Torolf Ternstrom Sturevagen 34 182 74 Stocksund Sweden
Subclassification of Lanthanides and Actinides
In "A Periodic Table" by the author1 a special subclassification of the lanthanides and actinides was suggested and . . some reasons for the groups in question were given. New and more c o m ~ l e t earguments for the suitability of such a subclassiiiratim are now presented below. In a modified table of the aforesaid type with extradotted lines, Eu-Sr, Cu-Na and Ce-Zr, Sc-AI, t h e groups are recognized as I-groups, I standing for Internal Transition Elements in contradistinction to E for External Transition Elements and N for Nontransition Elements. T h e I-group elements together with corresponding "section"-periods are tabulated in the figure. In this connection i t should also he noted that such a subclassification is not restricted to the aforesaid type of Periodic Tahle. but mav be a d a ~ t e dfor anv conventional tvDe .. of Periodic'~able. With reference to the 13-elements (Gd-group) it is wellknown that 3+ ions with half-filled or fully-filled antepenultimate shells have inert gas character, just as the 2+ ions with fully-filled penultimateshells-in thglast case this character is weak with half-filled shells (Mn). I n consequence the 13elements 1) show colorless simple ions of the 3+ oxidation state, which are
stable in aqueous solution, whereas simple ions of Gd(I1) and Cm(IV)cannot exist in such a solution, the elements Ce and Yb, which also show colorless ions of the 3+ oxidation state, being differentwith reference to Ce(1V)and Yh(II),and the observed 6+ oxidation state of Cm being due to a misunderstanding 2) show similar hex. crystal structures 3) form similar sesquioxidesof the hexagonal,monoclinic and cubic strmtlm?r .~~~ ~
3 h w wn~1arrlr~tronircunfigurations with one d-electron 5, hiwe pn~pertirsin the above respects similar t o rhuxe of the 41
Sc-group.-rf. the k2-elementstZn, Cd, Hg)and Be. Mg. The 12-elements (Eu-group) are no doubt different from those of the previous group, especially due to pronounced 2+ oxidation states. They are, however, each in this and other respects rather close to the preceding elements in every "section"-period (fig), but nevertheless also notably different from these elements. Thus with reference to Eu and Yb they form salt-like compounds of the 2+ oxidation state similar to Sm and Tm, but the compounds of Eu and Yb are more stable than those of the latter ones. More remarkable differerences, even in relation to other preceding elements and also in relation to the following 13-elements, are that Eu, Yb, e.g.
......
......
Ce Pr Nd P m T b DY H o Er T h Pa U Np Bk Cf Es Fm
I4 15
I6
S m Eu
Gd
T m Y b Lu Pu A m C m
Md No
Lr
I2
13
6A
. . . . . . . . . . 6B 7A . . . . . . . . . . 7B
Subclassificationof lsnmanidas and actinides into lqroups together with corresponding "section"-periods 1) show a metollie valence of 2+, in contrast to 3+ for the other lanthanides 2) show only cubic crystal structures, that of Pm being uncertain 3) show mueh lower melting points, densities, and residual resistivities at 4.ZPK 4) show mueh larger metallic radii and atomic volumes
5) show much higher compressibility 6) dissolve in liquid ammonia at -7E°C to give blue solutions,
and 7) are in several of the above respects closer to Ca, Sr than to the other lanthanides,-cf. paragraph 2 ahove
Similarly Am is also rather closely connected to the preceding elements with regard to oxidation states in general and corresponding compounds, but differs in other essential respects. Thus Am 1) is the first actinide being most stable in oxidation state 3+ and the 3+ ion is also similar to those of the lanthanides 2) appears in oxidation state 2+ in solid solution, as do most of the lanthanides, this, however, not being the case with reference to the preceding elements Np, Pu or the following element Cm, AmJnnow also having been prepared 3) shows crystal structures notably different from those of the preceding elements and close to those of the lanthanides-cf. U-metals below 4) shows mueh higher melting point than the preceding elements 5) shows much lower density than the preceding elements4f. Eu, Yb above 6) shows a much larger metallic radius than the preceding elements-cf. Eu. Yh above :I ii the fir%!nrtinlde shwing all the types of sesquioxidrv of hexagonal, mundmu, and cubic srrurtur? appearin!: among the lanthanides Regarding properties according to (I), (3)-(7) Am is no doubt also close to Cm and