# Explain why the results of the Geiger–Marsden (gold foil) experiment could not be accounted for by the Thomson 'plum pudding' model, and outline what the experimental observations revealed about the structure of the atom.

> IB DP Chemistry Higher Level (2023 syllabus) — S1.2 The nuclear atom · Explain · 4 marks

## Mark scheme (4 marks)

1. The plum pudding model predicted that alpha particles would pass straight through or be only slightly deflected, because positive charge was assumed to be spread diffusely throughout the atom.
2. A small fraction of alpha particles were deflected through large angles (greater than 90°), including some that bounced almost straight back, which the plum pudding model cannot explain.
3. The large-angle deflections showed that most of the positive charge (and mass) of the atom is concentrated in a very small, dense region called the nucleus.
4. The fact that most alpha particles passed straight through (were undeflected) showed that the atom is mostly empty space, with electrons occupying the region outside the nucleus.

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