# Explain why the line emission spectrum of hydrogen provides evidence for the existence of discrete electron energy levels within the atom.

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

> When a low-pressure sample of hydrogen gas is excited by an electrical discharge, light is emitted that, when passed through a diffraction grating, produces a series of sharp, coloured lines at specific wavelengths rather than a continuous band of colour.

## Mark scheme (4 marks)

1. Electrons in the atom exist only in discrete (quantised) energy levels, so they can only possess certain specific energies.
2. When an electron transitions from a higher energy level to a lower energy level, a photon is emitted with energy equal to the difference between the two levels.
3. Because the energy differences between levels are fixed/specific values, only photons of specific frequencies (or wavelengths) are emitted, since E = hf.
4. These specific frequencies correspond to the observed sharp spectral lines; a continuous spectrum would result if electrons could possess any energy value, so the line spectrum is direct evidence for quantisation.

## Key terms

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

## Related

- [Revision notes for IB DP Physics Standard 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)

---
Source: [GradeNine](https://www.gradenine.co.uk/q/explain-why-the-line-emission-spectrum-6bb2a1ae) · Published by Druglandscape Ltd.