A MODIFICATION OF THE PERIODIC TABLE L. M.SIMMONS The Scots College, Sydney, Australia
IN
THEIR review of types of graphical classifications of the elements, G . N. and Mary Battell Quam (5) refer to those two-dimensional tables in which the elements of each period are arranged in a single series as "long charts." Mendelcev's so-called vertical table, published in 1869, was of this type, while his better-known table of 1872 contains two series of elements in each long period and is therefore regarded by the Quams as a "short chart." The Quams subdivide the long charts into Werner and Bayley types, the difference being that in the latter an attempt is made to show the relationships within the short chart groups without destroying the advantage of the simplicity of the Werner type. Werner's original table was published in 1905 (7) and was designed to overcome the difficulties encountered in placing certain elements in the periodic system, especially the 8th group and rare earth e l e ments. Werner regarded the grouping of elements into a single space as an emergency measure necessitated by an endeavor to consider too many analogies. Werner's table therefore showed only one element in each tabular space. Modifications of Werner's table have been made in subsequent years; the latest of these is that rccommended by Laurence S. Foster (1). These modifications contravene Werner's implied principle of one element, one space, in that they group together the rare earth metals in one or two tabular spaces. An outstanding contribution made in Werner's table was the determination of the width of the table by the length of the longest period. However, this introduced in all but the longest periods many medial gaps which were not intended to represent undiscovered elements. Modifications of Werner's table, such as that of Foster, reduce the number of these medial gaps by the expedient of again grouping together the rare earth elements in one tabular space, without intending to imply that they are isotopes. The existence of even the reduced number of medial gaps, and the use of subsidiary tables for the rare earth elements, detract from the simplicity with which long charts display the periodic law, and verbal descriptions of the construction of the charts are very involved. Furthermore, the work of Seaborg (6),Kiess, Humphrey~,and Laun ($) makes it desirable that the transuranium elements shall be allocated to their correct positions in the main table. It is therefore proposed to show that the following simple prescription leads to the construction of a long chart which possesses only those medial gaps which
imply elements which have not yet been isolated, and which displays in full the rare earth elements and their known homologs. The resulting table transfers the emphasis from the inert gases to the alkaline earth metals, and for this some justification is offered on the basis of atomic structure. Construction of the New Long Chart. In order of increasing atomic number, starting with hydrogen, the elements are placed in a horizontal series from left to right until the first alkaline earth metal has been placed; a new series is then commenced below the former and sufficiently far to the left to allow the second alkaline earth metal to fall directly below the first. After the second alkaline earth metal has been placed, a third and succeeding horizontal series are similarly formed, in each case starting sufficiently far to the left to allow each alkaline eartkmetal to fall vertically below those terminating the previous series. The decision as to which elements constitute alkaline earth metals may rest either on the chemical properties of the elements, or may be made by determining their atomic numbers from the expression:
where y is any positive integer. This expfession is the sum t o y 1terms of the mathematical series:
+
2X1~,2Xl~;2X2~,2X2~,2X3:2X3P,2 ,... X4a
The Relationship, of the New Long C h r t to That of We-rner. In all modern modifications of Werner's table, such as that of Foster, the elements Be and Mg have been transferred to positions above Ca. If, now, H is transferred to the position above F, and the two left-hand groups, headed by Li and Be, are moved to the right of the table in such a way as to preserve the order of increasing atomic numbers, the new long chart (Figure 1) is obtained. Chemical Relationships Displayed by the New Long Chart. This derivation of the new lone chart from that of Werner ensures that the chemical-relationship8 displayed in the latter are retained, with the exception that H is now grouped with the halogens instead of with the alkali metals. In describing the periodic system, W. G. Palmer (4) wrote in 1944that: The ambiguity in the position of the lightest element, hydrogen, gave rise to lively dispute, and in the course of time opinion swayed between placing it as the first halogen, or as the first a l h l i metal.. .it is hardly worth while at the present time to rehearse the various arguments used to support the one or the other position.
