Cerium electronic configuration

Although rare-earth ions are mosdy trivalent, lanthanides can exist in the divalent or tetravalent state when the electronic configuration is close to the stable empty, half-fUed, or completely fiUed sheUs. Thus samarium, europium, thuUum, and ytterbium can exist as divalent cations in certain environments. On the other hand, tetravalent cerium, praseodymium, and terbium are found, even as oxides where trivalent and tetravalent states often coexist. The stabili2ation of the different valence states for particular rare earths is sometimes used for separation from the other trivalent lanthanides. The chemicals properties of the di- and tetravalent ions are significantly different. 

Cerium, at wt 140.12 electron configuration [Xe] is characterized chemically by having two stable valence states, Ce ", cerous, and Ce" ", ceric,... 

The rare earth (RE) ions most commonly used for applications as phosphors, lasers, and amplifiers are the so-called lanthanide ions. Lanthanide ions are formed by ionization of a nnmber of atoms located in periodic table after lanthanum from the cerium atom (atomic number 58), which has an onter electronic configuration 5s 5p 5d 4f 6s, to the ytterbium atom (atomic number 70), with an outer electronic configuration 5s 5p 4f " 6s. These atoms are nsnally incorporated in crystals as divalent or trivalent cations. In trivalent ions 5d, 6s, and some 4f electrons are removed and so (RE) + ions deal with transitions between electronic energy sublevels of the 4f" electroiuc configuration. Divalent lanthanide ions contain one more f electron (for instance, the Eu + ion has the same electronic configuration as the Gd + ion, the next element in the periodic table) but, at variance with trivalent ions, they tand use to show f d interconfigurational optical transitions. This aspect leads to quite different spectroscopic properties between divalent and trivalent ions, and so we will discuss them separately. 

Symbol Nd atomic number 60 atomic weight 144.24 a rare earth lanthanide element a hght rare earth metal of cerium group an inner transition metal characterized by partially filled 4/ subshell electron configuration [Xe]4/35di6s2 most common valence state -i-3 other oxidation state +2 standard electrode potential, Nd + -i- 3e -2.323 V atomic radius 1.821 A (for CN 12) ionic radius, Nd + 0.995A atomic volume 20.60 cc/mol ionization potential 6.31 eV seven stable isotopes Nd-142 (27.13%), Nd-143 (12.20%), Nd-144 (23.87%), Nd-145 (8.29%), Nd-146 (17.18%), Nd-148 (5.72%), Nd-150 (5.60%) twenty-three radioisotopes are known in the mass range 127-141, 147, 149, 151-156. 

Lanthanum, the first member of lanthanides has the configuration of 5d)6s2 and next member cerium, has 4fi6s2 while the next element praseodymium has the configuration 4f3 6s2. Although lanthanum itself does not possess any 4/electrons, it is customary to include this element in the series. The electronic configurations of the elements with fully filled (// and half-filled (f7)/-orbitals are relatively more stable. 

The first attempts to record the Bk(IV) solution absorption spectrum were hindered by the presence of cerium impurities (92). The positions of the Bk(IV) absorption bands, superimposed on the strong Ce(IV) bands, suggested the assignment of Sf7 for the electronic configuration of Bk(IV), in agreement with the actinide hypothesis. 

Three places after xenon there follows the remarkable group of the elements of the Rare Earths, because here, beginning with cerium, the Nf or 4f shell (/ — 3, m = —3, —2, —1, o, 1, 2, 3) is filled up. There is thus produced a group of 14 elements from cerium (58) to lutecium (71), which all possess the same electron configuration of the outermost shell as lanthanum and thus also show a great similarity in chemical properties [group of the lanthanides or lanthanons]. 

In solid solutions, rare earths impart colour to the solutions due to their electronic configurations. This property has been profitably used in the manufacture of ceramic pigments which are extensively used in colouring of wall and floor tiles, table-ware and sanitary-ware. Cerium and praseodymium are extensively used in ceramics pigments. The shades that result due to the use of cerium and praseodymium are yellow, orange and green. 

In aqueous solution, lanthanides are most stable in the tripositive oxidation state, making them difficult to separate and purify. The preference for this oxidation state is due in part to the energy of the 4f electrons being below those of the 5d and 6s electrons (except in the cases of La and Ce). When forming ions, electrons from the 6s and 5d orbitals are lost first so that all Ln + ions have [Xe] 4f electronic configurations. Under reducing conditions, certain lanthanides (europium, samarium, and ytterbium) can be stable as dipositive ions, and cerium can adopt a +4 oxidation state (5). 

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]. 

A t the time of Alfred Werner s birth, the only lanthanide elements that had been identified positively were lanthanum and cerium. Yttrium, a lanthanide element by all criteria except electronic configuration, was known also. However, in 1891 when Werner proposed the substance of the coordination theory, all of the elements of the lanthanide series except promethium, europium, and lutetium had been clearly identified and quite well characterized. Only promethium remained undiscovered at the time of Werner s demise. 

Cerium is a member of the lanthanides in the Periodic Table and adopts tetra- and tripositive states in its electronic configuration. Among cerium reagents, ceric ammonium nitrate (CAN) is most widely used in organic synthesis. It is well known to convert phenol derivatives to quinones in high yields under mild conditions. An excellent review on cerium(IV) oxidation of organic compounds is available, and only a few examples will be described herein. 

Following lanthanum are the 14 elements known as the lanthanides, or rare earth series [cerium (Z = 58) to lutetium (Z = 71)]. The rare earth metals have incompletely filled 4f subshells or readily give rise to cations that have incompletely filled 4f subshells. In this series, the added electrons are placed in 4/orbitals. After the 4/subshells are completely filled, the next electron enters the 5d subshell of lutetium. Note that the electron configuration of gadolinium (Z = 64) is [Xt 6s Af 5d rather than [Xe]6 4/ . Like chromium, gadolinium gains extra stability by having half-filled subshells Af). 

---