A scientist is investigating the emission of light from excited hydrogen gas. The gas emits light only at specific frequencies rather than a continuous range of frequencies. Explain why excited hydrogen atoms emit light only at specific frequencies, and what this tells us about the energy levels inside an atom.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
When hydrogen gas is supplied with energy, it emits light at a small number of distinct frequencies. This produces a line spectrum rather than a continuous spectrum.
Model answer (5 marks)
Electrons in a hydrogen atom can only occupy discrete energy levels; they cannot exist at energies between these levels.
When an electron is excited, it jumps to a higher level. When it falls back to a lower level it releases the energy difference as a photon.
The photon energy is E=hf, so the frequency f is determined by the energy gap between the two levels.
Because only certain energy gaps exist, only certain photon energies – and hence frequencies – are produced.
Thus the line spectrum shows that energy inside the atom is quantised and that each element has a fixed set of energy levels.
When an electron is excited, it jumps to a higher level. When it falls back to a lower level it releases the energy difference as a photon.
The photon energy is E=hf, so the frequency f is determined by the energy gap between the two levels.
Because only certain energy gaps exist, only certain photon energies – and hence frequencies – are produced.
Thus the line spectrum shows that energy inside the atom is quantised and that each element has a fixed set of energy levels.
Examiner tips
- Use the phrase "discrete energy levels" to gain the first point. Show the photon energy equation E=hf to link energy difference to frequency. Mention that the spectrum is unique to hydrogen to cover the final point.
Common mistakes
- Confusing continuous with discrete energy levels. Forgetting to state that the photon energy equals the difference between levels. Using the wrong equation (e.g. E=mc²) instead of E=hf.
Mark scheme (5 marks)
- Electrons in an atom can only exist at specific, discrete energy levels (not between them)
- When an electron falls from a higher energy level to a lower energy level, it releases energy as a photon
- The energy of the photon equals the difference between the two energy levels
- Because only certain energy level differences are possible, only certain photon energies (and therefore frequencies) can be emitted
- This shows that energy inside the atom is quantised / energy levels are fixed properties of each element, explaining the unique line spectrum
Key terms in this question
Related
- All AQA A-Level Physics (7408) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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