 Solution

For two valence electrons, the central-field shell structure should be the largest scale, residual electrostatic interactions should establish a reasonably definite configuration and atomic term, and spin-orbit coupling should be weaker than the separations between the relevant terms. Then $L$ and $S$ remain approximately good quantum numbers. Within a well-isolated term the leading fine structure interaction can be represented as $A\mathbf{L}\cdot \mathbf{S}$. Its coefficient must be effectively constant for all $J$ levels in that term. Mixing of neighboring terms, spin-spin interactions, higher-order corrections or strong applied fields can spoil the simple interval rule; hyperfine structure must be negligible or separately resolved.

A suitable approximate example is the magnesium $3s3p{}^3{P}_J$ term, with $L=1$, $S=1$ and $J=0,1,2$. The interval-rule prediction isAn arbitrary common term energy may be added. For $A>0$ the levels ascend in the order ${}^3{P}_0,{}^3{P}_1,{}^3{P}_2$, with gap ratio one to two. Each level still contains $2J+1$ degenerate magnetic sublevels in zero external field. The superscript three labels the spin multiplicity $2S+1$, while the different subscripts identify fine structure components of that same term. This is an approximate example of LS coupling, not a claim that every real two-electron atom has exact one-to-two fine splitting.