Contraction actinide

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). 

The only crystalline phase which has been isolated has the formula Pu2(OH)2(SO )3(HaO). The appearance of this phase is quite remarkable because under similar conditions the other actinides which have been examined form phases of different composition (M(OH)2SOit, M=Th,U,Np). Thus, plutonium apparently lies at that point in the actinide series where the actinide contraction influences the chemistry such that elements in identical oxidation states will behave differently. The chemistry of plutonium in this system resembles that of zirconium and hafnium more than that of the lighter tetravalent actinides. Structural studies do reveal a common feature among the various hydroxysulfate compounds, however, i.e., the existence of double hydroxide bridges between metal atoms. This structural feature persists from zirconium through plutonium for compounds of stoichiometry M(OH)2SOit to M2 (OH) 2 (S0O 3 (H20) i,. Spectroscopic studies show similarities between Pu2 (OH) 2 (SOO 3 (H20) i, and the Pu(IV) polymer and suggest that common structural features may be present. 

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. 

M = Am, Cm, Bk, Cf, and Es) has been investigated by solvent extraction processes using bis-(2-ethylhexyl)phosphoric acid. The stability of [M(NCS)] and [M(NCS)2] increased gradually across the series in accord with expectations based on the actinide contraction, and there was evidence for the tetrad... 

The measured metallic radii of the metals are drawn, in Fig. 4, on a network of dashed lines specifying the actinide contractions for f configurations progressively increasing by one electron at constant metallic valence (i.e. without change of the number of s, p, d electrons, which are thought to provide the only bonding). (The... 

In Fig. 5 a minimum as in the metals is observed in the curves of the compounds with more metalloidic elements. For AnAs and AnSh this minimum tends to disappear. After the minimum (see AnN), there is a decreasing trend, which can be explained in terms of actinide contraction. Between PuN and AmN, a jump is seen, which is similar to the one met in metals (see Fig. 2). 

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... 

A well known qualitative argument, often employed in discussions of the actinide series, takes the departure of metal radii from a simple actinide-contraction curve as a measure of 5f-itinerant bonding (see Figs. 3 and 5 of Chap. A). The model presented here justifies and gives a quantitative basis to this argument. 

Spin-polarization sets up in the second part of the series, as in the case of metals, and, correspondingly, Eq. (22) should be modified with the use of spin-polarized terms. This explains the onset of an actinide contraction trend in heavier actinide NaCl compounds, as shown in Fig. 5 of Chap. A for the AnN system. 

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. 

TRANSACTINIUM EARTHS. A group of chemical elements more frequently termed the Actinides. In order of increasing atomic number, they indude actinium, thorium, protactinium, uranium, neptunium, plutonium, americium, curium, berkelium, californium, einsteinium, fermium, mendelevium, nobelium. and lawrencium. See also Actinide Contraction. 

TRANSURANIUM ELEMENTS. The chemical elements widi an atomic number higher than 92 (uranium), commencing with 93 (neptunium) and through 110 (darmstadtium) frequendy are termed Transuranium elements, Any additional elements that may be identified will be a part of this series, See also Actinide Contraction and Chemical Elements. 

The ionic radii of the M3+ and M4+ ions of the actinides decrease with increasing positive charge of the nucleus (the actinide contraction) (Fig. 15.15). This contraction is due to the successive addition of electrons in an inner f shell where the incomplete screening of the nuclear charge by the added f electron leads to a contraction of the outer valence orbital. Because the ionic radii of ions of the same oxidation state are generally similar (Fig. 15.15), the ionic compounds of the actinides are isostructural. 

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. 

Ionic Radii and the Actinide Contraction - A Partial Relativistic Effect... 

Analogous and carbonates can be assumed to be isostructural with the well-characterized uranyl compounds, albeit with slightly shorter bond distances to reflect the actinide contraction. For the triscarbonato complexes of Np this has been confirmed by the structures of K4Np02(C03)3 by single-crystal structure determinationof [(CH3)4N]Np02(C03)3. " ... 

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... 

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