# 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

> 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.

## 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

- [classical model](https://www.gradenine.co.uk/glossary/classical-model)

## 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)
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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-why-the-existence-of-discrete-e7b19061) · Published by Druglandscape Ltd.