Explain why the line emission spectrum of hydrogen consists of series of discrete lines at specific frequencies, and why these lines are grouped into distinct series such as the Lyman and Balmer series.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Electrons in a hydrogen atom can occupy only quantised energy levels, not a continuum. When an electron falls from a higher level to a lower one it emits a photon whose energy equals the difference between the two levels, so the frequency is
f=ΔE/h
Only certain ΔE values are possible, giving only specific frequencies – hence discrete spectral lines. Each series is made up of all transitions that terminate at the same lower level. The Lyman series ends at n=1 (UV region) and the Balmer series ends at n=2 (visible region), so the lines appear in different parts of the spectrum.
f=ΔE/h
Only certain ΔE values are possible, giving only specific frequencies – hence discrete spectral lines. Each series is made up of all transitions that terminate at the same lower level. The Lyman series ends at n=1 (UV region) and the Balmer series ends at n=2 (visible region), so the lines appear in different parts of the spectrum.
Examiner tips
- Use the phrase "quantised energy levels" to show understanding of the first point.
- Show the equation f=ΔE/h or ΔE=hf to link energy difference to frequency.
- Explain that a series is defined by a common lower level and give the Lyman and Balmer examples.
Mark scheme (4 marks)
- Electrons in a hydrogen atom exist only in specific, quantised energy levels (shells/orbitals), not in a continuum of energies.
- When an electron falls from a higher energy level to a lower energy level, it emits a photon of electromagnetic radiation whose frequency (or energy) corresponds exactly to the difference in energy between those two levels.
- Because only specific energy level differences are possible, only specific frequencies of light are emitted, producing discrete lines rather than a continuous spectrum.
- Each series corresponds to all electron transitions that end (terminate) at the same lower energy level; the Lyman series involves transitions to n = 1 (ultraviolet region) and the Balmer series involves transitions to n = 2 (visible region), so different series appear in different regions of the electromagnetic spectrum.
Key terms in this question
line emission spectrum · Balmer series
Related
- All IB DP Chemistry Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
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