Explain how the line emission spectrum of an element provides evidence for electrons existing in discrete energy levels within an atom.

AQA A-Level Physics (7408) — 3.2.2 EM Radiation and Quantum Phenomena · Explain · 5 marks · View as Markdown

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

When a gas is heated or an electrical discharge is passed through it, the gas emits light at specific frequencies only. This produces a line emission spectrum unique to that element.

Model answer (5 marks)

Electrons in atoms can only occupy certain fixed energy levels.
When energy is supplied, an electron is excited to a higher level.
The electron then relaxes to a lower level, emitting a photon.
The photon energy equals the difference between the two levels.
Because the levels are discrete, only specific photon energies (frequencies) are emitted, giving a line spectrum.

Examiner tips

Use the exact wording: "fixed energy levels", "excited", "relaxes", "photon", "energy difference".
Show the logical sequence: allowed levels → excitation → emission → energy difference → discrete frequencies.
Keep each point brief but complete; 5 marks allow one sentence per point.

Common mistakes

Confusing continuous with discrete spectra; writing that all frequencies are emitted.
Using vague terms like "possible" instead of "fixed" or "allowed".
Omitting the link between energy difference and photon frequency.

Mark scheme (5 marks)

Electrons in atoms can only occupy certain (discrete/fixed/allowed) energy levels
Electrons absorb energy and move to a higher energy level (are excited / move to a higher level)
When an electron falls back to a lower energy level it emits a photon
The energy of the photon equals the difference in energy between the two levels
Because energy levels are fixed/discrete, only specific photon energies (frequencies) are emitted, producing lines at specific frequencies rather than a continuous spectrum

Key terms in this question

line emission spectrum · discrete energy levels