Triads, periodic

When Mendeleef devised his periodic table the noble gases were unknown. Strictly, their properties indicate that they form a group beyond the halogens. Mendeleef had already used Group VIIl to describe his transitional triads and the noble gases were therefore placed in a new Group O. 

The platinum-group metals (PGMs), which consist of six elements in Groups 8— 10 (VIII) of the Periodic Table, are often found collectively in nature. They are mthenium, Ru rhodium, Rh and palladium, Pd, atomic numbers 44 to 46, and osmium. Os indium, Ir and platinum, Pt, atomic numbers 76 to 78. Corresponding members of each triad have similar properties, eg, palladium and platinum are both ductile metals and form active catalysts. Rhodium and iridium are both characterized by resistance to oxidation and chemical attack (see Platinum-GROUP metals, compounds). 

The nine elements, Fe, Ru, Os Co, Rh, Ir Ni, Pd and Pt, together formed Group VIII of Mendeleev s periodic table. They will be treated here, like the other transition elements, in vertical triads, but because of the marked horizontal similarities it is not uncommon for Fe, Co and Ni to be distinguished from the other six elements (known collectively as the platinum metals) and the two sets of elements considered separately. 

Like Rh and Ir, all three members of this triad have the fee structure predicted by band theory calculations for elements with nearly filled d shells. Also in this region of the periodic table, densities and mps are decreasing with increase in Z across the table thus, although by comparison... 

I wondered whether there might be some means of maximizing the number of atomic number triads that appear on the periodic table. One thing was immediately clear, namely that the relocation of helium, as proponents of the left-step favor, would lead to the loss rather than the gain of an atomic number triad. This was therefore one more argument against the relocation of helium. 

Paper nine is another one that appeared in American Scientist. In it I took a philosophical look at two important ideas that contributed to the evolution of the periodic system. These two ideas are Prout s hypothesis and the notion of triads, which was the subject of paper eight. Both hypotheses are interesting because they were extremely productive even though they both turned out to be refuted some time later. The fact that this should happen lends some support to the views of Karl Popper who always claimed that refutability was the all important aspect of good hypotheses and theories and not whether they turn out to be correct or not.23 For Popper, all that we really have is tentative theories and not theories that last forever. 

Instead I propose a more radical solution, namely that of not identifying bonded atoms with elements as basic substances, a view for which I claim support from the work of Mendeleev and Paneth. This does not solve the problem of redesigning a periodic table to reflect the behavior of bonded atoms. But if we are to retain the traditional periodic table of neutral atoms, we may still forge a connection with elements as basic substances by arranging the elements so as to maximize atomic number triads, where atomic number may now be interpreted to also mean element number . 

The Role of Triads in the Evolution of the Periodic Table Past and Present... 

However, one aspect, having to do with triads of elements, is troubling in this otherwise elegant left-step periodic system. The use of the left-step table results in the loss of a triad involving helium, neon, and argon. 

Although triads were highly instrumental in the discovery of the periodic system, the concept of atomic weight triads became somewhat neglected following the accurate determination of atomic weights. 

Dobereiner s observation had little impact on the chemical world at first but later became very influential. He is now regarded as one of the earliest pioneers of the development of the periodic system. Very little happened regarding triads... 

Suffice it to say that Dobereiner s research established the notion of triads as a powerful concept, which several other chemists were soon to take up with much effect. Indeed, Do-bereiner s triads, which would appear on the periodic table grouped in vertical columns, represented the first step in fitting the elements into a system that would account for their chemical properties and would reveal their physical relationships. 

It appears that, in the space of a single day, February 17 th 1869, Mendeleev not only began to make horizontal comparisons but also produced the first version of a full periodic table that included most of the known elements. Moreover, Mendeleev s overall approach consists of looking at atomic weight differences in conformity with the general principle of triads even though he was not specifically identifying triads in the manner of Dobereiner. 

