Explain how scientists' model of the atom changed as a result of the alpha particle scattering experiment. Your answer should include what the earlier model suggested and what the new evidence showed.

Eduqas A-Level Physics — 1.7 Particles and nuclear structure · Explain · 5 marks · View as Markdown

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

In a famous experiment, a beam of alpha particles was fired at a thin gold foil. Most passed straight through, but a small number were deflected at large angles, and a very small number bounced almost straight back.

Model answer (5 marks)

The earlier plum‑pudding model proposed that the atom was a diffuse sphere of positive charge with electrons embedded throughout. The scattering experiment showed that most alpha particles passed straight through, indicating that the atom is largely empty space. However, a small fraction were deflected at large angles and some even bounced back, revealing a concentrated positive charge at the centre. This demonstrated that the nucleus is very small compared with the overall size of the atom. Consequently, the new nuclear model places the mass and positive charge in a tiny central nucleus, with electrons orbiting around it.

Examiner tips

  • Use the word ‘plum‑pudding’ for the earlier model and ‘nuclear’ for the new model. Mention the key observations: most particles pass through, few deflected, few back‑scattered. Show the size comparison: nucleus is tiny compared with atom. Keep the answer concise and directly address each point in the mark scheme.

Common mistakes

  • Confusing the terms ‘plum‑pudding’ and ‘nuclear’ or using the wrong model name. Failing to mention that the nucleus is small compared to the atom. Over‑expanding the answer with unnecessary details or repeating the question.

Mark scheme (5 marks)

  1. The earlier model (plum pudding model) suggested the atom was a ball of positive charge with electrons embedded throughout it
  2. Most alpha particles passed straight through, showing the atom is mostly empty space
  3. A small number of alpha particles were deflected at large angles / bounced back, showing a concentrated positive charge at the centre
  4. The nucleus is very small compared to the overall size of the atom
  5. The new model (nuclear model) placed the mass and positive charge concentrated in a small central nucleus with electrons orbiting around it

Key terms in this question

alpha particle

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