The periodic system

The periodic table is one of the most potent icons in science. It lies at the core of chemistry and embodies the most fundamental principles of the field. It is often regarded a model of scientific classification/taxonomy:

“A well known, still used, and expanding classification is Mendelejew's Table of Elements. It can be viewed as a prototype of all taxonomies in that it satisfies the following evaluative criteria:

(a) Theoretical foundation: A theory determines the classes and their order.

(b) Objectivity: The elements can be observed and classified by anybody familiar with the table of elements.

(c) Completeness: All elements find a unique place in the system, and the system implies a list of all possible elements.

(d) Simplicity: Only a small amount of information is used to establish the system and identify an object.

(e) Predictions: The values of variables not used for classification can be predicted (number of electrons and atomic weight), as well as the existence of relations and of objects hitherto unobserved. Thus, the validity of the classification system itself becomes testable. '” (Feger, 2001, pp. 1967-1968; breaks with hanging indentations added)

Perhaps has this "icon" a somewhat more magical air than it deserves. According to Scerri (2005) has there been considerable debate within chemistry in recent years as to the placement of the elements hydrogen and helium within the periodic system.

"It is often supposed that one of the goods delivered by successful science is the right way of classifying the things in the world. . . .The standard paradigm for such a successful scientific classification is the periodic table of the elements." (Dupré, 2006, 30).

Dupré continues:

"However, there is also much potentially wrong with the supposition just mentioned. Most importantly, there is a highly questionable implication of there being some uniquely best classification. Classifications are good or bad for particular purposes, and different purposes will motivate different classifications. It may be that there is such an ideal classification for chemistry, but if so it is because of the specific aims implicit in the history of that discipline. Chemistry aims at the structural analysis of matter and if, as appears to be the case, all matter is composed of a small number of structural elements, a classification based on those elements will be best suited to these purposes. It is also often the case that chemical structure will be the best guide to the properties of kinds of matter, but not necessarily. Two quite distinct chemicals are referred to as ‘jade’ and, despite some serious debates on the issue, Chinese jade carvers have decided that both are real jade (LaPorte, 2004)." (Dupré, 2006, 30).

Hull in Routledge Encyclopedia of Philosophy relates classification in science to the development of scientific laws:

"Any set of entities can be classified in indefinitely many ways. Books can be classified according to author, title, subject matter, and so on. Although these various classifications can be integrated into a single reference system so that any book can be retrieved on a variety of different counts, books as physical objects can be arrayed on library shelves in only one order. Even here, contingent problems arise. For example, very large books may have to be shelved out of order, and expensive first editions may be locked away in separate collections.

More stringent requirements apply to scientific classifications. The ultimate goal for scientific classifications is to group entities so that these classes function in, or facilitate the formation of, scientific laws (see Laws, natural §1). Aristotle divided motion into super- and sub-lunar as well as forced and natural (see Mechanics, Aristotelian). The primary justification of his classification was the system of laws that he was able to generate using it. When Newton introduced his quite different system, his classification replaced Aristotle’s because Newton’s system of laws was more powerful, accurate and inclusive. In general, systems of scientific classification are intimately connected to scientific theories and cannot be evaluated independently of them. Different sorts of theories require different classifications.

One major difference is between structural and historical classifications. The periodic table of physical elements is structural. The elements are individuated and ordered linearly according to their atomic number. Hydrogen comes first, then helium, lithium, and so on. These elements in turn can be arranged hierarchically as metals, rare earths, and so on. Some of these arrangements are perfectly nested; others are not. A more contemporary classification would include reference to subatomic particles and their relations (see Chemistry, philosophical aspects of §4). In general, structural hierarchies do not include very many levels. They are not very deep. Although cosmology is a legitimate area of physics, no one has suggested a historical classification of the physical elements; for example, classifying them in the order in which they appeared after the Big Bang." (Hull, 1998).

Concerning realism versus pragmatism in the periodical system, Scerri (2005):

Are alternative classifications possible?

