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1.3.2Additional lessons

Emission Spectra — IB Chemistry SL Revision Notes & Topical Questions

5 min read 4 parts Lesson 8 of 98

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

1.3.2 Emission Spectra

Light from different sources produces different spectra.

Continuous
  • White light passes through a prism and gives all colours merging into each other with no gaps.
Line emission
  • Excited gaseous atoms give out only certain wavelengths, so the spectrum is bright lines on a dark background.
Fingerprint
  • Each element has its own set of lines, so a spectrum can be used to identify the element.
Meaning
  • Lines at only some wavelengths show that atoms have only certain allowed energies.

Part 2 of 4How a Line Is Produced

An electron moves between fixed energy levels by gaining or losing a photon.

1
Energy from heat or electricity excites an electron from a lower energy level to a higher one.
2
The excited electron is unstable and soon falls back to a lower level.
3
The energy lost is emitted as one photon, whose energy equals the difference between the two levels.
4
Since ΔE = hν, each size of energy gap corresponds to a particular frequency and hence one line in the spectrum.

Part 3 of 4The Hydrogen Spectrum

Lines are grouped into series by the level the electron falls to.

The Hydrogen Spectrum
Lyman series
  • Transitions to n = 1 release the largest energies and appear in the ultraviolet region.
Balmer series
  • Transitions to n = 2 give the lines in the visible region.
Paschen series
  • Transitions to n = 3 give lines in the infrared region.
Convergence
  • Lines in each series get closer together at higher frequency because the energy levels get closer together at higher energy.

Part 4 of 4Exam Checklist & Tips

1.3.2 – Key Points for the Exam

  • Emission spectrum The set of wavelengths emitted when excited atoms fall to lower energy levels.
  • Energy level A fixed energy that an electron in an atom is allowed to have.
  • Excited state A state in which an electron occupies a higher energy level than normal.
  • Convergence limit The frequency at which the lines of a series merge, corresponding to the electron leaving the atom.

Common Mistakes

  • Saying that electrons emit energy when they move up to a higher level.
  • Drawing the lines of the hydrogen spectrum evenly spaced rather than converging.
  • Thinking an emission line represents one electron emitting every wavelength.
  • Mixing up which series is visiblethe Balmer series is visible and the Lyman series is ultraviolet.

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