a sphere of uniformly spread positive charge with tiny negative electrons embedded in it like plums in a pudding. If this were true, the positive charge and mass would be smeared evenly throughout the atom, so any fast-moving positively charged particle fired at it should experience only small, gentle deflections and pass through almost undisturbed.
The discovery that overturned this picture was the alpha-particle scattering shown by large-angle deflection, observed in the Geiger–Marsden (Rutherford) gold foil experiment. When a beam of alpha particles was directed at a very thin gold foil:
- Most alpha particles passed straight through, indicating the atom is largely empty space.
- A small number were deflected through large angles, and a very few even bounced almost straight back.
A uniform, diffuse positive charge could never produce such strong back-scattering. The only way to explain it was that the atom's positive charge and nearly all of its mass are concentrated in an extremely small, dense central region — the nucleus — which can repel and sharply deflect an approaching alpha particle. This directly contradicted Thomson's model and led Rutherford to propose the nuclear model of the atom.
The other options are real discoveries but do not refute Thomson's model. X-ray diffraction measuring atomic radius concerns spacing in crystals, not the internal charge distribution. Bohr's observation of hydrogen line spectra refined Rutherford's model by explaining electron energy levels — it came later and addressed a different problem. The discovery of the neutron by Chadwick (1932) came long after Thomson's model had already been rejected and dealt with the composition of the nucleus, not with disproving the plum-pudding idea.
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