Lanthanides lanthanum

As a final introductory point, it should be noted that there is some confusion within the foundry industry, and its literature, regarding the specific rare earths being employed. Early work in this field was conducted using mischmetal. However, in many instances, only the cerium level was reported in these tests. The presence of the other rare earths was ignored. Even today, the elements most often mentioned are the first four lanthanides lanthanum, cerium, praseodymium and neodymium. That is not to say that the effects of the other elements in the series would not be similar to those of the first four or that they could not be utilized. Rather, their roles have not been studied individually. 

The aqueous chemistry of the two rows of f-block elements, the lanthanides (lanthanum to lutetium) and the actinides (actinium to lawrencium), are sufficiently different from each other to be dealt with in separate sections. Similarities between the two sets of elements are described in the actinide section. 

Under the same conditions, in contrast to what is observed for calix[4]arene-bearing CMPO moieties, with CPil2, distribution ratios of lanthanides increase from the lightest lanthanide, lanthanum, to europium. Americium can be easily separated from the lightest lanthanides (separation factor DAm/La > 20, DAm/Ce =15, /lAlll,Nd = 10, UAi /si = 7.5, DAm/Eu = 6), which are the most abundant lanthanides in fission-product solution. Cavitands bearing picolinamide (Cv5) or thiopicolin-amide (Cv6) residues seems much less selective than their calixarene counterparts, giving SAm/Eu < 2.18... 

Other elements may occur in the Earth s crust, but their concentration in (sea) water is negligible due to a very low solubility. Especially trivalent and tetravalent cations form insoluble hydroxides at neutral pH-values, which diminishes the bioavailability of many elements, especially of groups Ilia (Ga, In), IVa (Ge, Sn), Va (Bi), Illb (Sc, Y, La, Ac), IVb (Hf, Ti, Zr) and the lanthanides. Lanthanum was not accumulated into the cytoplasm of bacterial cells (Bayer and Bayer 1991), and this may also be true for the other elements of these groups. 

Many of the enzymes involved in the cleavage of organic phosphorus compounds require metal ions for activation. However many metal ions can also facilitate the hydrolysis of organic phosphorus compounds in the absence of enzymes. As early as 1938 it was shown that lanthanide (lanthanum and cerium) and actinide (thorium) hydroxides could accelerate the hydrolysis of a-glycerol phosphate in alkaline solution (Bamann and Mersenheimer, 1938). A similar study by Butcher and Westheimer (1955) showed that lanthanum hydroxides could accelerate the hydrolysis of three simple phosphate esters (methoxyethyl phosphate, hyrdroxyethyl phosphate and aminoethyl phosphate) by a factor of about 1000. Using... 

For the two common lanthanides lanthanum and neodymium that there are no quality heat capacity measurements available between 20 and 298 K and the selected entropy values at 298 K are therefore little more than educated guesses. Further measurements are also required on alpha cerium between 20 and 96 K in order to obtain a precision value for the entropy at 298 K. What is woiryingly being shown up by these reviews is that for many of these elements, selected values in a particular region are generally based on only one set of measurements since other measurements differ so significantly that they cannot be considered. It is possible that measurements on the pure metals will become fewer and fewer as there is a concentration oti commercially exploiting the many extraordinary properties of lanthanide compounds. 

Reference has been made already to the existence of a set of inner transition elements, following lanthanum, in which the quantum level being filled is neither the outer quantum level nor the penultimate level, but the next inner. These elements, together with yttrium (a transition metal), were called the rare earths , since they occurred in uncommon mixtures of what were believed to be earths or oxides. With the recognition of their special structure, the elements from lanthanum to lutetium were re-named the lanthanons or lanthanides. They resemble one another very closely, so much so that their separation presented a major problem, since all their compounds are very much alike. They exhibit oxidation state -i-3 and show in this state predominantly ionic characteristics—the ions. 

In general, the absorption bands of the actinide ions are some ten times more intense than those of the lanthanide ions. Fluorescence, for example, is observed in the trichlorides of uranium, neptunium, americium, and curium, diluted with lanthanum chloride (15). 

Lanthanides is the name given collectively to the fifteen elements, also called the elements, ranging from lanthanum. La, atomic number 57, to lutetium, Lu, atomic number 71. The rare earths comprise lanthanides, yttrium, Y, atomic number 39, and scandium. Sc, atomic number 21. The most abundant member of the rare earths is cerium, Ce, atomic number 58 (see Ceriumand cerium compounds). 

Cerium ranks ca 25th in abundance in the earth s cmst (3), and cerium, which occurs at 60 ppm cmstal abundance, not lanthanum at 30 ppm, is the most abundant lanthanide. 

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. 

Yttrium and lanthanum are both obtained from lanthanide minerals and the method of extraction depends on the particular mineral involved. Digestions with hydrochloric acid, sulfuric acid, or caustic soda are all used to extract the mixture of metal salts. Prior to the Second World War the separation of these mixtures was effected by fractional crystallizations, sometimes numbered in their thousands. However, during the period 1940-45 the main interest in separating these elements was in order to purify and characterize them more fully. The realization that they are also major constituents of the products of nuclear fission effected a dramatic sharpening of interest in the USA. As a result, ion-exchange techniques were developed and, together with selective complexation and solvent extraction, these have now completely supplanted the older methods of separation (p. 1228). In cases where the free metals are required, reduction of the trifluorides with metallic calcium can be used. 

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. 

Table 30.5 Stoichiometries and structures of reduced halides (X/M < 2) of scandium, yttrium, lanthanum and the lanthanides...

The actinides ( actinons or actinoids ) are the fourteen elements from thorium to lawren-cium inclusive, which follow actinium in the periodic table. They are analogous to the lanthanides and result from the filling of the 5f orbitals, as the lanthanides result from the filling of the 4f. The position of actinium, like that of lanthanum, is somewhat equivocal and, although not itself an actinide, it is often included with them for comparative purposes. 

Lanthanides Elements 57 (La) through 70 (Yb) in the periodic table, 146 Lanthanum, 147 Laser fusion, 528 Lattices in ionic crystals, 249 Lavoisier, Antoine, 14 Law of conservation of energy A natural law stating that energy can neither be created nor destroyed it can only be converted from one form to another, 214... 

The lanthanides, or rare earths, are lanthanum and the fourteen chemical elements following. These fourteen elements, all of which are very similar to La in chemical behavior, include... 

Table 23-1. some properties of lanthanum and the lanthanide elements... 

Lanthanide elements, 411, 389 contraction, 413 electron configurations, 415 occurrence and preparation, 413 oxidation numbers, 414 properties, 412 Lanthanum... 

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