Lanthanide/actinide contraction

Further important influences on the periodic trends arise from partial screening of nuclear charge (including lanthanide/actinide contraction, scandide contraction, and even a 2p-contraction) and from the effects of special relativity. Various aspects of main-group bonding are covered in more detail elsewhere in this book. 

The effects of the lanthanide/actinide contraction on the post-lanthanide/actinide main group elements have been investigated by Bagus et al. and Seth et al., respectively. The latter show that the interpretation of the observed total effect in terms of shell-structure and relativistic effects is dependent on the order in which these contributions are taken into account, i.e., a nonadditive behavior is observed (Figure 6). 

Crystal Structure and Ionic Radii. Crystal stmcture data have provided the basis for the ionic radii (coordination number = CN = 6), which are summarized in Table 9 (13,14,17). For both and ions there is an actinide contraction, analogous to the lanthanide contraction, with increasing positive charge on the nucleus. 

By contrast, the ionic radius in a given oxidation state falls steadily and, though the available data are less extensive, it is clear that an actinide contraction exists, especially for the -f3 state, which is closely similar to the lanthanide contraction (see p. 1232). 

Thus the rather easily obtained atomic sizes are the best indicator of what the f-electrons are doing. It has been noted that for all metallic compounds in the literature where an f-band is believed not to occur, that the lanthanide and actinide lattice parameters appear to be identical within experimental error (12). This actually raises the question as to why the lanthanide and actinide contractions (no f-bands) for the pure elements are different. Analogies to the compounds and to the identical sizes of the 4d- and 5d- electron metals would suggest otherwise. The useful point here is that since the 4f- and 5f-compounds have the same lattice parameters when f-bands are not present, it simplifies following the systematics and clearly demonstrates that actinides are worthy of that name. 

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 ionic radii of the commonest oxidation states are presented in Table 2. There is evidence of an actinide contraction of ionic radii as the 5/ orbitals are filled and this echoes the well established lanthanide contraction of ionic radii as the 4/orbitals are filled. Actinides and lanthanides in the same oxidation state have similar ionic radii and these similarities in radii are obviously paralleled by similarities in chemical behaviour in those cases where the ionic radius is relevant, such as the thermodynamic properties observed for halide hydrolysis. 

In this treatment, the line connecting the metallic radii of actinium and curium is considered as representative of an actinide contraction, analogous to the lanthanide contraction. This actinide contraction line may be considered as a trivalent basis line as for lanthanides therefore, the considerable departure to a lower value for the metallic... 

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. 

RARE-EARTH ELEMENTS AND METALS. Sometimes referred to as the fraternal fifteen," because of similarities in physical and chemical properties, the rare-earth elements actually are not so rare. This is attested by Fig. 1, which shows a dry lake bed in California that alone contains well in excess of one million pounds of two of die elements, neodymium and praseodymium. The world s largest rare earth body and mine near Baotou, Inner Mongolia, China is shown in Fig. 2. It contains 25 million tons of rare earth oxides (about one quarter of the world s human reserves. The term rare arises from the fact that these elements were discovered in scarce materials. The term earth stems from die tact that the elements were first isolated from their ores in the chemical form of oxides and that the old chemical terminology for oxide is earth. The rare-earth elements, also termed Lanthanides, are similar in that they share a valence of 3 and are treated as a separate side branch of the periodic table, much like die Actinides. See also Actinide Contraction Chemical Elements Lanthanide Series and Periodic Table of the Elements. 

The data of Table 4 and Figure 10 show that the IR of the 4d and 5d elements are almost equal due to the lanthanide contraction which is 86% a non-relativistic effect, while the IR of the transactinides are about 0.05 A larger than the IR of the 5d elements due to an orbital expansion of the 6p3/2 orbitals being the outer orbitals for the maximum oxidation state. The IR of the lighter 6d elements are however smaller than the IR of the actinides since the latter undergo the actinide contraction of 0.030 A which is mostly a relativistic effect [13,105]. 

The ionic radii for the commonest oxidation states (Table 20-1) are compared with those of the lanthanides in Fig. 20-1. There is clearly an actinide contraction, and the similarities in radii of both series correspond to similarities in their chemical behavior for properties that depend on the ionic radius, such as hydrolysis of halides. It is also generally the case that similar compounds in the same oxidation state have similar crystal structures that differ only metrically. 

