The Rare-Earth and Actinoid Elements

The fourth choice above, rare earths , historically refers to the oxides rather than the elements, and their literal rarity is quite variable, but rare earth can include scandium and yttrium, which are very similar chemically although not in having a low-lying 4f-electron subshell. Here then we prefer lanthanoid except when including scandium and yttrium as rate earths . 

For the 15 elements lanthanum through lutetium collectively we use the common symbol Ln. Actinium through lawrencium, the 15 actinoids , are likewise represented as An. For the rare-earth elements collectively we propose and use the symbol Rth. This, like other symbols for elements, has just one upper-case letter, unlike R.E., and does not conflict with Re for rhenium. 

These elements show a kind of mini-periodicity [2] of characteristic extreme oxidation states, as seen in Table 3.1. A few of these known oxidation states, which represent exactly empty, half-full or full f subshells of electrons, are nevertheless not stable in water, as will be seen in the descriptions below. Table 3.2 

4fl4 7oYb 71 Lu 72Hf 7sTa 74W 75Re 760s 

oiM 102N0 losLr lOiRf losDb loeSg io70h losHs 

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In this chapter we look at /-block metals and their compounds. There are two series of metals the lanthanoids (the 14 elements that follow lanthanum in the periodic table) and the actinoids (the 14 elements following actinium). The lanthanoids and actinoids (Table 24.1) are collectively known as the inner transition metals, while scandium, yttrium, lanthanum and the lanthanoids are together called the rare earth metals. Although La and Ac are strictly group 3 metals, the chemical similarity of La to the elements... 

The analytical chemistry of the transition elements see Transition Metals), that is, those with partly filled shells of d (see (f Configuration) or f electrons see f-Block Metals), should include that of the first transition period (Sc, Ti, V, Cr, Mn, Fe, Co, Ni, and Cu) and that of the second transition series (Y, Zr, Nb, Mo, Tc, Ru, Rh, Pd, and Ag). The third transition series embraces Hf, Ta, W, Re, Os, Ir, Pt, and An, and although it formally begins with lanthanum, for historical reasons this element is usually included with the lanthanoids (rare-earth elements) see Scandium, Yttrium the Lanthanides Inorganic Coordination Chemistry Rare Earth Elements). The actinoid elements see Actinides Inorganic Coordination Chemistry) are all radioactive see Radioactive Decay) and those with atomic number see Atomic Number) greater than uranium (Z = 92) are artificial the analytical chemistry of these elements is too specialized to consider here. 

Merinis and Boussieres [2,3] pioneered the method of thermochromatography (TC) in application to radiochemistry. With the equipment schematically pictured in Fig. 1.1, they investigated thermochromatographic behavior of some 20 elements, mostly in the form of chlorides. The elements were some alkaline, alkaline earth, rare earth, transition, noble, and actinoid (Th and Pa) metals. The authors experimental technique was based on slow batch chemical volatilization. They obtained data on the shift in position of TC peaks as a function of the experiment duration. 

The actinides, or actinoids, have atomic numbers between 89 and 102 and are named for the first element in that series, actinium. The actinides are often called the transuranium elements and include the three heaviest naturally occurring elements in the periodic table—thorium, protactinium, and uranium. Protactinium is rare, but uranium and thorium are found in significant amounts in the Earths crust. The remaining actinides are synthetic, which means they are produced through artificial means. 

On the other hand, two presumptions are responsible for the late development of organometallic chemistry with lanthanoids and actinoids. One is that rare-earth metals implies sparsity and high expenses as impediments for the use of these metals. However, the element concentrations in the continental crust (Figure 6.1) show that these elements are certainly very seldom compared to iron (abundance 43 200 ppm) yet the rarest, uranium and thulium, are far more common than the precious metals, e.g. silver, platinum, or frequently used transition metals such as palladium, rhodium or iridium. 

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