Bohr developed a modification of Rutherford's nuclear atomic model. Explain how Bohr's model differed from Rutherford's model, and explain why scientists accepted Bohr's model as an improvement.

OCR GCSE Chemistry A: Gateway Science (J248) — C1.2 Atomic structure · Explain · 4 marks · View as Markdown

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

In the early twentieth century, several scientists proposed different models of the atom. Each new model was developed to better explain experimental evidence that earlier models could not account for.

Model answer (4 marks)

Rutherford’s model described the atom as a small, positively charged nucleus surrounded by electrons that could orbit it, but it did not give any restriction on the electrons’ positions or energies.

Bohr modified this by proposing that electrons occupy only certain fixed orbits (shells) that have specific, quantised energy levels. Electrons can move between these shells only by absorbing or emitting a photon of a precise energy.

Because Bohr’s model could explain the discrete lines in the hydrogen spectrum and the stability of atoms, it resolved problems that Rutherford’s model could not. The quantised orbits matched experimental data, making the model more accurate.

Scientists accepted Bohr’s model because it was supported by new experimental evidence and provided a clearer, more predictive description of atomic behaviour, illustrating how scientific models evolve with better data.

Examiner tips

  • Use the word ‘quantised’ or ‘fixed energy levels’ to show understanding of Bohr’s key idea.
  • Show the link between the model and the hydrogen spectrum – this is a high‑value point.
  • Mention that Rutherford’s model lacked energy restrictions – a clear contrast.
  • Keep the answer concise and avoid unnecessary detail.

Common mistakes

  • Confusing Bohr’s model with the planetary model – remember it is about quantised orbits, not just any orbit.
  • Failing to mention the hydrogen spectrum – this is the main experimental evidence.
  • Using vague phrases like ‘better explanation’ without specifying what was explained (e.g., spectral lines).

Mark scheme (4 marks)

  1. Rutherford's model placed electrons orbiting the nucleus but did not specify fixed positions/energy levels for the electrons.
  2. Bohr proposed that electrons exist in shells (orbits) of fixed energy around the nucleus.
  3. Bohr's model better explained existing experimental observations/evidence that Rutherford's model could not explain.
  4. Scientific models are accepted/updated when new evidence supports them, showing the development of scientific understanding over time.

Key terms in this question

Rutherford · atomic model

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