Editing Selection rule (section)

{{Short description|Formal constraint on the possible transitions of a system from one quantum state to another}}
In [[physics]] and [[chemistry]], a '''selection rule''', or '''transition rule''', formally constrains the possible transitions of a system from one [[quantum state]] to another. Selection rules have been derived for [[electromagnetic]] transitions in [[molecule]]s, in [[atom]]s, in [[atomic nucleus|atomic nuclei]], 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 [[chemical reaction]]s, where some are formally [[spin-forbidden reactions]], that is, reactions where the spin state changes at least once from [[Reagent|reactants]] to [[Product (chemistry)|products]].

In the following, mainly atomic and molecular transitions are considered.