Ionic radii of actinides and lanthanides

Table 9. Ionic Radii of Actinide and Lanthanide Elements...

Figure 14.13. Ionic radii of actinide and lanthanide ions M- and M " as a function of the atomic number Z. (According to G. T. Seaborg The Transuranium Elements. Yale University Press 1958 Addison-Wesley Publ. Comp., S. 137.)...

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. 

The redox behaviour of Th, Pa and U is of the kind expected for d-transition elements which is why, prior to the 1940s, these elements were commonly placed respectively in groups 4, 5 and 6 of the periodic table. Behaviour obviously like that of the lanthanides is not evident until the second half of the series. However, even the early actinides resemble the lanthanides in showing close similarities with each other and gradual variations in properties, providing comparisons are restricted to those properties which do not entail a change in oxidation state. The smooth variation with atomic number found for stability constants, for instance, is like that of the lanthanides rather than the d-transition elements, as is the smooth variation in ionic radii noted in Fig. 31.4. This last factor is responsible for the close similarity in the structures of many actinide and lanthanide compounds especially noticeable in the 4-3 oxidation state for which... 

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. 

Table 2. Ionic radii (nm) of some actinides and lanthanides...

Figure 15.15 Variation of the ionic radii of trivalent lanthanide and actinide ions with increasing Z.

An empirical set of effective ionic radii in oxides and fluorides, taking into account the electronic spin state and coordination of both the cation and anion, have been calculated (114). For six-coordinate Bk(III), the radii values are 0.096 nm, based on a six-coordinate oxide ion radius of 0.140 nm, and 0.110 nm, based on a six-coordinate fluoride ion radius of 0.119 nm. For eight-coordinate Bk(IV), the corresponding values are 0.093 and 0.107 nm, based on the same anion radii (114). Other self-consistent sets of trivalent and tetravalent lanthanide and actinide ionic radii, based on isomorphous series of oxides (145, 157) and fluorides (148, 157), have been published. Based on a crystal radius for Cf(III), the ionic radius of isoelectronic Bk(II) was calculated to be 0.114 nm (158). It is important to note, however, that meaningful comparisons of ionic radii can be made only if the values compared are calculated in like fashion from the same type of compound, both with respect to composition and crystal structure. 

One probably can predict some of the crystallographic properties, of the tetrapositive element 104 by extrapolation from those of its homologs zirconium and hafnium. The ionic radii of tetrapositive zirconium (0.74 A) and hafnium (0.75 A) suggest an ionic radius of about 0.78 A for tetrapositive element 104, allowing for the smaller actinide rather than lanthanide contraction. Further one would expect the hydrolytic properties of element 104 and the solubilities of its compounds (such as the fluoride) to be similar to those of hafnium. The sum of... 

The ionic radii of the lower-valence states (An, An ) of the early actinides are similar to those of the trivalent states of the lanthanides [15]. As might be expected on the basis of the similarities in size and charge, the early actinides in their lower valence states play similar roles to the trivalent lanthanides and Zr in the stmctures of phosphates and arsenates. The chemistry of both trivalent- and tetravalent-actinide phosphates has recently been re-examined in detail [16-18]. This chapter takes a stmctural approach to the same material, and of necessity restricts its coverage to those compounds whose crystal stmctures have either been determined or whose stmctural affiliation may reasonably be inferred. It... 

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]. 

Fig. 3. Ionic radii of some actinides and lanthanides in various valences...

The closest redox-stable analogue of Ce(IV) is thorium(IV), for which a large data base of thermodynamic parameters is available for the carboxylic add complexes (Martell and Smith 1977). Using the ionic radii of Shannon (1976) and recalling that the stability of lanthanide and actinide complexes is derived almost exclusively from electrostatics, we can estimate that a 16% increase in the log of the stability quotients for thorium (since AG oc Z /r oc log should provide a reasonable estimate for the corresponding complexes of cerium(IV) [rce(CN = 8) = 0.97 A, r iCN = 10) = 1.13 A, (l/rce)/(l/ xh) = 116, CN = coordination number]. 

