The second part of the question which forms the title of this essay is simple enough and, to my mind, merits a simple and unequivocal answer: yes. As a corollary, one might then ask another question: how many atoms are required to make a molecule? Again, I would offer a simple and straightforward answer: more than one.
In support of my answers to these questions, I could quote the first sentence of the definition of a molecule taken from the International Union of Pure and Applied Chemistry (IUPAC) Compendium of Chemical Terminology, (a repository of hundreds of agreed definitions of chemical terms and a great deal more besides), usually referred to as the “Gold Book”, which is reproduced below with my emphasis [1,2]:
An electrically neutral entity consisting of more than one atom (n > 1).
You might ask where I’m going with all this since the definition above is precise and unambiguous and who, therefore, could reasonably contest the answers I’ve given to the questions I’ve posed. Well, several authors, in fact, and it was a trio of recent papers that prompted me to write this essay [3-5].
In the first of these papers, the authors Reina, This and Reina (hereafter RTR) note that, historically, the terms ‘atom’ and ‘molecule’ were often conflated but they also cite several contemporary studies which consider examples of student misconceptions that educators encounter when dealing with these terms [3]. More importantly, however, RTR then address the seemingly fundamental question of whether molecules are made of atoms with reference to the earlier work of Jensen [6], Taber [7], Clark, [8] and Matson [9,10]. Thus, RTR quote Jensen who argues that ‘…neutral atoms cannot serve as structural units within polyatomic molecules or ions’ and refer to Taber when they state that ‘Taber endorses Jensen’s position and suggests that chemists should abandon the inaccurate idea that molecules are made of atoms’ [6,7]. RTR also highlight the contributions of Clark and Matson who advocate, respectively, (i) that a distinction should be made between atomes (isolated atoms) and atoms (atoms in molecules) and (ii) that molecules are made from atoms rather than of atoms [8-10].
It is important to stress that RTR do not agree with all of the points made by the four authors whose work they discuss but they nevertheless proceed to offer a new definition of a molecule. Specifically, they ‘…propose to define a molecule in terms of the number of bonds (0 to n, with n a finite number), allowing one to differentiate molecules and lattices and helping one to understand monoatomic species’ [3]. This definition is clearly at odds with the IUPAC definition since isolated atoms of the noble gases (He, Ne, Ar etc.), which are present in bulk samples of these gases under ambient conditions, would now be classified as molecules. Indeed, RTR themselves state that ‘…helium (He) is a molecule formed from a single atom…’ and subsequently that ‘…atoms are building blocks in molecules’ which implies that in the case of helium, it is at one and the same time an atom and a molecule [3]. I’ll return to this matter below.
In the second of the trio of papers, Giunta offers a robust critique of much of what RTR propose which it is important to summarise [4,11]. First, Giunta points out that the new RTR definition of a molecule is not only contrary to the IUPAC definition quoted above, it is also inconsistent with current chemical usage, i.e., single atoms are not considered to be molecules. Another of Giunta’s objections is that the RTR definition defines a fundamental concept (molecule) in terms of a more complicated concept, namely that of a chemical bond. Certainly from a pedagogical point of view, students are likely to be introduced to atoms and molecules before they start to consider the nature of chemical bonds. As Giunta argues, a better definition of a molecule along the lines that RTR intend would be in terms of a chemical object which contains a finite and non-zero number of nuclei which avoids any mention of bonds but the issue of whether a single atom can be termed a molecule remains.
With regard to the question of whether molecules are made of atoms, I should again make it clear that RTR do take issue with some of the statements made by Jensen, Taber, Clark, and Matson and state that, on the basis of their new definition ‘…it is possible to conclude that atoms are building blocks in molecules’ and furthermore that ‘Molecules are made from atoms and molecules are constituted by atoms.’ Giunta concurs (as do I) that, at least conventionally, if not literally, molecules are made of atoms albeit with the proviso that modern teaching texts may not always make explicit how isolated atoms change once incorporated into molecules. Giunta does, nevertheless (and correctly in my opinion) question the logic of RTR’s statement, ‘Now, because stable free atoms are molecules, this inherently implies that molecules can only be constituted from and by atoms’ arguing that any definition of the term atom on the basis of this new definition of molecule is circular [4]. In the final paper of the three, RTR defend and maintain their original statements against Giunta’s criticisms [5].
