Periodic table/system 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. 

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. 

Oxidative additions are frequently observed with transition metal d8 systems such as iron(0), osmium(O), cobalt(I), rhodium(I), iridium(I), nickel(II), palladium(II) and platinum(II). The reactivity of d8 systems towards oxidative addition increases from right to left in the periodic table and from top to down within a triad. The concerted mechanism is most important and resembles a concerted cycloaddition in organic chemistry (Scheme 1.1). The reactivity of metal complexes is influenced by their... 

I postpone the full explanation for why perfect triads occur in parts of the periodic system until chapter 6, where I give an account of the discovery of isotopes and atomic number. From the perspective of the modern periodic table, about 50% of all possible vertical triads, using atomic numbers, are in fact exact. 

The discovery of atomic number provided one of the most clear-cut modifications the periodic system had undergone since its foundation had been laid by the hkes of Johann Dobereiner some 100 years previously. When the concept of atomic number was combined with the new understanding of isotopy, it became possible to appreciate why William Front s hypothesis (that all elements are composites of hydrogen) had been so tantalizing to the early pioneers of the periodic system. Indeed, Front s hypothesis could now be said to be valid in the somewhat modified form that all atoms in the periodic table were multiples of a single rmit of atomic number or, as it was subsequently named, the proton. It also became possible to explain why triads had been so enticing and so instrumental in the early evolution of the periodic system. 

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. 

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