Alternatively any triads taken from combinations of elements in rows 2, 3, 4 or 4, 5, 6 and so on, do not give perfect triads. The reason why this works so perfectly, albeit in only about 50% of possible triads, is because the length of each period repeats just once in the long-form periodic table, with the exception of the very first short period. The full sequence is 2, 8, 8, 18, 18, 32, presumably 32, and so forth. 

So if one selects any element at random there is a 50% chance that the element above and below the selected element, in the same column of the periodic table, will have atomic numbers at an equal interval away from the original element. If this is the case, then it follows trivially that the second element in the sequence will lie exactly mid-way between the first and third elements. In numerical terms, its atomic number will be the exact mean of the first and third elements, or in other words the atomic number triad will hold perfectly. All one needs to do is to pick a middle element from the first of a repeating pair of periods. Thus about half of all the elements are good candidates for beginning a triad. This phenomenon is therefore a mathematical consequence of the fact that all periods repeat (except for the first one) and that the elements are characterized by whole number integers. 

The aim of the present article is to elevate the role of triads to an even greater extent. Since triads are now expressed in terms of atomic numbers they coincidentally characterize the elements as basic substances. In other words they characterize the true basis for periodic classification compared with the elements as simple substances, as argued by Mendeleev and more recently by Paneth and other authors. 

Finally let us turn to the new periodic table, which it is claimed restores a fundamental role to triads. Rather than relocating helium to the. alkaline earths and thereby losing a perfect triad (He, Ne, Ar), we propose to relocate hydrogen into the halogen group, thereby gaining one completely new perfect triad (H, F, Cl) as shown in Figure 3-... 

As suggested in the title of the present article, we believe that the periodic table, which initially arose from the discovery of atomic weight triads, can now be further enhanced by recognizing the fundamental importance of atomic number triads. In addition one should recognize the more fundamental nature of the elements as basic substances rather than as simple substances, and that the periodic system is primarily a classification of the former. Whereas we previously suggested that these aims were best served by the left-step table we now favor the revised left-step table shown in Figure 3. 

Moreover, if we consider atomic numbers instead of atomic weights for the triads discovered in Ihe 19th century, it turns out that the atomic number of Ihe middle element is exactly the average vt the other two elements Indeed, about halt of al Ihe possible triads in the modern periodic table are exact in this sense However many other potential triads are not even approximately correct in that the atomic number of the middle dement is nowhere near Ihe average of Ihe other two... 

What will now be proposed is that in addition to its role in ordering the elements, the quantity Z may be used to also affect a secondary classification of the elements, that is, their placement into vertical groups in the sense of the conventional periodic table. In proposing this idea, I make use of what was historically the earliest hint of chemical periodicity, namely, the existence of triads of elements [35]. 

The realization that the better ordering criterion is atomic number rather than atomic weight invites us to consider triads of atomic numbers. This reveals a most remarkable fact, namely that —50% of all conceivable triads on a conventional periodic table are in fact exact. For example, the elements sulfur, selenium, and tellurium have atomic numbers of 16, 34, and 52, respectively, thus showing that the atomic number of the middle of these three... 

FIGURE 5. Long-form periodic table highlighting several atomic number triads. 

What is being suggested is that given the fundamental importance of concentrating on elements as basic substances, and given the fact that such elements are characterized by their atomic numbers, one should aim to maximize the number of perfect triads in displaying the periodic table. This proposal has an immediate consequence on the question of where the element helium, as well as other troublesome elements such as hydrogen, should be placed.13... 

Similarly, the desire to maximize the number of atomic number triads would suggest that helium should not be moved away from its traditional place at the top of the noble gases as has been suggested by proponents of the left-step periodic table such as Bent and Weinhold. 

One early attempt to organize the elements clustered them into groups of three, called triads, whose members display similar chemical properties. Lithium, sodium, and potassium, for example, have many common properties and were considered to be a triad. This model was severely limited, for many elements could not be grouped into triads. The triad model is just one of nearly 150 different periodic arrangements of the elements that have been proposed. 

Called the Iron Triad on the periodic chart, the elements are ... 

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