As a measure of the effort and ingenuity that human beings are prepared to devote to solving puzzles, few can rival that of chemists in their attempts to marry the conflicting demands of presenting the array of chemical elements succinctly and logically with the need to do justice to their physical and chemical properties and relationships. Mazurs listed 1050 variants while van Spronsen more recently recorded the history of these developments. Even so, despite these massive endeavours, Emsley still felt obliged to appeal, through the pages of "New Scientist", to chemists everywhere for a satisfactory solution to this problem. It is a bold man, therefore, who would venture yet another model and even more to claim it superior to all previous attempts.
The current IUPAC model is an eighteen-group extended version of Mendelejew's table with the lanthanides (Z=57 - 70) and actinides (Z=89 - 102) excluded and added on as a subgroup (afterthought?). There is no prima facie reason why the f-block of chemical elements should be exiled in this way when the d-block is equally difficult for traditional tables to accommodate, especially the Fe, Co and Ni groups.
The second major difficulty is that the most prominent feature of the periodic table in school education and inorganic chemical reactions is the variation in valency from zero up to seven. Traditional popular tables have reflected this by arranging the groups to, by and large, reflect valency. The loss of this in IUPAC's 18-group system is the shortcoming most commonly complained of (e.g. ) and which has led to IUPAC's system being virtually ignored and relegated to a footnote on the history of subcommittees on nomenclature.
Thirdly, the grouping of elements into s, p, d and f "blocks" has now been popular for nearly half a century and is a feature of most school and university texts. The reason for this is that it intelligently reflects electronic structure. However this too is ignored by IUPAC's 1-18 grouping.
Finally, the relationship of Y to La and Lu has at last been clarified by Jensen and seen to be analogous to the relationship of Mg to Ca and Zn. This realization reflects the 'splitting' of the energy levels in the sixth period to include the f-electrons and the analogous splitting in the fourth period to include the d-electrons. The first splitting, however, occurred in the second period to accommodate the p-electrons and the relationship of H to Li and F is the corresponding analogue. This is, of course, why H can reasonably be counted as an anomalous alkali metal ion (H+) or an anomalous halogen gas (H2), with corresponding hydride (H-) as anomalous halide. Unfortunately this observation is nowhere evident in the hapless IUPAC groupings but neither is it clear in traditional tables. Usually hydrogen is placed in IUPAC group 1 with the alkali metals or it floats in an uncomfortable limbo in a space somewhere above the table with many students puzzled by this strange disembodiment from the other elements of the table.
Scerri has shown the robust experimental basis of the periodic table as the core of chemistry and it behoves chemists to do the periodic table the justice it consequently deserves by interpreting it and representing it as clearly and coherently as possible to gain the maximum information and use from it.
Groups have previously been labelled IVa or IIb etc. with the 'a's and 'b's being arbitrary. If, instead, the 'a's and 'b's are replaced by s, p, d and f, depending upon which subshell is filling, enlightenment is obtained in two ways: group VId (Cr, Mo, W) immediately tells you (i) the group valency and (ii) which block the element is in, i.e. which electron subshell is filling. If the periods are designated by Arabic numerals, then 6VId identifies the element uniquely as W and indicates its position in the periodic table, its valency and likely physical and chemical properties.One important problem remains: while Y=5IIId, La=6IIIf and Lu=6IIId, what are the corresponding designations of Mg, Ca and Zn? Even more problematic, what are the designations of H, Li and F? Zn has a primary valency of two but our reasoning so far would suggest it be designated as 4Xd, i.e. the tenth d-element in the fourth period. Unfortunately this radically violates the principle of reflecting valency. The 'd' indicates which subshell is filling but the 'X' does not indicate the valency, although there should be ten groups in the d-block. So, let the Roman numeral reflect, as far as is reasonable, the valency and see what happens: in the fourth period (4-) we have: Sc=IIId, Ti=IVd, V=Vd, Cr=VId, Mn=VIId, and Cu=Id, Zn=IId, with Fe, Co and Ni variable. It seems sensible and reasonable to make Fe, Co and Ni the