Atoms with Multiple Valence Electrons
The spectra of atoms having only one valence electron, considered in the previous chapter, are relatively simple and constitute the only examples where the energy levels can be determined by the solution of the one-dimensional Schrödinger equation.
When we consider atoms with more valence electrons, the treatment becomes considerably more complex, and it is necessary to resort to a number of approximations to make the problem mathematically tractable. This chapter is devoted to introducing the physical basis of these approximations as well as the related concepts which form the basis for the complex terminology commonly used in spectroscopy (configurations, terms, multiplets, multiplicity, quantum numbers, etc.).