Rutherford Scattering

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During a gold-foil alpha scattering demonstration, only a very small fraction of alpha particles are deflected through large angles, which led Rutherford to conclude something about atomic structure.

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Rutherford's experiment, central to NCERT's treatment of the atom, fired alpha particles at a thin gold foil and recorded their deflections on a fluorescent screen. Most alphas passed nearly straight through, showing the atom is mostly empty space, but a rare few rebounded through very large angles. Such strong deflection can only occur if the alpha meets an intense, localised repulsive charge, so Rutherford concluded that all the positive charge and nearly all the mass are packed into a minute central nucleus. The uniform-charge (Thomson) picture is wrong because a spread-out positive cloud could never exert the concentrated force needed for back-scattering. The claim that electrons carry most of the mass is wrong because electrons are far too light to deflect a massive alpha particle. The absorption option is wrong because the alphas were detected after transmission, not absorbed. As stated in NCERT Class 12, Chapter 12 (Atoms), the rarity of large-angle scattering directly reveals the small size of the nucleus. A consistency check: the tiny fraction scattered matches the tiny fractional volume a nucleus occupies within an atom.

Rutherford Scattering

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