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Periodicity and the Lanthanides and Actinides

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Periodicity and the Lanthanides and Actinides Therald Moeller Periodicitv and the Lanthanides Arizona State University Tempe, Arizonio 85281 I and dctinides Periodicity is defined as the tendency to and Yt had been truly characterized as pure substances recur at regular intervals. As applied to the chemical (1). The Di of Mendeleev's table was not resolved into elements, the term indicates the regular recurrence of Pr and Nd until 1885. The Er listed was undoubtedly particular types of chemical and/or physical properties the mixture erbia, the resolution of which was not as a function of some atomic parameter such as ground- effected until many years later. state electronic configuration or nuclear charge. In The positions assigned in 1869 by Mendeleev bear discussing periodicity as it relates to the lanthanides and little relationship to those we now recognize. In part, actinides, or more generally to the f-type transition this situation resulted from inaccuracies in atomic elements, we are concerned, therefore, with systematiz- weight data produced by errors in combining weights. ing recurrent characteristics. In part, it resulted also from lack of purity in samples The f-type transition elements present problems that available for study and thus an incomplete knowledge are somewhat different from those encountered with of correct chemical properties. other types of elements as consequences both of the The Mendeleev table of 1872, as reproduced in Table general shielding of f orbitals by higher-level orbitals 2, listed no additional elements from the series in ques- and of the inherent differences in shielding experienced tion but did achieve a better correlation of properties, by the 4f and 5f orbitals. These problems render rea- as we now know them, by placing Yt, Di, and Er to- sonable a discussion under the somewhat separated gether in Group 111, Th in Group IV, and U in Group headings VI and basing this classification on a more nearly cor- rect assignment of atomic weights. The inclusion of Ce 1) Periodicity in relationship to the other elements 2) Periodicity between the lanthanide and actinide series in Group IV was undoubtedly based upon the better 3) Periodicity within each series characterization of the +4 oxidation state of this ele- ment, but the assignment of La to this group was proh- Periodicity in Relationship to the Other Elements ably more the result of yet another error in atomic The original Mendeleev table of 1869, as reproduced weight. in Table 1, listed, of the elements in question, only Er, Subsequent modifications of the Mendeleev table in- cluded other lanthanides as they were separated and ce,-,L~, , ~i, ~h,and ur, ~h~ element yt, which is not an f-transi'tion'element but which as a c&,sequence of characterized but could, in terms of their restrictive charge-size similarities is operationally included to ad- geometries, do little more than suggest that the lan- vantagen.ith the lanthanides, also appeared in this thanides are an unclassifiable group of elements which, table. Prior to 1869, only the elements La, Ce, Th, Ur, in terms of their very similar properties, OCCUPY only a sinnle uosition in Grou~111 followinn barium in Groun Presented at the Mendeleev Centennial Symposium held at the 11.- ~Adeed,even the brohlem of thYe total number df 157th ti,,,,^^ ti ti^^ of the ~~~~i~~~ chemical society, elements that can be in this series had no answer until Minneapolis, Minnesota, April, 1969. the theoretical considerations of N. Bohr set the Table 1. Mendeleev's Periodic Table of 1869 Volume 47, Number 6, June 1970 / 417 Table 2. Mendeleev's Periodic Table of 1872 Grovp IV Group V Group VI Group VII Grow Group I1 Grow I11 RHa RHI RHI RH Orou VIII Series R?O RO RaOz ROz R.06 RO* Rg07 ~84 1 H=l N = 14 0 - 18 F - 19 2 Li=1 Be = 9.4 B = 11 C = 12 S ir 32 Cl - 35.5 3 Na = 23 hlg = 24 A1 = 27.3 Si = 28 " PE Cr = 52 Mn = 55 Fs = 56 Co - 59 4 K=39 Ca = 40 = 44 Ti = 48 Ni = 59 Cu - 83 Aa - 75 6e - 18 Br - 80 5 (Cu = 63) Zn = 65 = 68 = 72 Nb = 94 Ma = 96 = 100 Ru = 104 Rh - 104 6 Rh = 85 Sr = 87 ?Yt = 88 Zr = 90 Pd = 108 Ag = 108 . Sb = 122 Te - 125 I - 127 7 (Ag = 108) Cd = 112 In = 113 So = 118 8 Cs = 133 Ba = 137 ?Di = 138 ?Ce = 140 9 Tn = 182 W = 184 Oa = 195 Ir = 191 10 ?Er = 178 ?La = 180 Pt = 198 Au = I99 Bi - 208 11 (Au = 199) Hg = 200 TI = 204 Pb = 207 U = 240 12 Th = 231 upper limit at nuclear charge (2) = 71 (2). These to show certain parallels in properties to those of the considerations and their subsequent extension to the d species, but differences that reflect the more extensive preferential and essentially sequential occupancy of the shielding of the f orbitals. In the sense that these ele- 4j orbitals as being responsible for both the existence ments follow directly those in Group IIA, their of and the similarities in properties among the lan- atoms and ions may be expected