Selection Rules
A set of quantum mechanical rules constraining allowed radiative transitions between atomic energy states.
Governing formula
Δl = ±1, Δm_l = 0, ±1
SI unit
Dimensionless Quantum Rules
In depth
Derived from the conservation of angular momentum during photon emission/absorption (since a photon carries intrinsic spin ℏ). Transitions violating selection rules are termed 'forbidden' and occur with vastly smaller probabilities.
Examples in the real world
Permitting an electron transition from 2p to 1s (Δl = -1), but forbidding a direct 2s to 1s dipole transition (Δl = 0).