= Term symbol
{title2=$^{2}S_{1/2}$}
{title2=$^{2}P_{1/2}$}
{title2=$^{2}P_{3/2}$}
{wiki}
This notation is so confusing! People often don't manage to explain the intuition behind it, why this is an useful notation. When you see Indian university entry exam level memorization classes about this, it makes you want to cry.
The key reason why term symbols matter are <Hund's rules>, which allow us to predict with some accuracy which <electron configurations> of those states has more energy than the other.
https://web.chem.ucsb.edu/~devries/chem218/Term%20symbols.pdf puts it well: <electron configuration notation> is not specific enough, as each such notation e.g. 1s2 2s2 2p2 contains several options of spins and z angular momentum. And those affect energy.
This is why those symbols are often used when talking about energy differences: they specify more precisely which levels you are talking about.
Basically, each term symbol appears to represent a group of possible electron configurations with a given <quantum angular momentum>.
We first fix the energy level by saying at which orbital each electron can be (<hyperfine structure> is ignored). It doesn't even have to be the ground state: we can make some electrons excited at will.
The best thing to learn this is likely to draw out all the possible configurations explicitly, and then understand what is the term symbol for each possible configuration, see e.g. <term symbols for carbon ground state>.
It also confusing how uppercase letters S, P and D are used, when they do not refer to orbitals s, p and d, but rather to states which have the same angular momentum as individual electrons in those states.
It is also very confusing how extremelly close it looks to <spectroscopic notation>!
The form of the term symbol is:
$$
^{2S + 1}L_J
$$
The $2S + 1$ can be understood directly as the degeneracy, how many configurations we have in that state.
\Video[https://www.youtube.com/watch?v=dhARbw8cdDE]
{title=Atomic Term Symbols by TMP Chem (2015)}
\Video[https://www.youtube.com/watch?v=DAgEmLWpYjs]
{title=Atomic Term Symbols by T. Daniel Crawford (2016)}
{description=
* https://youtu.be/DAgEmLWpYjs?t=2675 <term symbols for carbon ground state>
}
Bibliography:
* https://chem.libretexts.org/Bookshelves/Physical_and_Theoretical_Chemistry_Textbook_Maps/Supplemental_Modules_(Physical_and_Theoretical_Chemistry)/Spectroscopy/Electronic_Spectroscopy/Spin-orbit_Coupling/Atomic_Term_Symbols
* https://chem.libretexts.org/Courses/Pacific_Union_College/Quantum_Chemistry/08%3A_Multielectron_Atoms/8.08%3A_Term_Symbols_Gives_a_Detailed_Description_of_an_Electron_Configuration The PDF origin: https://web.chem.ucsb.edu/~devries/chem218/Term%20symbols.pdf
* https://chem.libretexts.org/Bookshelves/Inorganic_Chemistry/Inorganic_Coordination_Chemistry_(Landskron)/08%3A_Coordination_Chemistry_III_-_Electronic_Spectra/8.01%3A_Quantum_Numbers_of_Multielectron_Atoms
* https://physics.stackexchange.com/questions/8567/how-do-electron-configuration-microstates-map-to-term-symbols How do electron configuration microstates map to term symbols?