Szostak (2004, p. 13) writes: "Bryant (2000) identifies an "essentialist" attitude that the world divides naturally into classes, and strives to discredit this. She argues that there is always more than one way that any set of entities could be distinguished, and none of these merits priority (as do Bowker and Star, 1999, 322-3). While most of her examples are from biology, she does make passing reference to the periodic table, arguing that scientists could conceivably classify in terms of isotope number rather than atomic number (2000, 89-90). This brief argument seems highly questionable: there are good theoretical reasons for preferring atomic number, and classifying by isotope could be seen as merely as unpacking elements into their isotopes. Nevertheless, Bryant is likely correct that much of the time there is no unique system by which particular entities might be classified."

Bryant's pluralist view is sharply criticized by Stamos, 2004:

"she [Bryant, 2000] nevertheless argues (pp. 88–92) that even in the case of chemical elements more than one kind of causal essentialism is scientifically legitimate, that no one kind is privileged. To arrive at this conclusion, she employs a thought experiment developed by Donnellan. Suppose, first, that in addition to Earth there is Twin Earth. Second, scientists on both planets have the same modern atomic theory. Third, in addition to proton number, scientists on both planets have the concept of isotope number, which is the number of protons and neutrons in the nucleus of an atom (actually, contrary to Donnellan and Bryant, scientists on Earth call this the mass or nucleon number, not the isotope number). Fourth, on both planets, “Different isotopes of the same element display important differences in behavior” (p. 89). Fifth, “Each property [atomic number and isotope number]… accounts for certain uniform behaviors—radioactivity, breakdown and chemical reaction—which take place within or between particular substances” (p. 90). The upshot of the thought experiment is that “both atomic number and isotope number represent equally good candidates for the important property, the one which is relevant for dividing the natural world and so determining kind membership” (p. 90). Thus, while scientists on Earth classify the chemical elements according to proton number, scientists on Twin Earth classify them according to isotope number, and neither for   Bryant are wrong to do so, since “It is not at all clear on the Earth/Twin Earth scenario whether atomic number or isotope number is the more fundamental hidden property, so far as those substances that we call the ‘chemical elements’ are concerned” (p. 89). Accordingly the statement “Gold is atomic number 79,” for example, will be true for Earthians but false for Twin Earthians (p. 90). “Does this difference in truth values,” questions Bryant, “mean that we must judge one natural kind definition right and the other wrong? Of course not.…Since both highlight similarities or regularities which are ‘out there’ in the natural world…we must judge them equally objective, legitimate and correct” (p. 92). Moreover, “Since—as Donnellan shows—there can be more than one suitable candidate for defining a natural kind, scientists must make a choice…and so classification involves a synthesis of epistemology (human theorizing, decision and choice) and metaphysics” (p. 91). The whole argument is nothing but smoke and mirrors, and is easily exposed in a Randian fashion by a consideration of the rudiments of modern chemistry. First, the chemical behavior of an atom is causally determined by its negatively charged planetary electrons, the number of which is in turn causally determined by the positively charged protons in the atomic nucleus. While the number of neutrons in an atomic nucleus may vary in comparison to the number of protons (e.g. the most common isotope of gold has 79 protons and 118 neutrons in the nucleus), all the isotopes of a given element, as Uvarov et al. (1979, p. 226) put it, “are identical in chemical properties, and in all physical properties except those determined by the mass of the atom.” It follows from the principles of modern chemistry, then, built as they are upon an enormous empirical basis, that two atoms with different proton numbers but the same mass number ( _ Bryant’s isotope number) are not going to have the same chemical or physical properties and behaviors. Therefore mass number is not going to be deemed by competent scientists a suitable candidate for defining chemical elements, on Twin Earth as on Earth. The fact is, modern scientists classify atoms into elements based on proton number rather than anything else because it alone is the causally privileged factor. Thus nature itself has supplied the causal monistic essentialism. Scientists in their turn have simply discovered and followed (where “simply” ≠ “easily”). All that thought experiments to the contrary prove is either a desire to mislead or plain wishful thinking. " (Stamos, 2004, 138-139).  

Is a classification a true, objective reflection of structures of reality discovered by scientists? Or is it a social construction aimed at supporting some specific human interests and activities? Is it one among other possible ways to classify? The periodic table seems a strong case for essentialism and realism. But as Dupré, (2006, 30) wrote may this be the case because of the specific aims implicit in the history of chemistry.