Nevertheless, it may be pointed out that it does not
588
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RARE EARTH ELEMENTS Lanthsnide Series La Ce Pr Nd I1
c Sa Eu Gd Tb Dy Ho Er Tm Yb
57 58 59 60 61 62 63 Ac Th Pa U Np Pu Am Cm 89 90 91 92 93 94 95 96
Actinide
Swies
/
seem to he generally recognized that the hydrogen atom tends rather t o complete than to lose its electronic shell. Thus, the so-called hydrogen ion is nearly always solvated, e. g., as H:o:H+. . Unsolvated hyH drogen ions occuf as H- in molten metal hydrides. If the existence of protons in the gaseous state be used as an argument in favor of grouping H with the alkali metals, similar argument would class helium with the alkaline earth metals. Vertical and Horizontal Relationships. The elements a t the left of the new long chart show very strong chemical relationships along the two horizontal series; one of these horizontal groups of closely related elements is referred to by Foster as the Lanthanide Series of rare earth elements, while Seaborg has suggested that the second of these horizontal groups shall be called the Actinide Series. Proceeding toward the right of the chart, the transitional elements are encountered; these elements show only slight horizontal similarity. Still further t o the right the coinage metals (Cu, Ag, and Au) and volatile metals (Zn, Cd and Hg) show strong vertical relationships. To the right of the volatile metals the relationships become very strong along vertical lines, while horizontal relationships almost disappear. This gradual change from strong horizontal relationships a t the left of the chart, to strong vertical relationship a t the right, is marked by the stepped form of the new long chart. Integral Zones. The new' long chart displays the elements in integral zones (solid blocks) of related elements. Thus the lowest.step, at the left of the tahle,
.
E
contains the rare earth elements; the next step contains the transitional elements. At the right of the tahle are the alkali and alkaline earth metals; these may he regarded as constituting a zone of alkalme metals. To the left of the alkaline metal zone is one consisting of only the inert gases. At the left of these gases, omitting the contentious case of hydrogen, is a triangular zone of valencied nonmetals, cornered by B, F, and element 85 (Astatine). The elements As, Se, Sb, and Te, which show sufficient metallic and nonmetallic properties t o render classification difficult, form a square zone overlaying . .the middle of the hypotenuse .. of the triangle.' Between the triangle of valencied nonmetals and the volatile metals is another trianmlar zone. cornered hv Al, T1, and Po. This zone contains ail the metars having fragmentary p electron subgroups. The main zones may therefore be enumerated as follows : (i) Rare earth metals. (ii) Transitional metals. (iii) Coinage and volatile metals. (iv) Alkaline metals. (v) Inert gases. (vi) Valkncied nonmetals. (vii) p-valencied metals. The Periods and Grouys of the New Long Chart. The period numher of an atom in the new long chart may be found in the following way. The principal quantum number of'each electron in the atom is added to its serial quantum numher, and the maximum sum thus obtained is the period numher of the atom. The hydrogen atom possesses only one electron; its
JOURNAL OF CHEMICAL EDUCATION
principal quantum number (n) is 1, and its serial quantum number (1) is 0 : hence the period number of H is 1 0 = 1. Similarly, both electrons in He have n 1 = 1. Hence the first period of the new chart contains H and He. Lithium has three electrons having the following values for n, 1, and n 1:
+
+
+
n 1 1
1 0
2
0
n+l 1 1 2
0
from the inert gases to the alkaline earth metM group is so radical that it is not recommended. In this connection attention is drawn to the fact that in 1929 C. Janet (2) proposed such a transfer in his so-called helicoidal classification. Janet, wrote:' It has hitherto been customary to place helium at the head of the Group of inert gases. A number of considerations, however, point to this element being regarded at the head of the alkaline earth metals.
A further point of interest in Janet's table is that an intermediate step taken by him in the production of The maximum value of n 1 for Liis therefore 2, his helicoidal and spiral tables was the laying out of the hence Li belongs to period 2. Be also has a maximum elements in a manner closely similar to the present value of 2 for n 1, while all other atoms have (n proposed long chart. Janet's insistence on coiling the 1 ) . > 2 Thus Li and Be constitute the second period chart, together with the paucity of explanation, cause in the new long chart. Janet's table to be regarded as only touching the fringe The first two periods (H, He, Li, Be) are shown in the .of the present arrangement. same top horizontal series of the new chart; every The number of elements in the yth period of the new subsequent period occupies a new horizontal line. long chart is given by: T, = (y - (- I),)%; The start of each new period marks the formation of a this expression is the yth term of the series: new electronic subgroup, or the duplication of an 2 X 1 : 2 X 1 ~ , 2 X 2 : 2 X 2 2 , 2 X 3 = , 2 X 3 = , 2 X 4 a , ... existing subgroup in another shell. Thus the first period contains atoms possessing only one s subgroup and'dsers from the Rydberg series in having its fist of electrons; period 2 contains two s sub-groups with two terms identical, and therefore appears to be more .. no electrons of higher serial quantum number. Period symmetrical. 3 differs from the preceding by having one or more p It has already been mentioned that from this series electrons in one subgroup, while period 4 has two such the atomic number Z, of the yth alkaline earth metal p subgroups, but no d or f electrons. Period 5 marks may be expressed by: the formation of a d subgroup; in period 6 a second d subgroup exists, but there are no f electrons. In periods 7 and 8, atoms possess one and two f subgroups, respectively. The formation of a new period is thus marked by a In the same way the atomic number of the yth element suddenly increased complexity of the electron group- of any vertical group of elements is given by: ings, and this always occurs immediately after He or an For an alkali metal: Z,,= 2(, + 1) - 1 alkaline earth metal. In earlier tables the start of a For an inert gas: Z, = 2(, + 1) - 2 new period was marked by the formation of a new shell, For H or a halogen: Z, = 2(, + 1) - 3 little cognizance was taken of the electronic subgroupFor a member of the oxygen group: ings. There exists in this new long chart some lack of symmetry in that only the first two periods are written in the one horizontal line. This difficulty is probably no more serious than is usually found in placing H, For a member of the nitrogen group: Z, = 2 ( , 2) - 5 and sometimes He, in earlier tables. It is perhaps For a member of the carbon group: Z , = Z(, + 2) - 6 possible to write the first three periods thus: . For a member of the boron group: Z, = Z(, + 2) - 7
+
+
+
+
.
