Inner transition element origin

Moving across each period the f orbital is progressively filled the 4f orbital is filled for the lanthanides, and the 5f orbital is filled for the actinides. These elements are sometimes referred to as the rare earths, because it was originally difficult to separate and identify these elements. Rare earths are actually not scarce, but the term rare earths is still used for the lanthanides and actinides. A more accurate modern term for these two periods are the inner transition elements. 

Fig. 14.1). Only certain electronic transitions are permitted by quantum-mechanical selection rules, which are described in various text books on atomic physics. The x-ray spectral lines are designated by symbols such as Ni K i, Fe K 02. Sn Laa, and U Mcci. The symbol of an x-ray line represents the chemical element (Ni, Fe, Sn, and U) the notations K, L, or M indicate that the lines originate by the initial removal of an electron from the K, L, or M shell, respectively a particular line in the series is designated by the Greek letter a, j8, etc. (representing the subshell of the outer electron involved in the transition), plus a numerical subscript. This numerical subscript indicates the relative strength of each line in a particular series—for example, K i is more intense than Kota. Because there are a limited number of possible inner-shell transitions, the x-ray spectrum is much simpler than the complex optical spectrum that results from the removal or transition of valence electrons in addition. 

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