according to (1)-(4), (7) closer to the lanthanides in general than to Eu and Yh. However, with regard to all properties specified above as well as to the great volatility in relation to its neighbors on both sides and the electronic configuration it does not seem unreasonable to combine Am with Eu, Yb-cf. also end of next paragraphalthough the differences are obvious. Anyhow, a combination of Am with U, Np, Pu or possibly Cm does seem much less attractive. As to No, the chemical and physical properties are not yet much known. Nevertheless. it seems evident that the most stable oxidation state is 2+; in contrast to both Md and Lr, which fully justifies its place in the 12-group together with Eu and Yb. A comparison with Ag in the El-group (Cu, Ag, Au) may also be justified, inasmuch as No, behaving in aqueous solution like an alkaline earth metal, may be the I2-element that is closest to Ca, Sr, just as Ag is the El-element that is closest to Li, Na-cf. figure ahove. In this connection it may also he suitable to emphasize that divergences of the chemical and physical properties among the 1.'-elements have good correspondence with similar divergences of these propertiea among the El-elements. The 16-group, like the E8-group (Fe- and Pt-metals) is subdivided, the numeral 6 referring to the highest most stable oxidation state (U)-cf. I3-group. Dealing first with the Umetals (U, Np, Pu) it is obvious that these elements are different from other I6-elements as to oxidation states, but also TernstrBm, T., J. CHEM. EDUC. 41,190-191 (1964). Eyring, LeRoy in "The Encyclopedia of Chemical Elements," (Editor: Hampel, C. A,), Reinhold, New York, 1968, p. 562. Volume 53.Number 10, October 1976 / 629
different from the surrounding elements on both sides (Pa, Am)-cf. Am above-since they 1) show
higher oxidation states WVI), Np(VII), Pu(VIl) with reference to each side Pa(V),Am(VI), the announced 7+ oxidation state of Am evidently being based on a misinterpretation 21 show mutuallv simrlsr crystal types of at least the o-rhumbie, trlrnhedrd, and hcc structures, norahly d~fferrntfrom the trtrnhcdrnland hrcstrurturrsuf Paand thr hexagonaland t'cr structures of Am 3) show lower melting- points, much lower with reference to each side 4) show much higher densities sl ~. show hvdrated.hinaroxides of the 6+ oxidation state 6) show notably smaller metallic radii 7) show metal (111-IV) oxidation potentials closer to each other than to Pa and Am, respectively 8) show much longer half-livesof the most long-lived isotopes ~
Referring to the lanthanides of this group (Nd-metals), i t is well known that the 3+ ions of these elements are strongly colored, which however also refers to Pr, Dy in 15, whereas the 3+ ions of Ce, Yb are colorless and those of Eu, T b are only faintly colored. These facts may indicate that the lanthanides within groups I5 and I6 should be kept together. However, considering also other properties the present arrangement seems more reasonable, since the 16-elements of the lanthanides e.g. 1) show salt like stoichiometrichalides of the 2+ oxidation state
melting at higher temperatures, whereas the corresponding halides of Ce, Pr are metallic and the salt like DyC12 decompases at higher temperatures, halides of Pm(II), HOW),Er(I1) still being unknown-ef:also Eu, Yb above 2) show melting points in the two "sectionv-periods(fig.) somewhat closer to each other than to Pr and Dy,.respectively-cf. alsoEu, Yh above 3) show residual resistivities at 4.Z°K much higher than those of Pr and Dy-cf. also Eu, Yh above 4) show wavenumbers of the first intense 4f-5d transition of tri-
& t o Eu and Yb, respectively 6) show as regards Nd and Sm initial oxidation rates and carrwion rates in 1N NaOH much closer to each other than toPr, as well as to Eu 7) show as regards Ho, Er, Tm a) hexagonal crystal lattices possessing the lowest c/o ratios of all hexagonal metals, h) Curie temperatures very close together, the Curie temperature for Dy being much higher, and el Brinell hardnesses close together, the corresponding value for Dy and also for Yb being notably lower With regard to most of the above and other properties, it may also he reasonable to subdivide the lanthanides of I6 into two groups, lighter Nd, P m and Sm and heavier Ho, Er and Tm. As to the last elements of the 16-group the Es-metals (Es, Fm, Md) the chemical and physical properties are not much known, but they seem to be alike, since besides the most stable 3+ oxidation state the 2+ oxidation state is also known for all of them, but no higher oxidation states, although it is assumed that the 4+ oxidation state is possible for all of them. This is in contrast to Cf with a known 4+ and a probable 5+ oxidation state cf. below-and also in contrast to No with its most stable 2+ oxidation state and no supposed 4+ oxidation state. The metal (111-IV) oxidation potentials of Es, Fm, Md are also rather close together and rather different from those of Cf and No, respectively. Moreover, the half-lives of the most longlived isotopes of Es, Fm, Md are very close together indeed and 630 / Journal of Chemical Education