+
H He L i Be BCNOFNeNaMg In this way H is grouped with the alkali metals, indicating a tendency to form H + ions; sin& such ions are known probably only in gas discharge tubes, and He++ also occurs in such tubes, the argument leads to the grouping of He with the alkaline earth metals, and the marked differences in chemical properties between H and the alkali metals, and between He and the alkaline earth metals, can be ascribed to the considerable differences in atomic volumes. Acceptance of this view would produce an even more symmetrical table than that given in Figure 1,but the transfer of He
For a volatile metd:
For a coinage metal: ZZ,= 2 ( ,
+ 3) - 9
The remaining elemer~ts(the rare earth, transitional and transuranium elements) do not form well-related vertical groups. Advantages of the New Long Chart. The fact that the construction of the new long chart can be fully described in a simple sentence offers a considerable pedagogic advantage over the more commonly used tables which introduce what, a t least to a student,
DECEMBER, 1947
appear to be ad hoe adjustments. By avoiding the expedients of grouping elements into single spaces, subsidiary tables, and meaningless medial gaps, the new chart demonstrates more clearly the periodic law. The gradual change from horizontal to vertical relationships is clearly displayed, and the appearance of related elements in integral zones gives further regularity to the arrangement. The regularity is also improved by the fact that the numbers of elements constituting periods form a more complete mathematical series than Rydberg's in that all the known terms are duplicated. This has the further advantage that the algebraic relationships can be demonstrated between the period number and atomic number of any member of any related vertical group, and between the period number and the first two quantum numbers in any atom. By terminating the periods with alkaline earth metals, accent is thrown upon the effect of an increase in complexity of electron grouping on chemical properties. Summary. A modified Werner type of periodic table is constructed by placing the elements in a horizontal series from left to right in order of increasing atomic number until the f i s t alkaline earth metal has been placed. A second series is commenced below and snfficiently far to the left to allow the second alkaline
591
earth metal to fall directly below the first; subsequent series are formed by repeating this procedure. The resulting chart displays the usual groups of related elements and also shows a regular change from horizontal to vertical relationships, seven integral zones of related elements, and changes in the complexity of electronic grouping. The rare earth elements and their homologs are placed in the table without recoune to an isotopic grouping, subsidiary tables or medial gaps. The number of the period to which any atom b e longs is the maximum sum obtained by adding together the first two quantum numbers of each electron. The numbers of elements constituting the periods form a Rydberg series with the first term duplicated. LITERATURE CITED (1) FOSTER, L A ~ N C S., E J. CHEM.EDUCATION, 23, 602-3 (1946). (2) JANET,C., Chern. News, 138, 3 7 2 4 , 388-93 (1929). AND D. D. LAWN, J. Research (3) Kmss, C. C., C. J. RUMPHREYS, Nat. Bur. Standard+ 37, 57-72 (1946). W. G., "Valenoy-Classical and Modern," Cam(4) PALMER, bridge University Press, London, 1944, p. 28. (5) QWAM, G. N., AND MARYBATTELL QUAM,J. CHEM.EDUCATION, 11,27-31, 217-23,288-97 (1934). G., Chem. Eng. News, 23, 2190 (1945); 24, 1197 (6) SEABORQ, 11446) \.".-,. (7)WERNER, A,, BW. deut. chem. G-., 38,914-21 (1905).
