Actinide elements radius

Among the tetraborides, UB4 has the smallest volume and hence the smallest effective radius. Thus an actinide element having a metallic radius of 1.59 A (Pu) or smaller forms a diboride, while those having larger radii do not. As in the rare-earth series, the actinides able to form MB4, MBg and MB,2 borides form also MB2 diborides (Table 1). 

The redox chemistry of the actinide elements, especially plutonium, is complex (Katz et al., 1980). Disproportionation reactions are especially important for the +4 and +5 oxidation states. Some of the equilibria are kinetically slow and irreversible. All transuranium elements undergo extensive hydrolysis with the +4 cations reacting most readily due to their large charge/radius ratio. Pu (IV) hydrolyzes extensively in acid solution and forms polymers. The polymers are of colloidal dimensions and are a serious problem in nuclear fuel reprocessing. 

Lanthanides Element Valence shelP Ionic radius (pm) M3 Actinides Element Valence shell Ionic radius (pm) M3 M" ... 

The structures are formed only by the transition, lanthanide and actinide elements, and not by other metals of comparable atomic radius and electronegativity. 

The compounds formed are normally quite ionic. The ionic radii of the actinide elements of the different valency states decrease with increasing atomic number (the actinide contraction. Table 16.1). Consequently the charge density of the actinide ions increases with increasing atomic number and, therefore, the probability of formation of conq>lexes and of hydrolysis increases with atomic number. This is illustrated in Figure 16.7, where the heavier actinides are eluted before the lighter ones because the a-hydroxy-isobutyrate eluant forms stronger complexes as the cation radius decreases. 

The dodecahedron requires a larger radius ratio than the antiprism (i.e., 0.665 versus 0.645), and it is tempting to say that it is favored over the antiprism as a coordination type for the larger (with respect to the d-transition series) actinide elements. The lack of a sufficiently large number of well-determined structures makes such a suggestion tenuous at this time. However, there is some support for this thesis among the one dimensional polymers, as explained in the following section. 

The lanthanide and actinide elements are all metallic and are formally members of Group 3 of the Periodic Table. TThey all form 3+ ions in their compounds and in aqueous solution, with few exceptions. The actinide elements form a larger range of oxidation states than the lanthanides, but the 3+ ions are used in this section for comparison purposes. Figure 4.11 shows the lanthanides metallic and 3+ ionic radii. There is an almost regular decrease in metallic radius along the lanthanide series, with discontinuities at Eu and Yb. Most of the lanthanide metals contribute three electrons to bands of molecular orbitals con-... 

Electron configuration and ionic radius data for the actinide elements. 

One of the reasons to use other systems is that the systems proposed by Pauling were not complete. The data of many elements such as rare earth elements and actinide elements were absent. The description of the chief systems of atomic or ionic radius will be given as follows ... 

For actinide ions having a radius in the range of 0.9-1.0 A, coordination by nine fluoride ions is quite common. A number of structures in which this is true are listed in Table 20.6. Because ionic size is the important factor in yielding this coordination number, valences can range from 3 to 5, and the actinide elements involved can range from Pa to Am. In Li3ThF7 the F ions form a square antiprism with a pyramid on one face, but in all the other crystals listed the nine F neighbors... 

The ionic radius is a useful parameter with which to correlate numerous physical and thermodynamic properties of the actinide elemoits. Its usefulness for this purpose is not usually dependent on how it is d ned or on the absolute values that are used when comparing members of the series. Nevertheless, the tom radii implies spherical ions, and the modes of deriving such radii from crystallographic data usually assume that these spheres are in contact with spherical anions. When this assumption is not true, as in most real crystals, the derived radii depend on the method of calculation and are somewhat arbitrary. Consequently, there have been published for the actinide elements several tables of radii which differ both in absolute values and in the slope of the curve obtained when they are plotted against atomic number. All of these sets of radii have in common, however, two qualitative features a contraction of the radius with increasing atomic number and a cusp at the half-filled 5f-electron shell. Additional perturbations of the curve at the one-fourth- and three-fourths-fiUed shells have not been established for the actinides, although slight effects were shown to exist for the lanthanides... 

Symbol Pa atomic number 91 atomic weight 231.04 an actinide series radioactive element an inner-transition metal electron configuration [Rn]5/26di7s2 valence states +4 and +5 atomic radius 1.63A (for coordination number 12) twenty-two isotopes are known in the mass range 215-218,... 

Symbol Th atomic number 90 atomic weight 232.04 an actinide series radioactive element electron configuration XRn]6d27s2 valence state +4 atomic radius 1.80 A ionic radius, Th4+ 1.05 A for coordination number 8 standard electrode potential, E° for Th4+ -1- 4e Th is -1.899V all isotopes are radioactive the only naturally-occurring isotope, Th-232, ti/2 1.4xl0i° year twenty-six isotopes are known in the mass range 212-237. 

Fig. 3. Wigner-Seitz radii of d-transition metals and actinides vs atomic number Z. To the plot, elements displaying empty and full d- and f-shell have been added. In abscissae, the groups of the Periodic Chart of Elements have been indicated (see, e.g. Handbook of Chemistry and Physics). The figure shows the sudden jump in radius between Pu and Am discussed in this chapter, and, more deeply, in Chap. C...

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. 

Neutral extracting agents possessing oxygen-donor atoms (hard bases) in their structure easily coordinate trivalent lanthanide and actinide cations, but do not discriminate between the two families of elements, because the ion-dipole (or ion-induced dipole type) interactions mostly rely on the charge densities of the electron donor and acceptor atoms. As a result, the similar cation radii of some An(III) and Ln(III) and the constriction of the cation radius along the two series of /elements make An(III)/Ln(III) separation essentially impossible from nitric acid media. They can be separated, however, if soft-donor anions, such as thiocyanates, SCN-, are introduced in the feed (34, 35, 39, 77). 

Filling of the inner 4f electron shell across the lanthanide series results in decreases of ionic radii by as much as 15% from lanthanum to lutetimn, referred to as the lanthanide contraction (28). While atomic radius contraction is not rmique across a series (i.e., the actinides and the first two rows of the d-block), the fact that all lanthanides primarily adopt the tripositive oxidation state means that this particular row of elements exhibits a traceable change in properties in a way that is not observed elsewhere in the periodic table. Lanthanides behave similarly in reactions as long as the mnnber of 4f electrons is conserved (29). Thus, lanthanide substitution can be used as a tool to tune the ionic radius in a lanthanide complex to better elucidate physical properties. 

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... 

Similarities exist between the chemical characteristics of the actinides and those of the lanthanides. The metal ions are generally considered to be relatively hard Lewis acids, susceptible to complexation by hard (i.e., first row donor atom) ligands and to hydrolysis. Both actinide and lanthanide ions are affected by the lanthanide contraction, resulting in a contraction of ionic radius and an increasing reluctance to exhibit higher oxidation states later in the series. Most species are paramagnetic, although the electron spin-nuclear spin relaxation times often permit observation of NMR spectra, and disfavor observation of ESR spectra except at low temperatures. The elements display more than one accessible oxidation state, and one-electron redox chemistry is common. 

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