Lanthanides, term symbols

Atomic and ionic energy levels are characterized by a term symbol of the general form 25+1L/. The values of S, L, and J of lanthanide ions Ln3+ in the ground state can be deduced from the arrangement of the electrons in the 4f subshell, which are determined by Hund s rules and listed in Table 18.1.4. 

Table 1 Valence electron configurations of the lanthanides La-Lu in the oxidation states 0, +11, +III, and +1V and corresponding term symbols ...

Despite the presence of the 6s electrons in addition to the 4f and the Xe core for most lanthanides, the most common oxidation state is -I-III and not - -II, as is seen for the alkaline-earth metals which have a similar configuration. The electronic configurations for this oxidation state, along with the corresponding term symbols, are summarized in Table 1. In the oxidation state -f-III, the configuration is [Xe]4U for all of the ions, with n = 0 for La to n = 14 for Lu . The more common -fill state is justified by the additional stabilization gained in interactions of the triply charged... 

A complete diagram, showing the ground and excited states of all lanthanide ions in the +III oxidation state with corresponding term symbols, is displayed in Fig. 1.2. 

To avoid this confusion, and because many of the elements are actually far from rare, the terms lanthanide , lanthanon and lanthanoid have been introduced. Even now, however, there is no general agreement about the position of La, i.e, whether the group is made up of the elements La to Lu or Ce to Lu. Throughout this chapter the term lanthanide and the general symbol, Ln, will be used to refer to the fourteen elements cerium to lutetium inclusive, the Group 3 elements, scandium, yttrium and lanthanum having already been dealt with in Chapter 20. 

Inspection of Equation 1.23 and consideration of the properties of 3-y and 6-j symbols confirm that only even A--values contribute to crystal field splitting. Further, it indicates that mixing between levels belonging to different / multiplets can only occur if terms with k site symmetry of the lanthanide, in much the same way as discussed above for the Stevens formalism. 

The electronic configurations of the lanthanides are described by using the Russell-Saunders coupling scheme. Values of the quantum numbers S and L corresponding to the lowest energy are derived in the conventional manner. These are then expressed for each ion in the form of a ground term with the symbolism that S, P, D, F,. .. correspond to L = 0, 1,... 

The lanthanide elements comprise lanthanum and the 14 elements (cerium to lutetium) that follow lanthanum in the periodic table. Frequently, the terms lanthanides and lanthanons are used as synonyms for the rare earth elements (REEs), although strictly the REEs comprise the lanthanides and also yttrium, which together with gadolinium, the subject of Chap. 29, is omitted from this chapter. As in many accounts of the chemistry of the lanthanides, will be used as the collective chemical symbol for the trivalent cations. 

The /-block consists of the 4/ metals, La-Lu, and the 5/ metals, Ac-Lr. The common terms lanthanide and actinide derive from the names of the first elements of each series, and the symbol Ln, not assigned to any particular element, is a useful way to designate the lanthanides as a class. The older term for lanthanides, rare earths, is sometimes encountered. The actinides are radioactive, and only Th and U are sufficiently stable to be readily handled outside high-level radiochemical facilities t /2 = 4.5 x 10 years Th, ti/2 = 1.4 x 10 years). Even though they have no / electrons, scandium (Sc) and yttrium (Y) in group 3 are also typically considered with the /-block elements because of their rather similar chemistry. 

According to lUPAC rules, the following nomenclature should be used for the metal ions lanthanides for Ce to Lu, lanthanoids for La to Lu, and rare earths for Y, Sc, and La to Lu. In this contribution, however, we shall loosely use these terms as synonymous and represent them by the symbol R. Moreover, when the notation La-Lu will be used, La-Lu, except Pm will usually be meant. (Promethium is indeed radioactive and very few chemists work with this element.)... 

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