A Suggested Revision of the Position of Thorium in the Fourth Period of the Periodic Table G . E. VILLAR University of Montevideo, Montevideo, Uruguay used for the industrial production of thorium comCRUTINY of the succession formed by the atomic pounds is monazite sand, which is largely compoked of numbers of the noble gases, and the analogous complex phosphates of thorium and the rare earths (6). In other words, the primary source material for relation between corresponding elements of the second and third periods, and also the third and fourth thorium compounds is the same as that used preferperiods of the Periodic Table, has led us to accept and entially for obtaining the rare earths. In addition, the most important of the chemical chardevelop the hypothesis of J. Pemn (I)on the existence acteristics of the rare earths is their precipitability by in period 7 of a series of elements similar to the rare earths, whose position would correspond to that now oxalic acid from acid solutions, by which method the group can be separated easily from other substances. occupied solely by actinium (2). This belief is supported by the work of I. Curie and But this property is possessed also by thorium, whose P. Savitch on the transuranium element Rksh obtained oxalate is less soluble than the oxalates of the rare by them by neutron bombardment of uranium (3). earths in strongly acid media, a property which is utiThis element is characterized by properties similar to lized in the analysis of that element (7). At first glance, the tetravalency of thorium, which those of the rare earths, and by a close resemblance to justifies its position in subgroup (A) of Group IV, would actinium. On the other hand, the properties of neptune (93259), present difficulties in transferring this element to suba radioelement of 2.3 days' period obtained by Mc- group (A) of Group 111, composed of trivalent eleMillan and Abelson by activating uranium with neu- ments such as scandium, yttrium, and the rare earths; trons, have suggested to these investigators the prob- nevertheless, it is important to note that the element able existence of a second series of "rare earths" with which occupies the second place in the rare earth series which uranium and element 93*" would be associated and which would be the homolog of thorium in the actinium series is cerium, which is characterized by (4. G. Mayer has recently confirmed the possible exist- being tri- and tetravalent. The oxide of thorium (Tho*) has the same empirical ence of the second series of "rare earths" predicted by formula as cerium oxide (CeOz), the only stable anhyMcMillan and Abelson, coming to the conclusiou that drous oxide of cerium (8); besides, cerium compounds this series would be found in the proximity of element in general have the same empirical formulas as the Z = 92 (5). These experiments come close to confirming the pos- corresponding thorium compounds. The oxides of thorium and cerium are characterized sible existence of a second series of "rare earths" in the by being basic, while the oxides of titanium and zirlast part of the Periodic Table, which, according to what we have indicated, would begin with actinium (2 = 89). conium, elements in the thorium subgroup, behave like This hypothesis raises two questions: should the acid anhydrides (8). Salts of thorium, like those of cerium, have the propknown elements that follow in the natural classification occupy the position of actinium? That is, should erty of forming numerous addition compounds; thus, thorium, protoactinium, and uranium be transferred to for example, there are several series of derivatives of this position? Or what appears less probable, is it neces- cerium nitrate homologous to the corresponding derivasary to increase by fourteen units the atomic numbers tives of thorium nitrate. Among these are found: of the above elements, in order to maintain their usual Ce(N03t.2M'N08 and Th(NO&.2M'NOs position? In this regard, we have verified the fact in which the radical M' corresponds to monovalent that the atomic numbers and X-ray spectra of these radicals: NH4, K, Na, Rb, . . . . elements comply with Moseley's law. Ce(NOJ,.Mv(NO&.8Ha0 and T h ( N O s ) r . M " ( N 0 3 ~ ~ 8 H ~ O The considerationsto be detailed tend to justify moving thorium into the position occupied by actinium, in which the radical M" corresponds to the bivalent which would represent a modification of Groups I11 radicals: Mg, Zu, Ni, Co, and Mn. and IV of the Periodic Table. Also, the chlorides of cerium and thorium form with certain organic bases a series of homologous compounds CHEMICAL ANALOGIES BETWEEN THORIUM AND THE RARP. such as the following: EARTHS A NEW SERIES OF RARE EARTHS
S
The close similarity of thorium to the r q e earths is also manifested in nature, where the only mineral
CeCL.2(CJIsN .HC1) and ThCI,.Z(CsHaN.HCl) C ~ C I ~ . Z ( C Q .HC1) H ~ N and ThCI,.2(CgH,N. HCI)
Thorium, like the rare earths, combines with hydro- is less than the average for the atomic masses of the neighgen and nitrogen, forming the corresponding hydride boring elements. Thus, for example, (ThH,) and nitride ( T h a d ; in contrast, titanium AP AB, - 19.00 2 79.916 = 49,458 forms only adsorption compounds with those gases (8). Ao, = 35.457 < 2 The chemical analogies existing between thorium and the rare earths and especially between thorium and cerium, similar although not as marked as those shown between actinium and lanthanum, would justify transfemng thorium to subgroup (A) of Group I11 of the Periodic Table, in which the analogies are much better The only exceptions to this rule are presented by the than those that can be established between thorium and atomic masses of hafnium, tantalum, and tungsten: the elements which actually do adjoin it in subgroup (A) of Group IV.
