Explain why the existence of discrete atomic emission spectra provides evidence against the classical model of the atom, in which electrons orbit the nucleus like planets orbit the Sun.

IB DP Physics Higher Level (2023 syllabus) — E.1 Structure of the atom · Explain · 4 marks · View as Markdown

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

When the light emitted by excited hydrogen gas is passed through a diffraction grating, only a few specific wavelengths are observed rather than a continuous range of wavelengths.

Model answer (4 marks)

Classical physics predicts that an accelerating electron in orbit must continuously radiate energy, causing the electron to lose energy, spiral into the nucleus and the atom to emit a continuous spectrum. The observed discrete emission lines show that electrons can only occupy specific, quantised energy levels. When an electron falls from a higher to a lower level it emits a photon whose energy equals the difference between the two levels. Because only particular energy differences exist, only particular photon frequencies (and wavelengths) are produced, giving a line spectrum rather than a continuous one. This contradicts the classical prediction and therefore disproves the planetary‑orbit model.

Examiner tips

  • Use the phrase ‘discrete emission lines’ to show evidence of quantisation.
  • Explain the energy loss prediction of classical physics first.
  • Show the link between energy difference and photon wavelength.
  • Mention the contradiction to the continuous spectrum expectation.

Common mistakes

  • Failing to mention that classical physics predicts continuous radiation.
  • Confusing the emission spectrum with absorption spectrum.
  • Using vague terms like ‘quantum’ without explaining energy levels.

Mark scheme (4 marks)

  1. Classical physics predicts that an accelerating (orbiting) electron continuously radiates energy, so the electron should spiral inward and the atom should emit a continuous spectrum of radiation.
  2. Discrete spectral lines indicate that electrons can only occupy specific, quantised energy levels within the atom.
  3. When an electron transitions from a higher energy level to a lower energy level, it emits a photon whose energy equals the difference between the two levels.
  4. Because only specific energy differences exist between discrete levels, only specific photon frequencies (and therefore wavelengths) are emitted, producing a line spectrum rather than a continuous one, in direct contradiction to classical predictions.

Key terms in this question

classical model

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