In the Geiger-Marsden α-particle scattering experiment, the α-particles directed at the gold foil came from which radioactive source?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Bismuth \( \left({}^{214}_{83}\mathrm{Bi}\right) \)
At Rutherford’s suggestion, Geiger and Marsden directed a beam of 5.5 MeV α-particles from a radioactive bismuth source, \( {}^{214}_{83}\mathrm{Bi} \), at a thin gold foil. Lead bricks made the α-particles into a narrow beam. A zinc sulphide detector, which could be turned around the foil, counted the scattered particles.
NCERT (2026-27):
Alpha-particle Scattering and Rutherford’s Nuclear Model of Atom – “At the suggestion of Ernst Rutherford, in 1911, H. Geiger and E. Marsden performed some experiments. In one of their experiments, … they directed a beam of 5.5 MeV α-particles emitted from a 21483Bi radioactive source at a thin metal foil made of gold.”