Question

Describe briefly the Geiger-Marsden scattering experiment. Depict the graph showing the variation of the number of scattered particles detected with the scattering angle. How did this graph lead to the discovery of the nucleus?

Hint:

In the Geiger-Marsden experiment, most of the alpha particles passed through the foil, but a few were scattered at large angles. This helped Rutherford conclude that the atom has a small, dense, positively charged nucleus at its center.

Answer 1

Geiger-Marsden Scattering Experiment: The Geiger-Marsden experiment, conducted in 1909 by Hans Geiger and Ernest Marsden under the guidance of Ernest Rutherford, involved the scattering of alpha particles by a thin gold foil. The primary objective of the experiment was to investigate the structure of the atom by studying how alpha particles interact with matter. In the experiment: - A beam of alpha particles (helium nuclei) was directed at a thin gold foil. - A detector surrounded the foil to detect the scattered alpha particles at various angles. - The alpha particles were expected to pass through the gold foil with little deflection if the atom was composed of uniform matter. However, what they observed was surprising: - While most of the alpha particles passed through the foil with only slight deflections, some alpha particles were deflected by large angles, and some even bounced back at angles close to 180°. This observation led to the conclusion that the atom is not made of uniformly distributed matter, but rather has a small, dense, positively charged core at its center. This core was later identified as the nucleus, which contains most of the atom’s mass. Graph Showing Variation of Number of Scattered Particles with Scattering Angle: The graph showing the number of scattered particles as a function of scattering angle typically has the number of particles (N) on the y-axis and the scattering angle (\( \theta \)) on the x-axis. The graph is generally bell-shaped, with most of the alpha particles being scattered at small angles (close to 0°), while fewer particles are scattered at large angles. The number of particles scattered at very large angles (e.g., 180°) is minimal.

The sharp increase in the number of particles scattered at small angles and the small number of particles scattered at large angles led to a significant breakthrough in understanding atomic structure. Discovery of the Nucleus: The results of the Geiger-Marsden experiment led Rutherford to propose that the atom has a small, dense, positively charged nucleus at its center. The fact that a small fraction of alpha particles were scattered at large angles suggested that there was a very small, dense, positively charged center within the atom (the nucleus) that repelled the positively charged alpha particles. This was in stark contrast to the previously accepted plum pudding model, where the positive charge was assumed to be spread out evenly throughout the atom. Thus, the Geiger-Marsden experiment, combined with Rutherford’s analysis, directly contributed to the discovery of the atomic nucleus and the development of the nuclear model of the atom.