One of the most interesting developments in the history of chemistry has been the way in which theories of valency have evolved over the years. We are rapidly approaching the centenary of G.N. Lewis’ 1916 article in which he proposed the simple idea that a covalent bond consists of a shared pair of electrons. No doubt there will be celebrations and special issues of various journals that will be motivated by the arrival of this centenary. But as in all celebrations we tend to forget some lesser known contributors who provided important steps towards the eventually adopted theories. Here I would like to recall the work of one of these sub-alterns, the German chemist Richard Abegg.
Abegg had the good fortune of studying and working with Lothar Meyer, Ladenburg, A.W. Hoffman, Ostwald, Arrhenius, and Nernst before his life was tragically cut short at the age of 41 when he died in a ballooning accident. But before this untimely end Abegg provided what was perhaps the most important step in valence theory between the discovery of Mendeleev’s periodic system and G.N. Lewis’ notion of octets of electrons.
After publishing his periodic table in 1969, Mendeleev had noticed that the valences of many elements obeyed an important relationship that has been called his rule of eight. Mendeleev observed that the formulas of hydrides occurred in four forms, namely RH, EH2, RH3, EH4. Meanwhile, with oxygen the following forms are found,
R2O, RO, R2O3, RO2, R2O5, RO3, R2O7, RO4
Mendeleev noted that for no element does the sum of the hydrogen and oxygen equivalences exceed eight.
Oxide RO4 R2O7 RO3 R2O5 RO2
Hydride none RH RH2 RH3 EH4
Significantly, for what is to come, this relationship seems to only apply to elements from just four groups in the periodic table. Mendeleev’s rule remained obscure until it was noticed by Abegg who gave it a new lease of life by making it more general.
Instead of confining himself to compounds of oxygen and hydrogen, Abegg concentrated on the maximum and minimum valences available to each element, even if this was the case only in principle. The deeper significance of Abegg’s approach is that he did not face the same problem as Mendeleev when it came to compounds from groups I to III in the periodic table. It is as though Mendeleev was assuming that hydrogen was the most electronegative element whereas in fact elements such as those in groups I to III are typically more electropositive than hydrogen. Of course Mendeleev could not approach matters from an electrical point of view as Abegg did, since no such electrical views had yet been developed in the 1870s. And even when they were by the likes of Arrhenius, Mendeleev remained famously opposed to them.
According to Abegg’s rule of eight, elements are in principle capable of showing a maximum electropositive valence (normal valence) and a maximum electronegative valence (contravalence) in which the sum of the two valences is always equal to eight. Clearly Abegg had succeeded in generalizing Mendeleev’s rule, by making it more abstract and in removing the apparent problems that had prevented Mendeleev’s rule from being applicable to all eight groups of the periodic table.
In addition, in an article written in 1899 Abegg wrote,
The sum of eight of our normal and contra-valences has therefore the simple significance as the number which represents for all the atoms the points of attack of electrons; and the group number of positive valency indicates how many of the eight points of attack must hold electrons in order to make the element electrically neutral.
Abegg’s points of attack would soon become the eight electrons arranged at the corners of cubes, which would it turn become pairs of electrons at the corners of a tetrahedron and eventually a ring of eight electrons. And the rest, as they say, was history, especially in the hands of somebody as capable as G.N. Lewis.
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