E.1.1 Historical models and experimental evidence Notes

The Geiger–Marsden Experiment and the Discovery of the Nucleus

Why Does Matter Behave the Way It Does?

1. Consider you're playing a game of billiards, and one ball suddenly ricochets sharply backward after hitting another, as if it struck a hidden, rock-solid object.
2. This unexpected behavior mirrors what Hans Geiger and Ernest Marsden observed during their groundbreaking experiment under the guidance of Ernest Rutherford.

Their findings not only challenged the prevailing atomic model but also revealed the existence of the nucleus, a discovery that transformed our understanding of matter.

The Geiger–Marsden Experiment: Scattering Alpha Particles

The Setup

1. In 1911, Geiger and Marsden conducted an experiment to investigate the structure of the atom.
2. They directed a beam of alpha particles (positively charged helium nuclei) at an extremely thin sheet of gold foil.
3. Surrounding the foil was a screen coated with zinc sulfide, which produced tiny flashes of light whenever an alpha particle struck it.

Observations

The results of their experiment were astonishing:

1. Most alpha particles passed straight through the foil with little to no deflection.
2. A small number of particles were deflected at large angles, with some even rebounding toward the source.

Rutherford’s Interpretation

1. Rutherford concluded that the large-angle deflections could only occur if the atom’s positive charge was concentrated in a tiny, dense region.
2. He proposed a new model of the atom:
   1. The atom contains a compact, massive, positively charged nucleus at its center.
   2. Electrons orbit this nucleus, similar to how planets orbit the Sun.

This became known as the nuclear model of the atom.

Why the Plum Pudding Model Failed

The plum pudding model failed because it could not account for the large-angle scattering observed in the Geiger–Marsden experiment.

1. Small Deflections: