A review of Eric Scerri’s Selected Papers on the Periodic Table. This appeared in the International History, Philosophy and Science Teaching Group NEWSLETTER, February 2010

http://www.ihpst.org

Eric Scerri (2009) Selected Papers on The Periodic Table. Imperial College Press, London, ISBN: 13 978-1-84816-425-3, price: US$88

Reviewed by: Kevin C de Berg, Avondale College, Australia

The ten selected papers, preceded by a useful introduction, are arranged chronologically and cover a period from 1991 to 2009. Eric Scerri is the sole author of nine of the papers and is coauthor with John Worrall in the other paper. Three of the papers were originally published in the Journal of Chemical Education; two were originally published in American Scientist; and one each was originally published in The British Journal for the Philosophy of Science; Studies in History and Philosophy of Science; Chemical Heritage; International Journal of Quantum Chemistry; and Fundamental World of Quantum Chemistry: A Tribute to the Memory of Per-Olov Lowdin edited by E. Brandas and E. Kryachko.

The 2009 published version of the ten selected papers is hereafter referred to as Selected Papers. One might wonder why there was a need to publish this selection of papers given the excellent monograph publication on the Periodic Table by the same author in 2007. (The Periodic Table-Its Story and Its Significance by Eric R. Scerri, Oxford University Press).

What is important to realize is that the chronological presentation of the selected papers in the 2009 publication represents both Scerri’s consistent concerns relating to an understanding of the nature of the Periodic Table of the elements and his changing views on the nature of the Periodic Table over approximately a ten year period. The view that the Periodic Table should be thought of primarily as an organization of the macroscopic chemical properties of the elements rather than as a tool for accessing the electronic structure of the atoms of the elements is consistently presented over a ten year period. This does not mean that electronic structure is not important but Scerri consistently puts the primary emphasis on macroscopic chemical properties in the 2007 and the 2009 publications.

In the 2007 monograph Scerri favours a version of the Periodic Table known as the left-step version which places helium with the alkaline earths because of their similar outer electron configurations, but in the 2009 Selected Papers explains why he has now changed his mind and thinks a version which retains helium with the noble gases and which places hydrogen with the halogens is preferred.

Scerri explains in his 12-page introduction to Selected papers how he used to favour multiple versions of the Periodic Table, the choice of a version being a matter of convenience. He states that he now believes in a more objective picture which points to the existence of possibly one correct version, a version which most closely reveals the precise periodic relationship between as many elements as possible and for which agreement has not yet been reached. So Selected Papers provides an historical snapshot of Scerri’s own intellectual journey in matters relating to the Periodic Table. Four major themes seem to emerge from a reading of the ten selected papers.

1. The Dual Nature of an Element Scerri makes the point in at least five of the ten selected papers that elements can be thought of as simple substances or as basic substances and he insists that the Periodic Table should be thought of, in harmony with Mendeleev, in terms of the elements as basic substances. The example is given of the elements sodium and chlorine. As simple substances they exist as a soft silvery metal and a pale pungent greenish-yellow gas. In the salt sodium chloride, sodium and chlorine obviously do not exist as simple substances. Common salt shows no evidence of the presence of a soft metal or a pungent gas. In compound form sodium and chlorine are said to exist as basic substances. In Mendeleev’s mind, what remained constant about an element in simple and basic form was the property of atomic weight. In the early twentieth century it was seen to be more appropriate to organize the elements according to the property of atomic number. Focusing on elements as basic substances when considering the Periodic Table is consistent with Scerri’s opinion that the Periodic Table is an organization of the macroscopic chemical properties of the elements, rather than an organization of the electronic configurations of the atoms of the elements.

2. The question of the relationship between Quantum Mechanics and the Periodic Table This is a major topic in Selected Papers and is significantly addressed in seven of the ten papers. While Quantum Mechanics has undoubtedly proven to be a powerful explanatory tool in chemistry, and Scerri does not deny this, its strength as a theoretical tool can easily be overestimated when it comes to assessing the power of the Periodic Table as a chemical tool. The following quotes exemplify Scerri’s position. Quantum mechanics can generally be used to explain a particular atom s empirical electronic configuration, but that configuration usually cannot be deduced from quantum mechanics alone (Chemical Heritage 2007, p.27). The familiar sequence in which the s, p, d, and f orbitals are filled .has essentially been determined by empirical means. Indeed, Bohr s failure to derive the order for the filling of the orbitals has been described by some as one of the outstanding problems of quantum mechanics (Chemical Heritage 2007, p.26). The concept of electronic configurations cannot be derived from quantum mechanics Electrons in individual well-defined quantum states represents an approximation.

Chemistry textbooks often fail to stress the approximate nature of atomic orbitals and imply that a solution to all difficult chemistry problems lies in quantum mechanics (The British Journal for the Philosophy of Science 1991, pp.309- 325). The sequence of atomic numbers denoting the closing of the periods has never been derived from the principles of quantum mechanics (Journal of Chemical Education 2008, pp.585-589). The periodicity in the chemical properties of the elements is a complicated matter and is only approximately reflected in the electronic configurations of the atoms (Journal of Chemical Education 1991, pp.122-126).

Some of the details in the published selected papers will prove rather complicated for scholars not trained in computational chemistry but statements such as those chosen above provide a reasonably clear orientation of the arguments advanced. The 4s / 3d electron occupation scenario is discussed in at least five of the selected articles and will be of interest particularly to those who teach this topic.

Scerri questions whether the periodic classification of the chemical properties of the elements can be ultimately reduced to an understanding of atomic structure. With our current knowledge of the Periodic Table it would appear, according to Scerri, that chemistry has not been reduced to physics . In the Fundamental World of Quantum Chemistry, the challenging task for chemists is stated as follows: To what extent the Periodic Table can be explained strictly from the first principles of quantum mechanics without assuming any experimental data whatsoever.

3. Prediction and Accommodation The question of whether Mendeleev’s fame was due to his prediction, using the data from his periodic table, of the existence of elements yet to be discovered, or whether his periodic organization of the elements already known, equally or even more so contributed to his fame is dealt with largely in the longest of the Selected Papers in Studies in History and Philosophy of Science.

The following are some highlights of the discussion.

(a) There is little historical support for the standard story that predictive successes were outstandingly important in the success of Mendeleev’s scheme. Accommodations played at least an equal role.

(b) In all, Mendeleev predicted a total of eighteen elements of which only nine were subsequently isolated. So there were failed predictions as well as successful ones.

(c) As well as failed predictions of the existence of elements, Mendeleev’s sometimes failed in predicting the properties of elements subsequently found to exist. For example, he predicted the melting point of gallium to lie between that of Al (660oC) and In (157oC). [Note that the paper mistakenly quotes the molar mass of indium (115 g mol-1) as the melting point but the argument being put still holds]. Gallium was subsequently found to have a melting point of 30oC.

(d) De Boisbaudran did not discover gallium as a result of Mendeleev’s prediction but operated independently using empirical means and in ignorance of Mendeleev’s prediction.

(e) When the Davy medal was awarded to Mendeleev in 1882, no mention was made of his successful predictions in the oration. At this time two successful predictions had been made over and against sixty accommodations.

4. Representations of the Periodic Table What is clear from Selected Papers is that Scerri has progressively moved toward adopting a version of the periodic table that gives priority to atomic number triads. An atomic number triad occurs when the middle element of the triad has an atomic number which is the average of that of the other two members of the triad. In Scerri’s favored version helium retains its position with the noble gases thus retaining the atomic number triad (He, Ne, Ar) and hydrogen is added to the halogen group thus adding an additional triad (H, F, Cl) to the table. The significance of atomic number triads is an interesting proposition and the extent to which it remains significant awaits further development. This arrangement also leads to a very symmetrical table with four groups to the left and four groups to the right of the transition series. Scerri also advances the proposition that the third and fourth transition series should commence with the elements lutetium and lawrencium rather than lanthanum and actinium. Chemistry teachers should take note of this fact because chemistry textbooks published as recently as 2009 still place lanthanum and actinium in these positions. Scerri argued against this as early as 1991.

Conclusion

While Selected Papers covers similar topics to those dealt with in the monograph on the periodic table by the same author, Selected Papers demonstrates how an author s perceptions of a single topic (the periodic table) have materialized historically. There is a sense in which the question of the periodic organization of the elements has not yet been satisfactorily resolved, contrary to the impression many may receive on studying chemistry. The Selected Papers confirms that this is still an active research area and is a worthy addition to a library of materials on the periodic table. The publication adds significantly to the historical and philosophical dimensions of the topic.

References

Scerri, E.: 1991, Chemistry, Spectroscopy, and the Question of Reduction , Journal of Chemical Education 68(2): 122-126. Scerri, E.R.: 1991, The Electronic Configuration Model, Quantum Mechanics and Reduction , British Journal for the Philosophy of Science 42: 309-325. Scerri, E.R.: 1997, The Periodic Table and the Electron , American Scientist 85: 546- 553. Scerri, E.R.: 1998, How Good is the Quantum Mechanical Explanation of the Periodic System? , Journal of Chemical Education 75(11): 1384-1385. Scerri, E.R.& Worrall, J.: 2001, Prediction and the Periodic Table , Studies in the History and Philosophy of Science 32(3): 407-452. Scerri, E.R.: 2003, Lowdin s Remarks on the Aufbau Principle and a Philosopher s View of AB INITIO Quantum Chemistry, in E.J.Brandas and E.S.Kryachko (eds.), Fundamental World of Quantum Chemistry Vol II, Kluwer, Netherlands, pp.675-694. Scerri, E.R.:2007, Mendeleev’s Legacy-The Periodic System, Chemical Heritage, Spring: 22-27. Scerri, E.: 2008, The Role of Triads in the Evolution of the Periodic Table: Past and Present, Journal of Chemical Education 85(4): 585-589. Scerri, E.R.: 2008, The Past and Future of the Periodic Table, American Scientist 96: 52-59. Scerri, E.R.: 2009, The Dual Sense of the Term Element, Attempts to Derive the Madelung Rule, and the Optimal Form of the Periodic Table, If Any , International Journal of Quantum Chemistry 109: 959-971.