Lanthanide contraction elements

The effect of the lanthanide contraction on the metal and ionic radii of hafnium has already been mentioned. That these radii are virtually identical for zirconium and hafnium has the result that the ratio of their densities, like that of their atomic weights, is very close to Zr Hf = 1 2.0. Indeed, the densities, the transition temperatures and the neutron-absorbing abilities are the only common properties of these two elements which differ... 

The total lanthanide contraction is of a similar magnitude to the expansion found in passing from the first to the second transition series, and which might therefore have been expected to occur also in passing from second to third. The interpolation of the lanthanides in fact almost exactly cancels this anticipated increase with the result, noted in preceding chapters, that in each group of transition elements the second and third members have very similar sizes and properties. 

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. 

FIGURE 16.4 The atomic radii of the d-block elements (in picometers). Notice the similarity of all the values and, in particular, the close similarity between the second and the third rows as a result of the lanthanide contraction. 

In the sequence of structures from the large to the small rare-earth elements, the lanthanide contraction is manifested as shown in Figs. 19a and 19b. Within a structure, the cell volume diminishes linearly with the atomic number. If a certain, limiting value is reached, there is... 

Element 114 will be a metal in the same group as Pb, element 82 (18 cm3/mol) Sn, element 50 (18 cm3 /mol) and Ge, element 32(14 cm3 /mol). We note that the atomic volume of Pb and Sn are essentially equal, probably due to the lanthanide contraction. If there is also an actinide contraction, element 114 will have an atomic volume of 18 cm3 / mol. If there is no actinide contraction, we would predict a molar volume of 22 cm3 / mol. This need to estimate atomic volume is what makes the value for density inaccurate. 

The computational bond-length variations in Table 4.53 exhibit the expected periodic trends. Most noticeably, third- and second-series elements for groups 4, 6, and 10 exhibit similar bond lengths, i.e., the post-lanthanide contraction with respect to the ordinary increase of atomic size with increasing Z. 

The rare earths have ion radii that vary 1.5% from one element to the next - the size decreases while the atomic number increases from lanthanum to lutetium. This is what is commonly called the "lanthanides contraction", see Figure 10. 

As we go from left to right across the transition metals in the periodic table, the metal atoms become smaller, much as in the lanthanide contraction (Section 2.6). Furthermore, the atoms of elements of the first transition series are smaller than those of corresponding members of the second and third. Consequently, interstitial carbides are particularly important for metals toward the lower left of the series, as with TiC, ZrC, TaC, and the extremely hard tungsten carbide WC, which is used industrially as an abrasive or cutting material of almost diamond like hardness. The parallel with trends in chemisorption (Section 6.1) will be apparent. 

Owing to lanthanide contraction, niobium and tantalum have virtually identical atomic rad (1.47 A) and close ionization energies (Nb6.67, Ta7.3eV), and usually display very similt chemical behavior. Some definite differences can however be noted these can usually be trace to the lower sensitivity of tantalum to reduction and to its higher affinity for dioxygen. lb tantalum-element multiple bonds are usually stabler, while MfiXg arrangements are so ft known only for niobium. 

The relativistic effect goes approximately as Z2, and this is the reason for its importance in the heavier elements. In terms of energy and size, it starts to become important in the vicinity of Z = 60-70. contributing perhaps an additional 10% to the nonrelativistic lanthanide contraction (see Chapter I4).39 As we have seen, this results in an almost exact cancellation of the expected increase in size with increase in n from zirconium to hafnium. 

Although in many respects relativistic effects and the lanthanide contraction serve to make the postlanlhanide elements less reactive than would otherwise be the case, in other respects their bonding ability is increased. For example, bis(phosphine)plati-num(0) complexes react with molecular hydrogen, but the analogous palladium complexes do not 3... 

ACTINIDE CONTRACTION. An effect analogous to the Lanthanide contraction, which lias been found in certain elements of the Actinide series. Those elements from thorium (atomic number 90) to curium (atomic number 96) exhibit a decreasing molecular volume in certain compounds, such as those which the actinide tetrafluoiides form with alkali metal fluorides, plotted in Eig. 1. The effect here is due to the decreasing crystal radius of the tetrapositive actinide ions as the atomic number increases. Note that in the Actinides the tetravalent ions are compared instead of the trivalent ones as in the case of the Lanthanides, in which the trivalent state is by far the most common. 

LANTHANIDE CONTRACTION. The decreasing sequence of crystal radii of the triposiiivc rare-earth ions with increasing atomic number in the group of elements 157) lanthanum through (71) lutetiunt of the Lanthanide Series in the periodic table. 

FIGURE 16.5 The densities (in grams per centimeter cubed, g-cm 3) of the d-metals at 25°C. The lanthanide contraction has a pronounced effect on the densities of the elements in Period 6 (front row in this illustration), which are among the densest of all the elements. 

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