Split in energy levels due to interaction between electron up or down spin and the electron orbitals.
Numerically explained by the Dirac equation when solving it for the hydrogen atom, and it is one of the main triumphs of the theory.
Small splits present in all levels due to interaction between the electron spin and the nuclear spin if it is present, i.e. the nucleus has an even number of nucleons.
As the name suggests, this energy split is very small, since the influence of the nucleus spin on the electron spin is relatively small compared to other fine structure.
TODO confirm: does it need quantum electrodynamics or is the Dirac equation enough?
The most important examples:
21 cm is very long and very low energy, because he energy split is very small!
Compare it e.g. with the hydrogen 1-2 spectral line which is 121.6 nm!

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Fine structure by Donald Trump 29 Updated +Created
This is a section about Fine structure!
Fine structure is a very important subject about which there is a lot to say.
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Fine structure by Barack Obama 11 Updated +Created
This is a section about Fine structure!
Fine structure is a very important subject about which there is a lot to say.
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Fine structure by Wikipedia Bot 0
Fine structure refers to the small splittings and details observed in the spectral lines of atoms due to the interaction of the electron spin with the orbital angular momentum of the electrons in an atom. This phenomenon arises from the following effects: 1. **Spin-Orbit Coupling**: In an atom, electrons have intrinsic angular momentum (spin) and also orbital angular momentum from their motion around the nucleus.