The quantum numbers n, l, and j are appropriate <4>. A particular level is denoted either by nlj or bynl 2LJ with L = l and
J = j. The latter notation is somewhat redundant for one-electron spectra, but isuseful for consistency with more complex structures. The L values are written with the same letter codeused for l values, but with roman capital letters. The multiplicity of the L term is equal to2S + 1 = 2s + 1 = 2. Written as a superscript, this number expresses the doublet character of thestructure: each term for L 1 has two levels, with J = L ± 1/2, respectively.The Coulomb
interaction between the nucleus and the single electron is dominant, so that the largestenergy
separations are associated with levels having different n. The hyperfine splitting of the 1H 1sground level <1420.405 751 766 7(10) MHz> results from the interaction of the proton and electronmagnetic moments and gives rise to the famous 21 cm line. The separations of the 2n - 1 excited levelshaving the same n are largely determined by relativistic contributions, including the spin-orbit interaction,with the result that each of the n - 1 pairs of levels having the same j value is almost degenerate; theseparation of the two levels in each pair is mainly due to relatively small Lamb shifts.