Valence lanthanide contraction

A technologically important effect of the lanthanide contraction is the high density of the Period 6 elements (Fig. 16.5). The atomic radii of these elements are comparable to those of the Period 5 elements, but their atomic masses are about twice as large so more mass is packed into the same volume. A block of iridium, for example, contains about as many atoms as a block of rhodium of the same volume. However, each iridium atom is nearly twice as heavy as a rhodium atom, and so the density of the sample is nearly twice as great. In fact, iridium is one of the two densest elements its neighbor osmium is the other. Another effect of the contraction is the low reactivity—the nobility —of gold and platinum. Because their valence electrons are relatively close to the nucleus, they are tightly bound and not readily available for chemical reactions. 

The spectacular irregularity in the metallic radii of Eu and Yb occurs because they have only two valence electrons in the conduction band, whereas the other lanthanide metals have three valence electrons in the 5d/6s conduction band. Therefore, lanthanide contraction is not manifested by Eu and Yb in the metallic... 

When the coordination number and the valence state remain the same, the effective ionic radius will decrease as the atomic number increases. This is caused by lanthanide contraction. 

In addition to making the third-series transition metals smaller, the lanthanide contraction also makes them less reactive because the valence electrons are relatively close to the nucleus and less susceptible to chemical reactions. This accounts for the relative inertness—or nobility—of these metals, particularly gold and platinum. Moreover, the third-series transition metals are the densest known elements, having about the same atomic size as the second-series transition metals but twice the atomic weight. The densest element is iridium (Ir, Z = 77) at 22.65 g/cm. ... 

Even more striking is the anomalous position of Th in the entropy-radius relationship of Fig. 4. In following the IVA elements, the shift to smaller radius at Hf corresponds to the gross effect of the lanthanide contraction in the previous row. Note that Th is far over into the trivalent metal area, corresponding to a very large radius Ci.e., lower valence for a supposedly tetravalent metal). 

The most common oxidation state of the Ln ions is +3, although many divalent and tetravalent species are known. The predominance of trivalent ions can be seen from the electron configurations of these elements Xe Af 5d (>s (n = 0 for La, = 14 for Lu), where successive ionizations of 5d and 6 electrons leave the 4/ electrons as the valence. This also explains the well-known lanthanide contraction, wherein a small ( 16%), yet steady reduction in ionic radii is observed across this series with increasing nuclear charge (36, 37). 

Fig. 11 shows that the IR of the 4d and 5d elements are, as expected, almost equal due to the well-known lanthanide contraction (of 0.020 A) which is roughly 86% a nonrelativistic effect The diminished shielding of the nucleus charge by the 4f electrons causes the contraction of the valence shells. The IR of the transactinides are about 0.05 A larger than the IR of the 5d elements. This is due to an orbital expansion of the outer 6p3/2 orbitals responsible for the size of the ions. The IR of the transactinides are, however, still smaller than the IR of the actinides due to the actinide contraction (0.030 A, being larger than the lanthanide contraction) which is mostly a relativistic effect The 5f shells are more diffuse than the 4f shells, so that the contraction of the outermore valence shells is increased by relativity to a larger extent in the case of the 6d elements as compared to the 5d elements. This has first been shown for elements 104-118 by DF and DS calculations of atomic and ionic radii by Fricke and Waber [20]. 

The lanthanide contraction, however, has also effects for the rest of the transition metals in the lower part of the periodic system. The lanthanide contraction is of sufficient magnitude to cause the elements which follow in the third transition series to have sizes very similar to those of the second row of transition elements. Due to this, for instance hafnium (Hf ) has a 4" -ionic radius similar to that of zirconium, leading to similar behavior of these elements. Likewise, the elements Nb and Ta and the elements Mo and W have nearly identical sizes. Ruthenium, rhodium and palladium have similar sizes to osmium iridium and platinum. They also have similar chemical properties and they are difficult to separate. The effect of the lanthanide contraction is noticeable up to platinum (Z = 78), after which it no longer noticeable due to the so-called Inert Pair Effect (Encyclopedia Britannica 2015). The inert pair effect describes the preference of post-transition metals to form ions whose oxidation state is 2 less than the group valence. 

The lattice parameter of the compounds R3 MC in the lanthanide series for a given element M changes linearly with the radius of the trivalent rare earth ions, following the lanthanide contraction regularity, except for europium and ytterbium which appear to have a variable-valence state. In the ytterbium-M (Al, Ga, In, Tl)-carbon system, the unit cell volume of the compound YbgMC, is larger than the value expected from the relationship mentioned above. In the europium-M(Al, Ga, In, Tl)-carbon system no compound was found. For the compounds of yttrium, as expected, the lattice parameter falls between those of the respective terbium and dysprosium compounds. 

The lanthanide contraction explains which of the following periodic trends (a) The atomic radii of the transition metals first decrease and then increase when moving horizontally across each period, (b) When forming ions the transition metals lose their valence s orbitals before their valence d orbitals, (c) The radii of the period 5 transition metals (Y-Cd) are very similar to the radii of the period 6 transition metals (Lu-Hg). 

The atomic volumes of the lanthanides, as calculated from their room-temperature lattice parameters, are shown in fig. 3. Basically this is the same plot as given by Klemm and Bommer where the lanthanide contraction is evident, and also the anomalous valence states for cerium (slightly greater than three) and europium and ytterbium (both divalent). Anomalies due to divalency are also evident in many of the physical properties and these will be duly noted throughout the chapter. The occurrence of divalency in europium and ytterbium is a striking confirmation of Hund s rule that half-filled (in the case of divalent europium with a 4f con-... 

Today the lanthanide eontraction is still one of the most important tools available to the scientist in applying systematics to the behavior of lanthanide materials. Deviations from the lanthanide contraction established for a given compound series gives a measure of anomalous valences for cerium, samarium, europiun, thulium and ytterbium (see section 3.2) which are important in evaluating the nature of these elements in valence fluctuation, heavy fermion, and spin fluctuation behaviors (see section 4.4.4). 

Figure 4 presents the change of the crystall( raphic unit cell volume for the (R, An)T4 Alg-type compounds for different T elements (Buschow et al. 1976, Baran et al. 1987). This change is a monotonically decreasing function of increasing atomic number for actinides (only light ones) but for landianides the known lanthanide contraction has some exceptions, which is most pronounced for cerium compounds, and is probably due to the valence of cerium being different from 3+, as has been documented by X-ray spectroscopy experiments (Shcherba et al. 1992) table 3 lists the valence values. 

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