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1.3.2Additional lessons

Emission Spectra — IB Chemistry HL Revision Notes & Topical Questions

5 min read 4 parts Lesson 9 of 172

📚 IB Chemistry HL Hub / Topical Revision
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Part 1 of 4Continuous and Line Spectra

1.3.2 Emission Spectra

The light from a hot source can show two different types of spectrum.

Continuous
  • White light split by a prism shows a continuous band of colours with every wavelength present.
Line emission
  • Excited atoms of an element emit light only at specific wavelengths, giving bright lines on a dark background.
Fingerprint
  • Each element has its own pattern of lines, which allows it to be identified.
Meaning
  • Only certain energies are possible in the atom, so energy levels are quantised.

Part 2 of 4Origin of the Lines

The hydrogen atom spectrum can be explained in terms of electron transitions.

1
Energy supplied by an electric discharge promotes an electron from a low level to a higher energy level.
2
The excited state is unstable, so the electron falls back to a lower level.
3
The energy difference between the two levels is released as one photon, with ΔE = hν.
4
Because the levels have fixed energies, only photons of certain frequencies are emitted, giving distinct lines.
5
The lines get closer together towards higher frequency, since energy levels converge as n increases.

Part 3 of 4Hydrogen Series

Hydrogen Series
Lyman series
  • Transitions ending at n = 1 give lines in the ultraviolet region, as these have the largest energy gaps.
Balmer series
  • Transitions ending at n = 2 give lines in the visible region.
Paschen series
  • Transitions ending at n = 3 give lines in the infrared region.
Convergence
  • In each series the lines converge at high frequency, where the electron leaves the atom.
SeriesEnds atRegion
Lymann = 1Ultraviolet
Balmern = 2Visible
Paschenn = 3Infrared

Part 4 of 4Exam Checklist & Tips

1.3.2 – Key Points for the Exam

  • Emission spectrum The set of frequencies of radiation emitted by excited atoms.
  • Energy level A fixed energy that an electron in an atom can have.
  • Excited state A state with an electron in a higher energy level than normal.
  • Convergence The lines in a series getting closer together at high frequency.

Common Mistakes

  • Saying that the electron emits light when it moves up to a higher level.
  • Thinking that line spectra show every colour of the rainbow.
  • Stating that bigger electron jumps give longer wavelengths.
  • Describing the lines as having equal spacing.

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