Selection rule

<p>In <a href="/facts/Physics/a7aH2y08">physics</a> and <a href="/facts/Chemistry/SrNqkGVm">chemistry</a>, a selection rule, or transition rule, formally constrains the possible transitions of a system from one <a href="/facts/Quantum_state/7jbxd7Dx">quantum state</a> to another. Selection rules have been derived for <a href="/facts/Electromagnetic/oAkmALHT">electromagnetic</a> transitions in <a href="/facts/Molecule/N9ljSfFq">molecules</a>, in <a href="/facts/Atom/FoQ4kGN5">atoms</a>, in <a href="/facts/Atomic_nucleus/orpIm5Xl">atomic nuclei</a>, and so on. The selection rules may differ according to the technique used to observe the transition. The selection rule also plays a role in <a href="/facts/Chemical_reaction/GpqWlKrz">chemical reactions</a>, where some are formally <a href="/facts/Spin-forbidden_reactions/WJVIEu4w">spin-forbidden reactions</a>, that is, reactions where the spin state changes at least once from <a href="/facts/Reagent/c3GLInCu">reactants</a> to <a href="/facts/Product_(chemistry)/8uCoDUdv">products</a>.
</p><p>In the following, mainly atomic and molecular transitions are considered.
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Selection rule open-in-new

Selection rule