Describe how the atomic model has changed from the 'plum pudding' model to the nuclear model, and state one piece of experimental evidence that led to this change.

Edexcel GCSE Chemistry (1CH0) — 1.3 Atomic structure · Describe · 4 marks · View as Markdown

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

Scientists in the early twentieth century carried out experiments that changed our understanding of atomic structure. The 'plum pudding' model described atoms as a ball of positive charge with electrons embedded throughout it.

Model answer (4 marks)

In the plum‑pudding model the atom was a sphere of positive charge with electrons embedded throughout it.

In the nuclear model the positive charge (and most of the mass) is concentrated in a tiny central nucleus, and the electrons orbit or surround this nucleus.

The change was driven by Rutherford’s alpha‑particle scattering experiment: most alpha particles passed straight through, but a few were deflected sharply or even rebounded, showing that the atom contains a small, dense, positively charged nucleus.

Examiner tips

  • Use the exact wording from the mark scheme – e.g. ‘sphere of positive charge’, ‘tiny central nucleus’, ‘orbit/surround the nucleus’.
  • Mention the alpha‑particle experiment explicitly and describe the key observation (deflection).
  • Keep the answer concise – 4 points, one sentence each.

Common mistakes

  • Writing that the nucleus contains electrons instead of positive charge.
  • Failing to mention that the nucleus is small, dense and positively charged.
  • Giving a vague or incorrect description of the scattering experiment (e.g. saying all particles were deflected).

Mark scheme (4 marks)

  1. In the plum pudding model, the positive charge is spread throughout the atom (accept: the atom is a ball/sphere of positive charge with electrons embedded in it)
  2. In the nuclear model, the positive charge (and most of the mass) is concentrated in a small central nucleus
  3. In the nuclear model, electrons orbit/surround the nucleus (and most of the atom is empty space)
  4. Reference to the alpha particle scattering experiment as evidence (e.g. most alpha particles passed straight through but some were deflected/bounced back, showing the nucleus is small, dense and positively charged)

Key terms in this question

nuclear model

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