I cannot do justice to all of the points made and discussed in the various papers referred to above, but I remain unconvinced that RTR’s new definition of a molecule is an improvement on the current IUPAC definition. Quite the contrary, and it seems to me that the example of helium given above conflates the terms atom and molecule in just the manner that existed a century or so ago [12]. At a deeper level, however, I must admit to finding arguments concerning whether molecules are made of atoms (or not) – such as those made by Jensen [6] and Taber [7] noted above – rather sterile and remote from common chemical usage and practice in both teaching and research. Everyone recognises that an atom in a molecule is not quite the same as an isolated atom. Atoms in molecules share some or all (usually some) of their electrons with other atoms and in certain circumstances, one or two electrons may even be transferred in compounds best described as ionic (although they’re never completely transferred: there’s always some degree of sharing however slight). Practicing chemists find no difficulties with this and I am not persuaded that the teaching of a model which considers molecules to be made of atoms need present any difficulties in the chemistry classroom. To argue that it is incorrect or problematic to talk abouts molecules being made of atoms because the atoms in a molecule differ from isolated atoms is akin to arguing that a brick wall is not really made of bricks because the bricks in the wall are not quite the same as isolated bricks: their surface has been modified (indeed, chemically modified) by the mortar or cement [13]. Does anyone worry about whether brick walls are actually made of or constituted from bricks? No. Certainly not bricklayers (I presume but admit to not having checked). Furthermore, to take the analogy one step further, I would maintain that one brick does not a wall make.
The practice of chemistry is replete with the use of simple or approximate models of varying degrees of sophistication. All chemists make use of these models and most are content to employ the simplest model that allows one a level of understanding appropriate to the topic under examination and this is as true in research as it is in teaching. Certainly, in the classroom one must start with atoms in order to reveal the rich tapestry of chemistry which results from the myriad ways in which atoms interact with each other. Explaining the nature of those interactions, covalent, ionic, metallic etc., comes later and one quickly realises that the atoms are modified in some way depending on the nature of the interaction. To worry about whether they should still be called atoms, however, is to allow oneself to fall quite unnecessarily into a rabbit hole. Likewise, to blur the distinction between atoms and molecules (or between atoms and ionic solids, metals, ions etc.) serves no useful purpose. Let’s keep things simple: molecules, and everything else in the material universe, are made of atoms and individual atoms are not molecules.
References
[1]. IUPAC. Compendium of Chemical Terminology, 2nd Ed. (the “Gold Book”). A. D. McNaught and A. Wilkinson (Eds.), Blackwell Scientific Publications, 1997, (https://goldbook.iupac.org).
[2]. The IUPAC definition of ‘molecule’, https://goldbook.iupac.org/terms/view/M04002 (accessed on 17 April 2024). The full text of the definition is: An electrically neutral entity consisting of more than one atom (n > 1). Rigorously, a molecule, in which n > 1must correspond to a depression on the potential energy surface that is deep enough to confine at least one vibrational state.
[3]. Improving the Understanding of Chemistry by Using the Right Words: A Clear-Cut Strategy to Avoid Misconceptions when Talking about Element, Atoms, and Molecules. M. Reina, H. This and A. Reina, J. Chem. Ed., 2022, 99, 2999-3006.
[4]. Using the Right Words in the Right Way: Atom, Molecule, Element and Compound. C. J. Giunta, J. Chem. Ed., 2023, 100, 4-6.
[5]. Reply to “Using the Right Words in the Right Way: Atom, Molecule, Element and Compound”. M. Reina, H. This and A. Reina, J. Chem. Ed., 2023, 100, 7-9.
[6]. Logic, History and the Chemistry Textbook, II. Can we Unmuddle the Chemistry Textbook? W. B. Jensen, J. Chem. Ed., 1998, 75, 817-828.
[7]. The Atom in the Chemistry Curriculum: Fundamental Concept, Teaching Model or Epistemological Obstacle? K. S. Taber, Foundations of Chemistry, 2003, 5, 43-84.
[8]. Logic Lessons Lost. R. W. Clark, J. Chem. Ed., 2010, 87, 901-902.
[9]. Atoms versus Nuclei: Response to Logic Lessons Lost. M. L. Matson, J. Chem. Ed., 2011, 88, 381.
[10]. Atoms versus Nuclei: The Author of Logic Lessons Lost Responds. R. W. Clark, J. Chem. Ed., 2011, 88, 381-382.
[11]. It should be noted that in their papers, both RTR and Giunta also discuss definitions of the terms ‘element’ and ‘compound’, the arguments around neither of which I will consider here.
[12]. In ref. [4], Giunta makes the point that ‘When the noble gases were discovered, their molecules were said to consist of a single atom.’
[13]. I thank James Ladyman for suggesting this analogy.
** Nick Norman is Professor of Inorganic Chemistry (Emeritus) at the University of Bristol, UK.
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