Explain how the atomic model changed after the discovery of the nucleus, and describe the evidence from the alpha particle scattering experiment that led to this change.

AQA A-Level Chemistry (7405) — 3.1.1 Atomic structure · Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Before 1911, scientists used the plum pudding model to describe the atom. This model was later replaced following experimental evidence collected by Geiger and Marsden.

Model answer (5 marks)

In the plum‑pudding model the atom was a uniformly positive sphere with electrons embedded in it. The alpha‑particle experiment showed that most alpha particles passed straight through the gold foil, indicating that the atom is largely empty space. However, a small fraction were deflected at very large angles or even rebounded. These large‑angle deflections could only be explained if a small, dense region of positive charge and most of the mass – the nucleus – existed at the centre of the atom. Consequently the plum‑pudding model was replaced by the nuclear model, which accounts for the observed scattering.

Examiner tips

  • Use the exact phrases from the mark scheme (e.g. ‘ball of positive charge’, ‘large‑angle deflections’).
  • Explain the two key observations: most particles pass through, few are deflected.
  • Show how the deflections imply a small, dense nucleus.
  • Mention that the plum‑pudding model cannot explain the deflections.

Common mistakes

  • Confusing the plum‑pudding model with the Thomson model; not stating that electrons are embedded in a positive sphere.
  • Failing to mention that most alpha particles pass straight through, only a few are deflected.
  • Using vague terms like ‘small centre’ instead of ‘small, dense nucleus’.

Mark scheme (5 marks)

  1. In the plum pudding model, the atom was described as a ball of positive charge with electrons embedded in it
  2. Most alpha particles passed straight through the gold foil, showing the atom is mostly empty space
  3. A small number of alpha particles were deflected at large angles / some bounced back
  4. The large-angle deflections showed that most of the mass / positive charge is concentrated in a small, dense nucleus
  5. The nuclear model replaced the plum pudding model because the plum pudding model could not explain the large-angle deflections

Key terms in this question

alpha particle scattering · nucleus

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