Scandium subgroup

Shkolnikov, E.V. (2009) Thermodynamic characterisation of the amphoterism of oxides and hydroxides of scandium subgroup elements in aqueous media. Zh. PrikL Khim., 82, 1957-1962. 

The strangest section of the periodic chart comes in the first transition subgroup. Under scandium and yttrium (marked with stars on the periodic chart) fall two long horizontal lists of elements so much alike that they are squeezed into two squares of the chart. Elements with the atomic numbers 57-71 are called the lanthanides. The actinides are elements with the atomic numbers 89-103, and they are all radioactive. These transition elements are as follows ... 

Lanthanide elements (referred to as Ln) have atomic numbers that range from 57 to 71. They are lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), promethium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), and lutetium (Lu). With the inclusion of scandium (Sc) and yttrium (Y), which are in the same subgroup, this total of 17 elements are referred to as the rare earth elements (RE). They are similar in some aspects but very different in many others. Based on the electronic configuration of the rare earth elements, in this chapter we will discuss the lanthanide contraction phenomenon and the consequential effects on the chemical and physical properties of these elements. The coordination chemistry of lanthanide complexes containing small inorganic ligands is also briefly introduced here [1-5]. 

The second subgroup. Sc, Y, Sm, Gd, Tb, Dy, Ho, Er, Tm and Lu, also forms fluorite-type dihydrides, but their trihydrides are hexagonal and the dihydride undergoes a phase change on formation of the trihydride. Hexagonal scandium trihydride, ScH 5, can be prepared only at > 30 MPa H,. 

Compounds of equiatomic composition with rare-earth metals of the yttric and cerium subgroups crystallize with the ZrNiAl type whereas scandium germanide of equiatomic composition forms the MgAgAs type. 

Considering the crystal structures of the compounds it is possible to conclude that the presence of scandium leads to the formation of various kinds of superstructures to the structure types of the binary compounds (Sm5Gc4, Ca2As). RScGe ternary germanides are isostructural and are formed with the elements of the cerium subgroup (La, Ce, Pr, Nd, Sm, Eu), i.e. at the ratio r /rsc 1.1 for R=Y, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu the compounds of this composition are not observed. 

Only a httle information exists about the interaction of scandium with the elements of seletuum subgroup (Se, Te and Po). No phase diagrams are available for any of these binary systems. 

The complete group of rare earth metals consists of the three elements scandium Sc, yttrium Y and lanthanum La and the fourteen lanthanides cerium to lutetium. The total number of REMs is thus 17. They are, for practical reasons, divided into two subgroups, the cerium group with the lighter elements and the yttrium group with the heavier ones (Table 17.1). 

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