Fig. 14.4), it is best shown by the radii of the -1-3 cations (Fig. 14.5). There are two noticeable dilTerenccs between the two series of ions (1) although the actinide contraction initially parallels that of the lanthanides, the elements from curium on are smaller than might be expected, probably resulting from poorer shielding by 5/ electrons in these elements (2) the lanthanide curve consists of two very shallow arcs with a discontinuity at the spherically symmetrical Gd " (4/ ) ion. A similar discontinuity is not clearly. seen at... 

From the results shown in Fig. 6, some conclusions may be drawn. A comparison of the lanthanide curve with the summary curve for actinides suggests that the doubledouble effect is more pronounced in actinides than in lanthanides. A comparison of curves 3 and 4 reveals that the effect is the stronger, the higher the oxidation state of the actinide ions. Moreover, curves relating to actinides show, that the mean specific unit cell volumes decreases with Z more rapidly for this series than for lanthanides. This suggests that the actinide contraction is larger than that of lanthanides. Such a conclusion also results from the data of Zachariasen64). 

Crystal structure data obtained by X-ray diffraction methods for the actinide element halides are collected in Table IV. Crystal structure determinations have been most important in identifying new compounds of the actinide elements the data are sufficiently extensive now for use in drawing conclusions regarding systematic trends and relations among the actinide elements. The tetrafluorides, for instance, supply one of the best illustrations of an actinide contraction that is entirely similar to the well-known lanthanide contraction (Table V). 

Speciation and reactivity of actinide compounds comprise an important area for quantum chemical research. Even more so than in the case of lanthanides, f-type atomic orbitals of actinides can affect the chemistry of these elements [185,186] the more diffuse 5f-orbitals [187] lead to a larger number of accessible oxidation states and to a richer chemistry [188]. The obvious importance of relativistic effects for a proper description of actinides is often stressed [189-192]. A major differences in chemical behavior predicted by relativistic models in comparison to nonrelativistic models are bond contraction and changes in valency. The relativistic contribution to the actinide contraction [189,190] is more pronounced than in the case of the lanthanides [191,192]. For the 5f elements, the stabilization of valence s and p orbitals and the destabilization of d and f orbitals due to relativity as well as the spin-orbit interaction are directly reflected in the different chemical properties of this family of elements as compared with their lighter 4f congeners. Aside from a fundamental interest, radioactivity and toxicity of actinide compounds as well as associated experimental difficulties motivate theoretical studies as an independent or complementary tool, capable of providing useful chemical information. 

O Table 18.10 shows ionic radii of actinide elements together with those of lanthanide elements (Seaborg and Loveland 1990). The usefiil data on the ionic radii and coordination number are given by Shannon and Prewitt (1969). They carried out comprehensive study of crystal, or ionic, radii by analyzing the crystal structures of many fluoride, oxide, chloride, and sulfide compounds. Marcus published a data book on the properties of ions (Marcus 1997). The book covers a wide range of information on ionic radii of the actinide elements and other ions. Ionic radii of actinide elements decrease with increasing atomic number. This behavior is called actinide contraction and is one of the important examples of the actinide concept. 

Values for the lanthanide and actinide contraction from various atomic calculations"... 

Values for the lanthanide and actinide contraction derived from bond lengths from various relativistic (nonrelativistic) molecular calculations and experimental data... 

Pyykko (1979b) used the Dirac-Hartree-Fock one-centre expansion method for the monohydrides to calculate relativistic values for the lanthanide and actinide contraction, i.e. 0.209 A for LaH to LuH and 0.330A for AcH to LrH. The corresponding nonrelativistic value derived from Hartree-Fock one-center expansions for LaH and LuH is 0.191 A, i.e., for this case 9.4% of the lanthanide contraction is due to relativistic effects. The experimental value of 0.179 A would suggest a correlation contribution of-14.4% to the lanthanide contraction if one assumes that the relativistic theoretical values are close to the Dirac-Hartree-Fock limit, which is certainly not true for the absolute values of the bond lengths themselves. Moreover, it is well known that for heavy elements relativistic and correlation contributions are not exactly additive. Corresponding nonrelativistic calculations for AcH and LrH have not been performed and experimental data are not available to determine relativistic and electron correlation effects for the actinide contraction. Table 8 summarizes values for the lanthanide and actinide contraction derived from theoretical or experimental molecular bond lengths. It is evident from Ihese results... 

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