+
+
PHYSICAL ANALOGIES BETWEEN THORIUM AND THE RARE EARTHS
The crystal structure of thorium is that of a facecentered cube, similar to that of lanthanum, cerium a, and presumably similar to that of scandium and yttrium of subgroup (A) of Group 111; in contrast titanium, zirconium, and hafnium which form with thorium subgroup (A) of Group IV have a hexagonal crystal structure. Thorium carbide (ThG) crystallizes according to the tetragonal system, the same as the carbides of lanthanum (LaCz), praseodymium (PrCp), and neodymium (NdG), while the carbide of titanium (Tic) crystallizes in the cubic system the same as the carbide of zirconium (ZrC). Furthermore, the oxide of thorium (Thoz) crystallizes in the cubic system; cerium oxide (Ce02) and praseodymium oxide (Proz) crystallize in the same way, while the oxide of titanium (TiOJ crystallizes in the orthorhombic or tetragonal systems and the oxide of zirconium (ZrO,) in the monoclinic, tetragonal, hexagonal, or cubic systems (9). The isomorphism between cerium and thorium has been placed in evidence by the preparation of mixed crystals of thorium and cerium sulfates ThCe(S0a)z. 4Hz0; similarly the double nitrates of cerium, thorium, and ammonium (Ce, Th)(NH4)z(N08)s. This isomerism has been further confirmed by the X-ray analysis of the oxides CeOz and Thoz (10). ABNORMAL RELATION BETWEEN THE ATOMIC MASSES O F THORIUM, PROTOACTINIUM, AND URANIUM AND THOSE OF THE ELEMENTS WHICH COMPOSE WITH THEM GROUPS IV, V, AND VI OF THE PERIODIC TABLE
If one compares the values for atomic masses within a given group of the Periodic Table, a general rule will be observed: The atomic mass of an intermediate element
These three exceptions to so general a rule must be ascribed to the incorrect positions of thorium, protoactinium, and uranium in subgroups (A) of Groups IV, V. and VI. It is interesting to note that if one accepts the value 229 for the atomic mass of actinium, the atomic mass of lanthanum agrees with the rule previously euunciated: AL.
=
138.92 < Au
+ A*, 2
- 88.92 -
+ 229 = 158,96
2
which confirms the correct position of the last-cited radioelement. CONCLUSIONS
The hypothesis of the existence of a second series of rare earths in the position of actinium (Z = 89) raises the question whether the place now occupied by actinium should be filled by the elements which follow i t in the Periodic Table, or whether their respective atomic numbers should be increased by 14 units. As for thorium, study of its chemical and physical properties justifies its transfer to the position occupied by actinium. In its new position thorium would be found in subgroup (A) of Group 111, composed of elements whose similarity to i t is greater than that found by those forming subgroup (A) of Group IV, in which thorium is placed a t present. Furthermore, a comparative study of the atomic masses of the elements grouped with thorium, protoactinium, and uranium would render manifest the incorrect position of the elements mentioned and would suggest their transfer to the position occupied by actinium.
LITERATURE CITED
PERRIN, "Grainsde matiPre et delumike," Hermann et Cie., Paris, 1935. VELAR. Ann. Acad. B r a d Sci.. 12. 51-7 (1940). CURIEAND SAYITCA. Comfit. rend.. 206,906 (1938). MCMILLAN AND ABELSON, Phys. Rev., 57, 1185-6 (1940). MAYER, ibid., 60, 18P7 (1941). PASCAL, "Trait6 de chimie min6rale." Vol. XI, Masson et Cie., Paris, 1932.
(7) Scorr, "Standard methods of chemical analysis." 5th ed., D. Van Nostrand Company. Inc., edited by F-N, , New York C ~ t y1939. (8) SPENCER, "The metals of the rare earths," Longmans, Green and Company, New York City and London, 1919. (9) WYcKoaa, "The structure of crystals," 2nd ed., The Chemical Catalog Company, Inc., New York City, 1931. (10) PASCAL, "Trait6 de chimie minbale," Vol. VIII, Masson et Cie., Paris. 1933.
