• Chris Campbell

Are chemical models iconic?

Updated: Jun 9

Accounts of how diagrams and models represent scientific objects, such as molecules and crystal structures, can be captured under two broad headings (Suárez 2010). Firstly, the approach that focusses on an analysis of the relationship between a scientific object, such as a salt crystal, and its representation as a three-dimensional model – the so-called ‘analytical inquiry’. Secondly, a focus on the practice of representation: diagrams and model-making – the so-called ‘practical inquiry’. Such a brief description is susceptible to oversimplification; the two approaches are not necessarily mutually exclusive. Nevertheless, it is the practice of diagrammatic and model-making representation we will look at here and the way this can reveal new knowledge.

The North American philosopher and semiotician Charles Sanders Peirce was a great exponent of the epistemic fruitfulness of diagrams, famously stating: “I do not think I ever reflect in words: I employ visual diagrams, firstly because this way of thinking is my natural language of self-communion, and secondly, because I am convinced that it is the best system for the purpose” (quoted in Ambrosio, 2020). Peirce’s semiotics offers a useful analytical framework when considering the practice of scientific representation.

In Britain, gas street lighting was introduced in the early 19th century with much of London illuminated in this way by 1816. Much of the gas was obtained by the pyrolysis (thermal decomposition) of whale oil. In 1825, Michael Faraday discovered what he called bicarburet of hydrogen and is now known as benzene (C6H6), as an oily residue remaining behind in spent cylinders of compressed illumination gas. Following Faraday’s discovery, the structure and properties of benzene presented a great puzzle to the relatively new science of organic chemistry. In 1866, August Kekulé proposed a model for benzene as (in today’s terms) a 1,3,5-cyclohexatriene structure. The six carbon atoms are arranged as an irregular hexagon and connected by an alternating series of single and double bonds:

Later that same year (1866), Adolph Claus published the now more familiar representation of the benzene molecule which is adapted and captured here:

Kekulé’s representation was soon challenged by a number of his former students - Adolf Baeyer, Wilhelm Körner, and Albert Ladenburg.

References to this model of the benzene molecule as an icon - a visual image recognised across many cultures and understood to convey a particular meaning or spread of meanings - can be found in a number of articles – such as that by the research chemist Henry Rzepa. Indeed, the hexagonal shaped structure can be found as a design on coffee mugs and t-shirts. In this piece, the term icon will have quite a different meaning - one formulated within Charles Peirce’s semiotics.

Peirce’s (1895) formulation of iconicity is associated with epistemic fruitfulness: ‘A great distinguishing property of the Icon [such as Kekulé’s representation of benzene] is that by the direct observation of it other truths concerning its object can be discovered than those which suffice to determine its construction.’ (CP, 2.279)

The epistemic-iconic character of this model can be seen in Ladenburg’s (1896) challenge to his former mentor’s model and in Kekulé’s subsequent response. Ladenburg’s objection was the result of a deductive thought experiment on Kekulé’s iconic model which subsequently revealed information initially hidden or undisclosed. In brief, Ladenburg claimed whilst there was only one known di-substituted isomer of benzene Kekulé ’s formula would suggest two:

Landenberg proposed three alternative structures, each without Kekulé’s carbon-to-carbon double bonds:

On the basis of his new models, Ladenburg states: ‘Of course, this assumption destroys one of the foundations of the theory of aromatic bodies, according to which they contain doubly bonded carbon atoms.’

It took three years for Kekulé to respond in defence of his formula and to save his developing theory of aromaticity. In 1872 he suggested a new model for the structure of benzene predicated on his original proposal. Kekulé (1872) reproduces the following two models for benzene:

One of the most useful features of an iconic representation is its capacity for enabling surrogative reasoning, a concept developed by Chris Swoyer (1991) to explain a situation where an inquirer, such as Kekulé, uses a particular visual representation to disclose new information about the object it represents. Kekulé’s method of saving his model is founded on its ability to function as an icon, as described by Peirce’s (1893) formulation: ‘remember it is by icons only that we really reason, and abstract statements are valueless in reasoning except so far as they aid us to construct diagrams.’ (CP 4.127)

Ladenburg’s challenge was based on there being only one known di-substituted benzene isomer, where Kekulé’s original model suggested two. In defence of his model, Kekulé (1872) reasons that each carbon atom oscillates about an equilibrium position, colliding sequentially with its neighbours, and that these collisions are ‘repeated periodically’. In this way, Kekulé argues that the difference Ladenburg attributes to the two models shown above ‘is apparent [but] not real’. The two depictions are in fact equivalent. This was an entirely novel theory, not derived from any experimental result, but reasoned by Kekulé on the basis of his iconic model of the benzene structure. Peirce describes the power of an icon as enabling an inquirer to ‘deduce any other truth than that which it explicitly signifies’. (CP, 2.279).

are equivalent structures.

William Brock (1992) compares Kekulé to Picasso, who ‘transformed art by allowing the viewer to see within and behind things’, where Kekulé demonstrated that ‘chemical properties arose from the internal structures of his molecules’ which could now be ‘seen and read’ by the experienced chemist. The future of chemistry after 1865, writes Brock, lies ‘in structural chemistry and the sign of this hexagon’ - Kekulé’s iconic hexagonal model.

This piece was prompted by Accorinti and González’s excellent essay in Jargonium, ‘Are chemical models representative?’.

Key words: Kekulé, Peirce, benzene, icon, model.


Ambrosio, C. (2020). “Toward an Integrated History and Philosophy of Diagrammatic Practices”. East Asian Science, Technology and Society. 14(2): 347-376

Brock, W.H. (1992). “The Fontana History of Chemistry”. Fontana Press

Kekulé, A. (1866). "Untersuchungen uber aromatische Verbindungen". Annalen der Chemie und Pharmacie, 137(2): 129–196.

Kekulé, A. (1872). "Ueber einige Condensationsproducte des Aldehyds”, Annalen der Chemie und Pharmacie, 162 (2–3): 309–320.

Ladenburg, A. (1869) "Bemerkungen zur aromatischen Theorie", Berichte der Deutschen Chemischen Gesellschaft, 2: 140–142.

Peirce, C.S. Collected Papers of Charles S. Peirce, 8 vols., ed. Charles Hartshorne, Paul Weiss, and Arthur Burks. Cambridge, MA: Harvard University Press, 1931–58. Referenced by CP followed by volume and paragraph number, separated by a full stop Example: CP 4.123

Suárez, M. 2010. ‘Scientific Representation’. Philosophy Compass, 5(1): 91-101.

Swoyer, C. (1991). “Structural Representation and Surrogative Reasoning”. Synthese, 87: 449–508.

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