Emission and absorption spectra show that atoms have discrete energy levels, rather than a continuous range of possible electron energies. Each spectral line corresponds to a photon emitted or absorbed during a transition between two allowed levels.
The Reasoning and Mechanism
An electron in an atom can occupy only certain allowed energy states. When it moves between levels, the photon energy must equal the difference between them:
Here, is the energy-level difference, is the Planck constant, is the photon frequency, and is its wavelength.
| Spectrum | Atomic process | Appearance and meaning |
|---|---|---|
| Emission spectrum | An electron moves from a higher energy level to a lower level and emits a photon. | Bright lines occur at wavelengths corresponding to the allowed energy differences. |
| Absorption spectrum | An electron absorbs a photon and moves from a lower energy level to a higher level. | Dark lines occur where photons of the required energies have been removed from a continuous spectrum. |
For example, if two levels differ by , only a photon with energy can cause that transition. Photons with other energies are not absorbed for that particular transition.
The emission and absorption lines of the same element occur at matching characteristic wavelengths because they involve the same energy-level differences. Consequently, each element has a distinctive spectral pattern that can be used to identify its chemical composition, including in stars and gases.
A common misconception is that the lines represent the energies of the levels themselves. They actually represent the differences between energy levels.
IB Exam Technique
This is SL and HL content in E.1 Structure of the atom. In an Explain question, link discrete levels, electron transitions, and ; then distinguish emission from absorption and state that line patterns can identify elements.