Ionic radii of lanthanide and actinide elements (Seaborg and Loveland 1990)... 

One aspect of similarity in the two series is the contraetion of the ionic radii with increasing atomic number. For any oxidation state, the ionic radii decrease regularly along the series, due to the increase of the nuclear electric field. The variations of with atomic number for the same coordination number are parallel for the lanthanides and actinides and the difference (rL —r ) is essentially constant from Am " to Cf " (David et al. 1985). This property allows extrapolation to obtain the values of ionic radii of the heaviest actinide elements which have not been measured (table 1) (David 1986). 

In the case of lanthanides and actinides, the ionic radii decrease greatly and the differences between the atomic radii and ionic radii are significant. They are much bigger than those of other elements. If other elements display 10-20% decreases, in lanthanides and actinides the ionic radii would decrease by 50-60%. Also, among lanthanide elements and actinide elements, respectively, the differences between the atomic and the ionic radii are quite small. They are practically equal in most instances (Figures 2.11 and 2.12). 

For any oxidation state, the ionic radii decrease regularly with increasing atomic number as a consequence of the decreased shielding by / electrons of the outer valence electrons from the increasing effective nuclear charge. This actinide contraction is very similar to the corresponding lanthanide contraction. Table VII summarizes crystallographic ionic radii of lanthanide and actinide ions for coordination numbers 6 and 8. 

TABLE VII Crystallographic Ionic Radii of Lanthanide and Actinide Ions... 

This chapter is intended to provide a unified view of selected aspects of the physical, chemical, and biological properties of the actinide elements. The f block elements have many unique features, and a comparison of the lanthanide and actinide transition series provides valuable insights into the properties of both. Comparative data are presented on the electronic configurations, oxidation states, redox potentials, thermochemical data, crystal structures, and ionic radii of the actinide elements, together with a miscellany of topics related to their environmental and health aspects. Much of this material is assembled in tabular and graphical form to facilitate rapid access. Many of the topics covered in this chapter, and some that are not discussed here, are the subjects of subsequent chapters of this work, and these may be consulted for more comprehensive treatments. This chapter provides a welcome opportunity to discuss the biological and environmental aspects of the actinide elements, subjects that were barely mentioned in the first edition of this work but have assumed great importance in recent times. 

In Table 20.7 are listed radii of trivalent actinide ions (coordination number CN 6) derived from measurements on trichlorides by the method of Bums, Peterson, and Baybarz [288]. Determinations of M-Cl distances have been made for M = U, Pu, Am, Cm, and Cf the other values were estimated by use of unitcell data and curve fitting. All these radii are relative to the trivalent lanthanide radii of Templeton and Dauben [396], who employed data from cubic sesquioxides and assumed atomic positions to establish M-O distances. Also included in Table 20.7 are radii of tetravalent actinide ions obtained from the M-O distances calculated from unit-cell parameters of the dioxides [1] by subtracting 1.38 A for oxygen (the value used [396] for the sesquioxides). For comparison. Shannon s ionic radii, derived from oxides and fluorides, and Peterson s tetravalent radii, derived from dioxides, are shown [537,538]. As... 

Most transition metals of the three d-series in all their valency states exhibit ionic radii within the limits of 0.55 and 0.86 A, favourable to octahedral coordination. In fact higher coordination numbers are observed only in fluorides of the largest transition ions, above all in compounds of the lanthanide and actinide series. Therefore fluorides of those elements, though sometimes isostructural with compounds of the d-series, will not be discussed here. For information the books and reviews written by Spedding and Daane (291), Katz and Seaborg (181) and Kaiz and Sheft (182) may be consulted. 

Crystal Structure and Ionic Radii. Crystal structure data have provided the basis for the ionic radii (coordination number — C-N — 6). For both M3+ and M4 10ns there is an actinide contraction, analogous to the lanthanide contraction, with increasing positive charge on the nucleus. As a consequence of the ionic character of most actinide compounds and of the similarity of the ionic radii for a given oxidation state, analogous compounds are generally isostmctural. 

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