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.

 Table of contents
- Fine structure constant link
- Hyperfine structure link
  - Hydrogen line link Hyperfine structure

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Fine structure constant ( $α$ )

 toc

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Hyperfine structure

 toc

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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.

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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.

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TODO confirm: does it need quantum electrodynamics or is the Dirac equation enough?

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The most important examples:

hydrogen line useful in astronomy, and also the simplest possible case between 1s
caesium standard, which is used to define the second in the International System of Units since 1967.

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