GEORGE GLOCKLER and ALEXANDER I. POPOV State University of Iowa, Iowa City, Iowa
T H E value of the periodic table in the teaching of all chemistry need hardly be emphasized. A single page summarizes thousands of facts concerning physical and chemical properties of the 98 known elements, and elevates inorganic chemistry from a collection of seemingly unrelated data t o a systematic science. So great is the success of Mendeleev's and Lothar Meyer's discovery that its limitations are not always recognized and stressed. It is becoming more and more evident that in many cases the analogies predicted by the periodic table are only approximate, and that the generalizations cannot be carried too far. Only in the light elements do the chemical and physical properties strictly obey the periodic law. As the electronic stmcture becomes more complex, and the inner orbits begin to fill, the chemical analogies become less pronounced. The greatest difficulty in drawing up a periodic table is the assignment of position to the rare earth elements. I n the more conventional types of periodic tables the rare earths are usually separated from the main body of the table and are presented apart in a horizontal line. After the discovery of the transuranium elements Seaborg (1) postulated the existence of a second rare earth series beginning with thorium, mhich he proposed t o call the "actinide series." The second incomplete rare earth series was separated from the table and placed in a horizontal line below the lanthanides. This procedure resulted in thorium, protactinium, and uranium losing their positions as homologues of hafnium, tantalum, and wolfram, respectively. Recently it was pointed out by Haissinsky (t,5 ) , Paneth (4, 5), Sidgwick (6), and others, that in doing so Seaborg has completely neglected all the chemical evidence relating the withdrawn three elements to columns IV, V, and VI of the periodic table. Such separation of these two series of elements from the main table %-auld indicate that their chemical properties are as similar as the chemical properties of the isotopes of one element. This statement is not tme for the lanthanides, where not only is there considerable difference in chemical properties between, say, praseodymium and lutetium, but also some of the elements possess stable oxidation states different from three. These differences are still more pronounced in the actinides. The chemical properties of neighboring elements of the two series may be similar, but in many cases no more so than the neighboring transition elements, and in the case of thorium, protactinium, and uranium, certainly much less. Since the periodic table should represent the variation of chemical properties of the elements as a function
of the atomic number, it seems reasonable to expect that all the elements should be included in a single picture. Such attempts have been made in the past. For the most part they were rather complicated arrangements, often in three dimensions (7, 8). Another table of this type is the modification of the Bohr-Thomsen-Akhumov (9,10, 11) periodic table shown in the figure. It includes all the known elements and is thought to be of a much simpler form than previous attempts in this direction. The inclusion of the neutron can be justified by regarding it as an "element" with atomic number 0. The number of electrons in a shell is given by the expression 2n2 (IZ), where n is the principal quantum number. In the case of the neutron it can be said that n = 0 and the neutron is the first member of the periodic table with zero nuclear charge (IS). While the addition of electrons occurs in the outer shells, the variation of the chemical properties smoothly follows the periodic law. Elements in the same family have, in general, similar outer electronic configurations and, therefore, the same spectral terms for the ground state of the atom. The first major discrepancy occurs in the sixth row. The first member, .cesium, has xenon configuration and one 6s electron; the 6s shell is completed in the next element, barium, and the next one, lanthanum, has one 6d electron. Cerium adds two 4f electrons while losing the 5d one. Its electronic configuration is quite different from its homologues, zirconium, hafnium, or thorium. The 4f electrons are somewhat more strongly bound than the 5d ones and, therefore, it is possible for cerium to form a trivalent ion with the configuration Xe4f1 (Xe = xenon configuration). However, cerium also readily loses the last 4f electron and in doing so the tetravalent ion becomes a homologue of the elements in the fourth column and hAs the same electronic