1. Structural realism and essentialism in chemistry A Lecture given at the International Society for the Philosophy of Chemistry. Eric Scerri Department of Chemistry UCLA www.ericscerri.com/

2. Structural Realism An intermediate position between entity realism and anti-realism What survives across theory change is mathematical structure not scientific entities Originally due to Henri Poincaré, more recently discussed by John Worrall. More recently, Every Thing Must Go, Ladyman and Ross.

3. Advantages of S.R. Avoids the criticism against realism that many entities (phlogiston, ether) have turned out to be non-referring. Avoids going as far as anti-realism that denies any form of realism. SR recommends that we should be realists about just ‘structure’

4. SR in chemistry? So far SR has been discussed only in physics. Many examples of mathematical structures that persist after theory change. What about chemistry, the next most mathematical science?

5. The Periodic Table as ‘structure’ in chemistry May not be a manifestly mathematical structure but ‘structure’ nevertheless. It’s literally a structure! Although there is also a mathematical structure underlying the PT (group theory, information theory, similarity studies etc.) It is the structural relationship between the elements, embodied in the periodic table, that has survived major scientific revolutions like QM and relativity theory

6. Rule of 8 - Octet Rule as an element of ‘structure’ One structural aspect of the P.T. that goes beyond entities and even theories. Mendeleev and Newlands 1860s (rule of 8, law of octaves) Kossell and Abegg (rule of 8) Lewis and Langmuir (octet rule) Bohr, Schrödinger etc. atomic shell structure ( ns 2 np 6 ) The rule of 8 precedes any entities such as electrons. It was discovered before the electron. It survives to this day. Admittedly, there are exceptions to the octet rule.

14. Concentrating on structural aspects such as the rule of 8, in the periodic table and chemical bonding, is consistent with the appeal to elements as basic substances rather than as concrete entities, such as atoms or bonded atoms. Basic substances operate at an abstract level, as does mathematical structure. Neither involves entities.

15. Aspects of the octet rule The octet rule is of course broken by many compounds but this does not diminish it’s importance as a form of ‘magic number’ in chemistry. One might object by saying that many periods in the modern periodic table consist of more than 8 elements. This is perfectly correct and yet all the noble gases, except He, have eight outer electrons

16. 18 and 32-electron rules If one insists on the fact that other periods contain 18 or 32 elements one may appeal to other magic numbers in chemistry, namely 18 and 32 and the associated 18-electron and 32-electron rules which are very useful in rationalizing transition metal and f block compounds respectively. The fact remains that the periodic table, and the chemistry of the elements is governed by a few simple magic numbers, namely 2, 8, 18 and 32

17. Essentialism Now out of favor in biological classification Most authors think it still has a place in chemistry, Mendeleev ’ s periodic table if often cited as a model for essentialism. All and only members of a particular element share a common essence – their unique and common atomic structure. And knowledge of that structure enables us to predict and explain the behavior of instances of that element (Ereshefsky, 2007. p. 1) Of course there are problems. It works for elements, but as soon as we move to molecules Z fails as a criterion for identification. Structure must be added to distinguish e.g.CH 3 OCH 3 from CH 3 CH 2 OH

18. Also what Ereshefsky says is not strictly true when applied to classification of elements. Atomic structure gives primary classification or ordering of elements but is sometimes ambiguous on the question of secondary classification or placement into groups. Consider H and He (a)For H we usually consider the number of electrons present – one. So we place H in group 1. (b) For He we usually consider the number of electrons missing to obtain a full shell (zero). So we place He in the noble gases. If we applied (a) to He we would place it in group 2 If we applied (b) to H we would place it in group 17 (halogens) Why the inconsistent criterion for group membership?

25. Conclusions What really matters in the debate about ‘ element ’ is to clearly distinguish between 1. Combined element, 2. Element as simple substance, 3. Element as basic substance. Structural realism about the P.T. is consistent with recognizing the importance of elements as ‘ basic substances ’ abstractly not as entities. It is just the rule of 8 that survives across theory changes.

26. Chemical essentialism via atomic number, and/or atomic structure, is decisive for ordering the elements but not for putting them into groups (classification in literal sense) The nature of elements as basic substances is inferred from the nature of the other two senses of ‘ element ’ (combined elements and elements as simple substances) maybe in equal measure. To be explored further.