remarkably different from those of Cf and No. The 14-elements are all known in the 4+ oxidation state as maximum, the stability of the corresponding compounds being in the order T h > Bk > Ce > Tb-Bk(1V) as Ce(1V) thus heing stable in aqueous solution in contradistinction to Cm(1V)-which suggests the Th-group as designation for this group. Since recently the 2+ and 3+ oxidation states of T h have been verified, although the 2+ oxidation state is only of formal character, and particularly since the gaseous Th3+ unambiguously favors a configuration with an f-electron instead of a d-electron. T h has come closer to the other elements within the group with their most stable 3+ oxidation states. Possible alternative configurations of T h and Bk including a d-electron also improve the similarities within the group. Other similarities may also be referred to, e.g., crystal structures and binary oxides, the mutual connections Ce-Th and Tb-Bk in different respects not to be forgotten. The rather close relationship between the 14-elements and Zr, just as between the E3-elements (Sc-group) and Al, should also be noted-cf. figure above. On the other hand, there are no doubt reasons to combine a t least Ce and T h with P r and Dy and possibly also Nd, in view of the 4+ oxidation states of these last three elements. However, as already pointed out above, there are also good reasons to combine all the lanthanide elements in I5 and 16. In this connection. i t was also emohasized that obvious differences do exist n i t only between P r and Nd but also between Ce and Tb,and P r and Dy (3+ ions). The paralinear oxidation of Ce and Th, as distinguished from other lanthanides, is another difference between Ce and Th, and P r and Dv. It is also assumed that the 4+ oxidation state of ~m and Sm is possible. These facts seem to favor the present suggestion with respect to P r and Dy occupying an intermediate position. See also below with reference to Pr and Dy. Among the 15-elements P a has for a long time been the only element within the lanthanides and actinides having a well established maximum 5+ oxidation state. Recently, however, a higher oxidation state, probably 5+, has been reported for Cf, which would positively connect this element with Pa, Cf(IV), to be compared with Pa(IV), having been known for some time. Furthermore. since the 2+ and 3+ oxidation states of P a have now been verified, although the2+ oxidation state is onlv of formal character. and can be c o m ~ a r e dwith the known 2+ and the most stable 3+ oxidation state of Cf, it is evident that there is a good correspondence between existing oxidation states of P a and Cf. The 5+ oxidation state for Pr, previously reported, has not been verified up till now, but according to Eyringz "Pr5+ is not completely ruled out." Anvhow it does not seem unreasonable to connect Pr. Dv. h&ing distinyuished characteristics between Ce, T h and h.d; Ho.t u Pa. Cf. e r m :f the similarities betu,een the 15-elrmen~5, taken all tugethrr, in one respect or a n o t l w , are not pnrtlcu1,irlv oronouncrd. A suhnrou~inrwithin 15 with 1me I'r-yrouo (pry ~ yand ) one pa-gro;p (Pa, CD could perhaps he jugifieh for the time being. but since there are also certain connections I'r-l'n and I ) ? - ~ fe, g . the electron configurations, especiillly with regard to i)\,-(:i,and possihl\~alsuwith regard to I'r-Pa. as a ~r&nfigurkinn inciuding H d-electron has also been supgested, it seems more adequate to include all the 15-elem G t s in one group, designated the Pa-group, for similar reasons as with recard to the Th-croup (cf. above). The fact that all the elements in the 15-group do not show the maximum oxidation state for the group does not seem to constitute a hindranceeither, especially in regard to the already existing groups N6 (chalcogens), N7 (halogens), and N8 (noble cases).