eighth (group VIIId), ninth (group IXd) and tenth (group Xd) groups respectively, of the d-block. It is sensible because this preserves the valency label where it is relevant and, where it is not (as in Fe, Co and Ni), it serves to indicate the maximum number of chemical groups in a subshell (ten for the d-block, fourteen for the f-block, etc.), which is reasonable. In the case of H, Li and F our system would suggest H=1Is, Li=2Is and F=2VIIp respectively. This has several drawbacks. While H and Li are in the first group as in IUPAC, the halogens end up in group VII with no suggestion of their joint and symmetrical relationship, with the alkali metals, to H. Furthermore, if we look at He (1IIs) our system would suggest placing it with Be and, worse, suggest a valency of two. For this reason the first period is split simply into two groups, group I (H) and group 0 (He), reflecting valency. A full s-electronic (radial) subshell thus accords with group 0, the inert gases, at least in the first period. How then are Be, Mg, Ca, Sr, Ba and Ra to be designated? The solution of this takes advantage of the fact that, in each shell, the s and p electrons share the same radial quantum number (K,L,M,N,O,P or Q). Be is the last s-element of the second period but the radially quantized energy levels (first quantum number) in this and succeeding periods are not fully aufgebaut [Pauli] until the azimuthally quantized energy levels (second quantum number) are also full. So, from the second period onwards, it is a full p-electronic (azimuthal) subshell that accords with group 0, the inert gases. The alkaline earths are thus designated group IIs. B, C, N, and O are then in groups IIIp, IVp, Vp and VIp respectively. What about F and Ne? The fourth, radially quantized, N-shell “splits” to accommodate the fourth type of quantization, f-electrons, so that Y=5IIId yields La=6IIIf and Lu=6IIId. Similarly, the M-shell “splits” to accommodate the third type, d-electrons, so that Mg=3IIs yields Ca=4IIs and Zn=4IId. Analogously, the L-shell “splits” to accommodate the second type, p-electrons, so that H=1Is yields Li=2Is and F=2Ip. Ne is clearly group 0 so that a full azimuthal subshell is designated group 0. As before, the maximum chemical group number in a subshell corresponds to the total number of electrons in the new subshell and, when under eight, to the valency. In summary, just as Y (5IIId) 'splits' into La (6IIIf) and Lu (6IIId), and Mg (3IIs) 'splits' into Ca (4IIs) and Zn (4IId), so H (1Is) 'splits' into Li (2Is) and F(2Ip). The new table is identical to that of Bayley but with rational labelling and explanation of the groups that was not known in Bayley's time:
What is the heuristic value of this table?
H should give rise in its 'daughter groups' Is and Ip, to a group with one s-electron more than a completed period (the alkali metals, group Is) and to a group with one p-electron less than a completed period (the halogens, group Ip). This is, indeed, found to be the case. Mg, similarly, in its daughter groups, gives rise to the alkaline earth metals, group IIs (Ca, Sr, Ba, Ra) and the galvanic metals, group IId (Zn, Cd, Hg). In like manner, Y gives rise to the first of the inner transition groups (La and Ac: group IIIf) and to the transition metal Lu, last member of group IIId. These are also observed phenomena. The system creates groups such as IVf (Ce and Θ) and VIId (Mn, Da, Re) with valencies of four and seven respectively for these groups. Again, this is a well-known feature of these elements' chemistries.
If it has any validity, our table should be predictive. Besides groups such as Is (Li, Na, K, Rb, Cs, Fr), Ip (F, Cl, Br, J, Θe) and Id (Cu, Ag, Au), it creates group If (Θu and Bo). Similarly it creates a group IIf (Sp and Cy).
Therefore we make this prediction:
There exists Θu2O (Θu[+1]) as well as Θu2O3 (Θu[+3]) and SpO (Sp[+2]) as well as Sp2O3 (Sp[+3]).
1. Mazurs EG "Graphic Representations of the Periodic System during 100 years" Alabama UP, 1957
2. van Spronsen JW "The Periodic System of the Chemical Elements: a History of the First 100 Years" Elsevier, Amsterdam 1969
3. Emsley J "A Periodic Problem" New Scientist 1984, 101: 38 (no.1392, 12th January)
4. James MH "Chemical Elements" New Scientist 1985, 105: 49 (no.1449, 28th March)
5. Jensen WB "The Positions of Lanthanum (Actinium) and Lutetium (Lawrencium) in the Periodic Table" Jnl. Chemical Education 1982, 59: 634-636
6. Scerri ER "Plus ça change. . ." Chemistry in Britain 1994, May: 379-381
7. Bayley T in Mazurs EG (above) fig. 92 p.84. The original reference is: Bayley T "On the Connexion between the atomic Weight and the chemical and physical Properties of Elements" Philosophical Magazine 1882, 5:13, 26-37