a photon the atom will need to change state to conserve angular momentum. The term symbol selection rules are: = 0; = 0, ±1; = ± 1; = 0, ±1 . The expression "term symbol" is derived from the "term series" associated with the Rydberg states of an atom and their 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 spectroscopy were designated sharp, principal, diffuse, and fundamental and consequently the letters and were used to represent the orbital angular momentum states of an atom. Relativistic effects In very heavy atoms, relativistic shifting of the energies of the electron energy levels accentuates spin–orbit coupling effect. Thus, for example, uranium molecular orbital diagrams must directly incorporate relativistic symbols when considering interactions with other atoms. Nuclear coupling In atomic nuclei, the spin–orbit interaction is much stronger than for atomic electrons, and is incorporated directly into the nuclear shell model. In addition, unlike atomic–electron term symbols, the lowest energy state is not , but rather, . All nuclear levels whose value (orbital angular momentum) is greater than zero are thus split in the shell model to create states designated by and . Due to the nature of the shell model, which assumes an average potential rather than a central Coulombic potential, the nucleons that go into the and nuclear states are considered degenerate within each orbital (e.g. The 2 contains four nucleons, all of the same energy. Higher in energy is the 2 which contains two equal-energy nucleons). See also Clebsch–Gordan coefficients Angular momentum diagrams (quantum mechanics) Spherical basis Notes External links LS and jj coupling Term symbol Web calculator of spin couplings: shell model, atomic term symbol Angular momentum Atomic physics Rotational symmetry ar:ترابط مغزلي مداري it:Interazione spin-orbita