Describe how the nuclear model of the atom developed from the plum pudding model, following the results of the alpha particle scattering experiment.

Edexcel A-Level Chemistry (9CH0) — 1.1 Atomic structure · Describe · 5 marks · View as Markdown

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

In 1909, Geiger and Marsden fired positively charged alpha particles at a very thin sheet of gold foil and detected where the particles went after hitting the foil. Most passed straight through, some were deflected by small angles, and a very small number bounced almost directly back.

Model answer (5 marks)

The plum pudding model described the atom as a sphere of positive charge with electrons embedded throughout it. It predicted that alpha particles would pass straight through or be only slightly deflected, so the large‑angle deflections and back‑scattering observed were unexpected. The experiment showed that most alpha particles passed straight through, indicating that most of the atom is empty space. The large‑angle deflections and back‑scattering were explained by the presence of a small, dense, positively charged nucleus. In the nuclear model electrons orbit the nucleus at a distance outside the nucleus.

Examiner tips

  • Use the exact wording from the mark scheme – e.g. "plum pudding model" and "nuclear model".
  • Explain the unexpected result before describing the new model.
  • Show the logical progression: prediction → observation → explanation.
  • Include the key features: empty space, dense nucleus, electrons orbiting outside the nucleus.

Common mistakes

  • Confusing the plum pudding model with the Rutherford model; using the wrong model name.
  • Omitting the explanation that most alpha particles passed straight through because the atom is mostly empty space.
  • Failing to mention that the nucleus is small, dense and positively charged.

Mark scheme (5 marks)

  1. The plum pudding model described the atom as a ball of positive charge with electrons embedded throughout it (like plums in a pudding)
  2. The plum pudding model predicted all alpha particles should pass straight through or be only slightly deflected, so the large-angle deflections / back-scattering were unexpected
  3. Most alpha particles passed straight through because most of the atom is empty space
  4. A small, dense, positively charged nucleus was proposed to explain the large-angle deflections / back-scattering
  5. In the nuclear model electrons orbit the nucleus at a distance / outside the nucleus

Key terms in this question

plum pudding model · alpha particle scattering · nuclear model

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