also to reflect trends thanides provided the rationale for the logical inclusion in properties noted in the sequence Group IA-Group of these elements in a suitably modified form of the IIA. Both of these expectations have abundant experi- periodic table. mental support. It is unnecessary in this brief summa- Earlier theoretical and semitheoretical postulations tion to delineate in any more detail periodic relation- as to the existence of a second f-type transition series ships to other elements. were verified by both a reexamination of the naturally occurring elements Ac-U and an evaluation of the Periodicity between the Lanthanide and Actinide Series synthetically produced transuranium species (3). Elec- tronically, of course, the elements in the series Ac-Lr Periodicity between the two series should be a con- sequence of recurring electronic configurations of the (2 = 89-103), i.e., the actinides, proved to be exact or nearly exact analogs of the lanthanides. Except for the inevitable geometrical problems im- posed by the necessity of crowding, the modern extended tabulations based upon ground-state electronic con- figurations of the atoms accommodate both the lan- thanides and the actinides without problems and show their periodic relationships to the other elements. The table proposed by Seaborg (4), and included here as Figure 1, does this quite clearly and in addition in- dicates similar periodic relationships based' upon yet undiscovered elements of the 6f type. In the majority of these tabulations, lanthanum and actinium are iu- cluded as direct electronic analogs and congeners of scandium and yttrium. The lanthanides are then the elements Ce-Lu, inclusive; the actinides Th-Lr, inclu- sive. However, propertywise, and thus operationally, lanthanum and actinium may be considered in these categories. It is apparent, of course, that the lan- thanides provide a bridge between the elements of Groups IIA and IVB. The actinides should then do likewise, with Element 104 being a congener of hafnium. Extended forms of the periodic table, such as that shown in Figure 1, have the advantage of pointing out clearly the electronically separate classes of elements and relating each class to the other. Thus the lan- thanides and actinides are clearly members of transition series within d-transition series, or of inner transition Figure 1. Extended form of the periodic tabla showing accommodation series (5). As such, their atoms and ions may be expected of the Ionthanides and actinide.. Reproduced from reference 14). 41 8 / Journal of Chemical Education same type. The j-transition series owe their existence discernible tendency, wherever possible, to achieve to decreases in both the potential energies and spatial the nf7 and nf" configurations sooner than a strict extensions of the f orbitals immediately after the ele- nuclear charge sequence would dictate. This tendency ments lanthanum and actinium, which allow preferen- can be associated with the enhanced spectroscopic tial occupancy of these orbitals by the differentiating stabilities of the half and completely filled sub shells. electrons. The effect is particularly evident after In Figure 2 (lo), a comparison of the energies of the lanthanum (6-S), where, for example, the binding 4f"-15d16sz and 5f"-16d17s2configurations with those of energy of a single 4f electron drops from -0.95 eV for the 4f"6s2and 5f"6szconfigurations, respectively, shows the La atom to -5 eV for the Nd atom. Although the this tendency rather clearly. 4f orbitals lie well outside the xenon structure in the Comparisons between the lanthanide and actinide La atom and are thus unoccupied, they are within that series yield the regularities summarized in Table 5. In structure in the Nd atom and occupied. The situation considering these regularities, one must realize that the is comparable among the succeeding elements. With data used apply only to the gaseous ions in their lowest the actinides, the circumstance is similar hut much less energy states and thus do not necessarily reflect ob- pronounced. The net result is difficulty in establishing served situations. The +1, most of the +2, the lighter exact parallels between the lighter lanthanides and the +3 (for the actinides), and the heavier +4 (for the potentially corresponding actinides in both electronic actinides) ions have no existence in either solution or configuration and properties. This difficultyis reduced crystal lattices. Within these limitations, however, as the number of 5jelectrons present increases. periodicity between ions of the same charge type in a The ground-state electronic configurations and spec- given vertical column of Table 5 may he expected. troscopic terms characteristic of neutral atoms and Periodicity is indeed recognized in several areas of com- cationic species are summarized for the lanthanides parative chemistry, some of which are now considered (Ln) in Table 3 and for the actinides (An) in Table 4.
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