# Explain why electrons in an atom can only occupy discrete energy levels rather than a continuous range of energies, and outline one piece of experimental evidence that supports this conclusion.

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

## Mark scheme (4 marks)

1. Electrons in atoms exist in stationary states (allowed orbits/energy levels) in which they do not radiate energy.
2. Only specific values of electron energy are permitted because the de Broglie wavelength of the electron must form a standing wave that fits exactly around the orbit (i.e. the angular momentum is quantised / nλ = 2πr).
3. Identification of a valid experimental evidence: discrete line (emission or absorption) spectra of elements show that photons of only specific frequencies/energies are emitted or absorbed.
4. Explicit link between evidence and discrete levels: each spectral line corresponds to a photon of energy E = hf emitted (or absorbed) when an electron transitions between two specific energy levels, so the existence of discrete lines confirms the energy levels themselves are discrete.

## Key terms

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

## Related

- [Revision notes for IB DP Physics Higher Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-why-electrons-in-an-atom-08ecd052) · Published by Druglandscape Ltd.