Editing Angular momentum coupling (section)

== Term symbols ==
{{Main article|Term symbol}}

Term symbols are used to represent the states and spectral transitions of atoms, they are found from  coupling of angular momenta mentioned above. When the state of an atom has been specified with a term symbol, the allowed transitions can be found through [[selection rule]]s by considering which transitions would conserve [[angular momentum]]. A [[photon]] has spin 1, and when there is a transition with emission or absorption of a photon the atom will need to change state to conserve angular momentum. The term symbol selection rules are: {{math|Δ''S''}}&nbsp;=&nbsp;0; {{math|Δ''L''}}&nbsp;=&nbsp;0,&nbsp;±1; {{math|Δ''l''}}&nbsp;=&nbsp;±&nbsp;1; {{math|Δ''J''}}&nbsp;=&nbsp;0,&nbsp;±1&nbsp;.

The expression "term symbol" is derived from the "term series" associated with the [[Rydberg state]]s of an atom and their [[Atomic energy level#Orbital state energy level|energy levels]]. In the [[Rydberg formula]] the frequency or wave number of the light emitted by a hydrogen-like atom is proportional to the difference between the two terms of a transition. The series known to early [[Atomic spectral line|spectroscopy]] were designated ''sharp'', ''principal'', ''diffuse'', and ''fundamental'' and consequently the letters {{math|S, P, D,}} and {{math|F}} were used to represent the orbital angular momentum states of an atom.<ref>{{cite book |last=Herzberg |first=Gerhard |title=Atomic Spectra and Atomic Structure |url=https://archive.org/details/atomicspectraato00herz_877 |url-access=limited |publisher=Dover |location=New York |year=1945 |isbn=0-486-60115-3 |pages=[https://archive.org/details/atomicspectraato00herz_877/page/n73 54]–55}}</ref>