Scandium Group Elements

Scandium group elements as well as the lanthanides form allyl complexes in which the central atom has H- 3 oxidation state, while the actinides give compounds possessing + 4 oxidation states (Table 7.14). These complexes are formed by reactions of metal halides with allyl compounds of magnesium, lithium, tin, etc. " " ... 

We have not discussed in detail the transition metals of Groups Ilia, IVa, Va, or Vila. The first of these four, the scandium group, contains the rare and invariably tripositive metals, scandium (Z = 21), yttrium (Z = 39), and lanthanum (Z = 57) these elements are best considered with the rare earths and will not be taken up in this text. 

The ground state configuration of lanthanides, 4/" l5d 6s2 or 4/"6s2 does not determine the chemical behaviour of these elements. In forming Ln3+ ions, an / electron is removed. Lanthanides differ from transition elements in that 4/ orbitals are shielded and are not available for reactions. Thus lanthanides resemble more closely alkaline earths and the scandium group than transition elements in their chemistry. 

Scandium Group 3 element, symbol Sc, Z = 21, Ar = 44.9559. Technically, Scandium is a member of the first row transition metal series, but its chemistry is principally that of the Sd+ ion which has no d electrons. 

The rare earths are not really rare in nature. Cerium is reported to be more abundant in the earth s crust than lead and tin, and even the rarer elements, europium and lutetium are much more abundant than the platinum group elements. Except for scandium, these rare earths have never been found in nature as individual rare earths, but wherever they are found, they occur as mixtures of these elements in some combined form. The relative abundance of the individual rare earths can, however, vary considerably in these mixtures, depending on where they are found. In general, the even atomic numbered elements are from three to ten times as abundant as the odd numbered adjacent elements in the lanthanide series, and in the earth s crust, the light (lower atomic number) lanthanides are more abundant than the heavies. 

Meanwhile, as mentioned above, the metallurgist Habashi has proposed a periodic table in which the element aluminum k moved to the top of the scandium group. Moving to chemists, Rayner Canham has published an Inorganic Chemist s Table (see figure 10.12), in which he highlights a number of unusual relationships, several of which have been reviewed in thk chapter. One particularly unusual feature of his table is the inclusion of the ions CN and NH/ because of their similarities to certain elements. 

As Figure 22.1 shows, r/-block (B-group) elements occur in four series that lie within Periods 4 through 7. Each series represents the filling of five d orbitals and, thus, contains ten elements, for a total of 40 transition elements. The first of these series occurs in Period 4 and consists of scandium (Sc) through zinc (Zn) (Figure 22.2). Lying between the first and second members of the series in Periods 6 and 7 are the inner transition elements, in which / orbitals are filled. Two points are important to review ... 

Trends in atomic radii are of concern because chemical properties are determined in part by atomic size. Looking at the fourth-period covalent radii (one measure of atomic size) in Table 23.1, you see that they decrease quickly from scandium (144 pm) to titanium (132 pm) and vanadium (122 pm). This decrease in atomic size across a row is also observed in the main-group elements. It is due to an increase in effective nuclear charge that acts on the outer electrons and pulls them in more strongly. The effective nuclear charge is the positive charge Telf by an electron it equals the nuclear charge minus the... 

Scandium is very widely but thinly distributed and its only rich mineral is the rare thortveitite, Sc2Si20v (p. 348), found in Norway, but since scandium has only small-scale commercial use, and can be obtained as a byproduct in the extraction of other materials, this is not a critical problem. Yttrium and lanthanum are invariably associated with lanthanide elements, the former (Y) with the heavier or Yttrium group lanthanides in minerals such as xenotime, M "P04 and gadolinite, M M SijOio (M = Fe, Be), and the latter (La) with the lighter or cerium group lanthanides in minerals such as monazite, M P04 and bastnaesite, M C03F. This association of similar metals is a reflection of their ionic radii. While La is similar in size to the early lanthanides which immediately follow it in the periodic table, Y , because of the steady fall in ionic radius along the lanthanide series (p. 1234), is more akin to the later lanthanides. 

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. 

The transition elements comprise groups 3 to 12 and are found in the central region of the standard periodic table, an example of which is reproduced on the endpaper. This group is further subdivided into those of the first row (the elements scandium to zinc), the second row (the elements yttrium to cadmium) and the third row (the elements lanthanum to mercury). The term transition arises from the elements supposed transitional positions between the metallic elements of groups 1 and 2 and the predominantly non-metallic elements of groups 13 to 18. Nevertheless, the transition elements are also, and interchangeably, known as the transition metals in view of their typical metallic properties. 

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