configuration in the outer orbits. The next rare earth element, praseodymium, has three 4 j electrons in the ground state. It easily gives the trivalent ion with the stmcture Xe4p and can even lose another of the 4f electrons to give the tetravaledt oxide. The 4f shell becomes more stable, and, as w$s shown by the recent work of Marsh (Id), Rabidean and Glockler (15), and McCullough (16) the pentavalerit state of praseodymium, which wonld be homologous to the elements in the fifth column, is, as yet, unknown. In neodymium, the 4f shell becomes even more stable and the +3 oxidation state seems to be the only one found so far (17). With ytterbium, the 4f shell is complete and this explains the ease with mhich this element can be obtained
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APRIL, 1951
in the divalent state, analogous to zinc and cadmium. The next element, lutetium, is akin t o the trivalent elements in the column 111-B. The question as t o where do 5f electrons begin to be added is still largely unsettled. Spectroscopically they were found in uranium (IS), but not in thorium (19). Recent investigation of the +3 oxidation state of thorium (20) seems t o agree with the view that this element does not possess any 5f electrons. Seaborg (1) has stressed, however, that the differences in the energy levels of 5f and 6d electrons may he of the order of magnitude of a chemical reaction. Thus the presence or the absence of the 5f electrons in the neutral atoms of the first three members of the actinides is relatively unimportant, since the chemical analogies are based on their highest oxidation states. This view makes thorium, protactinium, and uranium bona jZe members of columns IV, V, and VI of the periodic table, without denying the possible existence in their neutral atoms of the 5f electrons. It is seen that the modified periodic table does in fact relate the disputed elements to the rare earths as well as to their more natural chemical homologues. It is quite true that neither the oxidation states of praseodymium and neobium, or neodymium and molybdenum, or their chemical properties are in any way similar; however, this is inevitable because of the sudden appearance of the 4f level at this point. A glance a t the conventional form of the periodic table shows that a t present the oxidation state is not necessarily the guiding principle in the assignment of positions to elements. In some cases, as with silver, the lowest valence determines the position of the element; in other cases, as with vanadium, it is the highest. It is also evident that the oxidation state determining the location of an element in the table is not necessarily the most stable one. Such is the case of Cu, Au, Sn, Pb, and some of the halogens. I n some cases (Ow, F+?,Br+', transition elements) the oxidation state indicated by the column remains unknown. With further development of inorganic chemistry and especially with the increased knowledge of the "unusual oxidation states" (Sf), valency ceases to be the
paramount criterion for the position of an element in the periodic table. Frequent compromises have to be made as to the location of an element in the periodic table, depending on the choice of the physical or chemical property used as a guide (3). Many different arrangements are possible, and the periodic table proposed herewith is one of them. LITERATURE CITED (1) SEABORG, G. T., Science, 104, 379 (1946). (2) HAISBINSKY, M.,Bull. Soe. Chim. France, 16, 668 (1949). (3) HAISSINSKY, M., J. de ehim. phys., 47, 415 (1950). (4) PANETH, M., LesIsotopes, "Congres Solvay" Brussels, 1947. (5) PANETH,M., Nature, 165, 748 (1950). (6) SIDGWICK, N. V., "The Chemical Elements and Their Compounds,'' Oxford University Press, London, 1950. (7) ESTEE,C. R., Ph.D. Thesis, State Uniwrsity of Iowa, August, 1947. (8) CWK, J. D.,Science, 111, 661 (1950). (9) BOHR,N.,2. physik., 9, 1 (1922). (10) THOMSEN, J., Z.amrg. Chem., 9, 192 (1895). (11) A K H U ME~.~I.,, J. Gen. Chem. U.S.S.R., 17, 1241 (1947). (12) RUARK,A. E., AND H. C. UREY,"Atoms, Molecules and Quanta," McGraw-Hill, New York, 1930, p. 273. (13) AKHUMO~, E. I., J . Gen. Chem. U.S.S.R., 16, 961 (1946). (14) MARSH,J. K..J. Chem. Soc., 1946, 17. S., AND G. GLOCKLER, J . Am. Chem. Soc. 73, (15) RABIDEAU, 488 (1951). (16) McCur,~ou~n, J. D., J. Am. Chem. Soc., 72, 1386 (1950). (17) Popov, A. I., AND G. GLOCKLER, ibid., 71, 4114 (1949). (18) SCHWRMANS, P., Physzea, 11,419 (1946). (19) MEGGERS, W. F.,Saenee, 105, 514 (1947). (20) WARF,J. C., p. 2Q Abstract, 118th Meetingof the A. C. S., Chicago, Illinois, Sept. 3-8 (1950). (21) KLEIXRERG, J., J. CHEM.EIIUC., 27, 32 (1950).
