Who was John David Main Smith?

This blog post concerns a virtually unknown chemist, John David Main Smith, who contributed a significant piece of research in atomic physics in the early 1920s at the time when knowledge of the field was undergoing very rapid changes. Main Smith is so little known that I had to search far and wide for a photograph of him before finally obtaining one from his son who is still living in the south of England.

Main Smith senior’s claim to fame rests mainly with a handful of articles that he published in the years 1924-25 in an unexpected place, the journal Chemistry & Industry. As a result, the papers did not have any serious influence on chemists and certainly not on physicists, with a few minor exceptions.

What Main Smith did was to take on the views of the mighty physicist, Niels Bohr, by proposing some improved electronic arrangements for the atoms of many elements. These new arrangements were independently, rediscovered by an almost equally obscure English theoretical physicist, Edmund Stoner, who at the time was a graduate student at the University of Cambridge.

Main Smith’s contribution may be put very simply by saying that he challenged Bohr’s view of a symmetrical distribution of electrons in each shell surrounding the nucleus of an atom. For example, in the case of the second shell, which had long been known to contain eight electrons, Bohr regarded the electronic structure to consist of two sub-shells each containing four electrons. Main Smith had the temerity to challenge this view and to claim that the second shell should be regarded as having a grouping of 2,2 and 4 electrons instead on the basis of detailed chemical evidence. Similarly Main Smith held that every shell begins with a sub-shell containing just two electrons. In contemporary terms he could be said to have discovered the existence of s-orbitals, and the fact that each shell begins with such an s-orbital.

On the other hand, here is one instance in which Main Smith turned out to be rather mistaken.

The existence of only one element, actinium, between thorium and radium renders it certain that the transition series of this last long period contains no transition sub-series similar to the analogous 14 “rare earth” elements, and it is consequently probable that, if all the elements of this period were known, they would number only 18 as in the case of the first and second periods.

Unfortunately this view stands in complete opposition to the currently accepted wisdom whereby there is indeed an analogous long period of 32 elements and starting with actinium at which point we do see a series of elements that are analogous to the rare earth elements. This modern view is generally attributed to the American chemist Glen Seaborg who proposed his amendment to the periodic table in the 1940s. In fact the idea was first proposed by Charles Janet, another lesser-known scientist who unambiguously anticipated this idea in the early 1930s.

Another concept that can definitely be attributed to Main Smith was later named the inert pair effect by the chemistry Neville Sidgwick. The essential idea is that as one descends several groups in the p-block of the periodic table there is an increasing tendency for two of the outermost electrons to not be involved in covalent bonding. For example the lower elements tin and lead situated in group 14 of the periodic table form di-chlorides whereas the higher members of the group such as carbon and silicon invariably form tetra-chlorides.

The original explanation of the inert pair effect, suggested by Sidgwick, was that the valence electrons in an s-orbital are more tightly bound and are of lower energy than electrons in p orbitals and therefore less likely to be involved in bonding. However this explanation was subsequently found wanting. If the total ionization potentials of the 2 electrons in s orbitals of elements in group 3 are examined it can be seen that they increase in the sequence, In < Al < Tl < Ga, which does not correlate with the fact that the inert pair effect supposedly becomes more pronounced as one descends the group. More recently, some of these anomalies have been explained as resulting from relativistic effects.

To conclude, Main Smith might be regarded as one of the missing links in the history of twentieth century atomic science. It is my sincere belief that such ‘minor players’ are in fact just as important as the heroic and better-known scientists and that the development of science should be regarded as an evolving single organism rather than as the product of isolated individuals.

Featured image credit: Niels Bohr in 1935. Public Domain via Wikimedia Commons.