Rare earth elements oxidation state

Halides of rare earth elements in a lower state of oxidation. G. I. Novikov and O. G. Polyachenok, Russ. Chem. Rev. (Engl. Transl.), 1964, 33,342-350 (86). 

Almost all of the rare-earth metal/rare-earth metal tri-iodide systems, R/RI3, contain binary phases with the rare-earth element in an oxidation state lower than -1-3 ( reduced rare-earth metal iodides) [3, 7, 10-13]. More common is the oxidation state -i-2. Elements that form di-iodides RI2 are illustrated in Fig. 4.1. 

The morning session was devoted to a general explanation of the areas of application in studying magnetic properties, oxidation states, compounds, and metal structure. In the afternoon, reviews of the Mossbauer investigations of iron, tin, iodine, tellurium, and some of the rare earth elements were presented. The meeting concluded with a discussion on the future of Mossbauer Spectroscopy in which an interested audience participated. 

The ores from which rare-earth elements are extracted are monazite, bastnasite, and oxides of yttrium and related fluorocarbonate minerals. These ores are found in South Africa, Australia, South America, India, and in the United States in Cahfomia, Florida, and the Carolinas. Several of the rare-earth elements are also produced as fission by-products during the decay of the radioactive elements uranium and plutonium. The elements of the lanthanide series that have an even atomic number are much more abundant than are those of the series that have an odd atomic number. 

Symbol Gd atomic number 64 atomic weight 157.25 a lanthanide series rare earth element electron configuration 4/ 5di6s2 partially filled / orbital common oxidation state -i-3 six stable natural isotopes Gd-152 (0.2%), Gd-154 (2.86%), Gd-155 (15.61%, Gd-156 (20.59%), Gd-157 (16.42%), Gd-157 (23.45%)... 

Indicate the position of the rare-earth elements in Mendeleev s periodic table, the electron configurations and sizes of their atoms, and their oxidation states. 

The solubility product of Pu(OH)3, Ksp = [Pu3+ ] [OH- ]3, has been estimated as 2 X 10-29 (108). The hydrolysis and complexation behavior of Pu(III) is similar to that of the rare-earth elements in the +3 oxidation state. This similarity is the result of the similarity in electronic structure, ionic radii, and electrical charge. 

Paris by the French scientist Paul-Emile Lecoq de Boisbaudran. Its isolation was made possible by the development of ion-exchange separation in the 1950s. Dysprosium belongs to a series of elements called rare earths, lanthanides, or 4f elements. The occurrence of dysprosium is low 4.5 ppm (parts per million), that is, 4.5 grams per metric ton in Earth s crust, and 2 x 10 7 ppm in seawater. Two minerals that contain many of the rare earth elements (including dysprosium) are commercially important mon-azite (found in Australia, Brazil, India, Malaysia, and South Africa) and bast-nasite (found in China and the United States). As a metal, dysprosium is reactive and yields easily oxides or salts of its triply oxidized form (Dy3+ ion). 

The crystallographic ionic radii of the rare-earth elements in oxidation states +2 (CN = 6), +3 (CN = 6), and +4 (CN = 6) are presented in Table 18.1.3. The data provide a set of conventional size parameters for the calculation of hydration energies. It should be noted that in most lanthanide(III) complexes the Ln3+ center is surrounded by eight or more ligands, and that in aqueous solution the primary coordination sphere has eight and nine aqua ligands for light and heavy Ln3+ ions, respectively. The crystal radii of Ln3+ ions with CN = 8 are listed in Table 18.1.1. 

Table 18.1.3. Crystallographic ionic radii (pm) and hydration entropies (kJ mol 1) of the rare-earth elements in oxidation states +2, +3 and +4...

The rare earth elements in the parent rocks associated with minerals are fairly resistant to weathering. But to the extent the weathering of rare earth minerals occurs, rare earths are released as M3+ ions. In the case of minerals containing Sm2+ and Eu2+ the ions are easily oxidized to trivalent state and their further speciation depends on the following secondary reactions. In principle meteoric water is capable of setting free rare earth ions or as a soluble M3+ complex. 

Although the +3 oxidation state is by far the most common one for the rare earth elements, for some of them others (+2, +4) are of importance. Cerium, and to a much lesser extent Pr and Tb, can form Ln4+ ions (formally speaking) but these are strongly oxidizing. Sm, Eu, and to a lesser extent Yb form Ln2+ ions. These deviations from normal behavior (i.e., formation of only Ln3+) are sometimes attributed to the special stability of empty, half-filled or filled shells Ce4+ (4/°), Eu2+ (4 f), Yb2+ (4/14), but Pr4+ (4 f) and Sm2+ (4/6) do not fully satisfy this criterion. This idea is better considered as a mnenonic than as an explanation. 

Oxidation states, ionic size of Ln3+ and redox potential of rare-earth elements... 

Recognition of the mechanisms by which trace elements are partitioned into minerals suggests the importance of looking at the relative distributions of groups of elements that have similar chemical behavior. The rare earth elements (REE), or lanthanides, have been particularly useful because they usually occur as trivalent cations that differ from each other only in ionic size. Each mineral, as it is formed, partitions the REE and other trace elements into its crystal lattice on the basis of ionic size and charge. The REE are distributed in minerals on the basis of size, and the total concentration in a rock depends upon the minerals that are present. In some cases, there is an anomaly in the behavior of Eu, which can be separated from the others when it is reduced partially to the 2 + oxidation state. 

For example, lanthanum was allotted an atomic weight of 94 (AWi) in Mendeleev s Attempted System. The modem value for lanthanum s atomic weight is 138.9 AW and as all the rare-earth elements typically exhibit the +III oxidation state, its modern valency is 3 (1 2). With the aid of Eq. (7), the valency number Vi as used by Mendeleev in the beginning of 1869 can be established ... 

Mendeleev drew five important conclusions from these rare-earth accommodations. The first one concerned the oxidation states of the rare-earth elements. Lanthanum, a typically trivalent element, was placed... 

Rare earth elements have similar configurations in the two outermost shells. They exhibit typical metallic properties in chemical reactions. They tend to lose three electrons and exhibit a 3+ valence state. From the Periodic Table of the elements, rare earth elements are classed as less reactive than alkali metals and alkaline earth metals but more reactive than other metals. They should be stored in an inert liquid otherwise they will be oxidized and lose their metal luster. The metal reactivity increases gradually from scandium to lanthanum and decreases gradually from lanthanum to lutetium. That is to say, lanthanum is the most reactive metal of the 17 rare earth elements. Rare earth metals can react with water and release hydrogen. They react more vigorously with acids but do not react with bases. 

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