In Library and Information Science was the periodical system dismissed as a classification system by Hulme (1911), originator of the principle of "literary warrant". Hulme wrote (p. 46-47):

"In Inorganic Chemistry what has philosophy to offer? [Philosophy here meaning science, which produced the periodical system]. Merely a classification by the names of the elements for which practically no literature in book form exists. No monograph, for instance, has yet been published on the Chemistry of Iron or Gold.

. . .

  Hence we must turn to our second alternative which bases definition upon a purely literary warrant. According to this principle definition is merely the result of an accurate survey and measurement of classes in literature. A class heading is warranted only when a literature in book form has been shown to exist, and the test of the validity of a heading is the degree of accuracy with which it describes the area of subject matter common to the class. Definition [of classes or subject headings], therefore, may be described as the plotting of areas pre-existing in literature. To this literary warrant a quantitative value can be assigned so soon as the bibliography of a subject has been definitely compiled. The real classifier of literature is the book-wright, the so-called book classifier is merely the recorder. " Hulme (1911, p. 46-47).

It should be said, however, that in the UDC classification is the periodic system visible in the classification of chemistry (although mixed with other criteria).

The periodical system is used in, for example, the MEDLINE database. Gold, for example, is a part of the "transition elements", which are the metallic elements situated in the center portion of the periodic table in the B groups. Displayed in the thesaurus in Dialog by the command e(gold) or, better, e(transition elements).
 

Literature:

Bowker, G. & Star, SL. (1999). Sorting Things Out: Classification and its Consequences. Cambridge, MA: MIT Press.

Bryant, R. (2001). Discovery and Decision: Exploring the Metaphysics and Epistemology of Scientific Classification. Madison, NJ : Fairleigh Dickinson University Press.
 

Cahn, R. M. (2002). Philosophische und historische Aspekte des Periodensystems der chemischen Elemente [Philosophical and Historical Aspects of the Periodic Systems of Chemical Elements]. Karlsruhe, Germany: HYLE Publications. Available:
http://www.hyle.org/publications/books/cahn/cahn.pdf (Visited March 9, 2004). 

Caldin, E. F. (1961). The structure of chemistry in relation to the philosophy of science. London & New York: Sheed and Ward, 49 p. Version reprinted 2002 in International Journal for Philosophy of Chemistry, 8(2), 103-121. Available at: http://www.hyle.org/journal/issues/8-2/caldin.html

Dupré, J. (2006). Scientific classification. Theory, Culture & Society, 23(2-3), 30-32.

Feger, H. (2001). Classification: Conceptions in the social sciences. IN: International Encyclopedia of the Social and Behavioral Sciences. (Vol. 3, pp. 1966-1973).

Hettema, H. & Kuipers, T. A. F. (1988). The Periodic Table - Its Formalization, Status and Relation to Atomic Theory. Erkenntnis, 28, 841-860.

Hull, D. L. (1998). Taxonomy. IN: Routledge Encyclopedia of Philosophy, Version 1.0, London: Routledge.

Hulme, E. W. (1911). Principles of Book Classification. Library Association Record, 13:354-358, oct. 1911; 389-394, Nov. 1911 & 444-449, Dec. 1911. Click for fulltext:Hulme_1911_354-358+389-394.pdf; Hulme_444-449.pdf

Scerri, E. R. (2005). Some Aspects of the Metaphysics of Chemistry and the Nature of the Elements. HYLE- International Journal for Philosophy of Chemistry, 11(2), 127-145.

Scerri, E. R. (2006). The Periodic Table: Its Story and Its Significance.  Oxford: Oxford University Press. 

Stamos, D. N. (2004). Book Review of: “Discovery and decision: exploring the metaphysics and epistemology of scientific classification”. Philosophical Psychology, 17(1), 135-139.

Szostak, R. (2004). Classifying science, Phenomena, data, theory, method, practice. Berlin: Springer.

WebElements™ periodic table http://www.webelements.com/

Wikipedia, the free encyclopedia. (2006). Periodic table. http://en.wikipedia.org/wiki/Periodic_table

http://www.colorado.edu/physics/2000/periodic_table/atomic_weight.html

Birger Hjørland

Last edited: 29-01-2008

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