# 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

> 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.

## Mark scheme (5 marks)

1. Electrons in atoms can only occupy certain (discrete/fixed/allowed) energy levels
2. Electrons absorb energy and move to a higher energy level (are excited / move to a higher level)
3. When an electron falls back to a lower energy level it emits a photon
4. The energy of the photon equals the difference in energy between the two levels
5. 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

- [line emission spectrum](https://www.gradenine.co.uk/glossary/line-emission-spectrum)
- [discrete energy levels](https://www.gradenine.co.uk/glossary/discrete-energy-levels)

## Related

- [Revision notes for AQA A-Level Physics (7408)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-how-the-line-emission-spectrum-f7b5f79c) · Published by Druglandscape Ltd.