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What Do Chemists Mean When They Talk about Elements? Elena Ghibaudi,* Alberto Regis, and Ezio Roletto Department of Chemistry, Gruppo SENDS, Universita di Torino, Torino I-10125, Italy ABSTRACT: Throughout the centuries, the concept of element has undergone a clear evolution from a strictly philosophical area to the scientific domain, although the conceptual progress has not always gone along with a terminological evolution. Some inconsistencies in the definition of element can be found in several precollege and university-level textbooks, which creates confusion about the concepts of element and of simple substance and raises a teaching problem. In this paper, a survey of the historical evolution of the idea of element is followed by the critical review of some definitions of element found in the scientific literature and in textbooks. We discuss a definition of element consistent with current scientific knowledge that overcomes the ambiguity between the concepts of element and simple substance at the level of instruction. KEYWORDS: High School/Introductory Chemistry, First-Year Undergraduate/General, History/Philosophy, Misconceptions/Discrepant Events, Nomenclature/Units/Symbols, Periodicity/Periodic Table

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THE HISTORICAL EVOLUTION OF THE CONCEPT OF ELEMENT

[E]lements of bodies mean the things into which bodies are ultimately divided, while they are no longer divided into other things differing in kind; and whether the things of this sort are one or more, they call these elements....[W]hat is small and simple and indivisible is called an element....Hence come the facts that the most universal things are elements, because each of them being one and simple, is present in a plurality of things. For ancient philosophers and their disciples, “element” meant

The philosophical problem of seeking the “ultimate constituents” of physical reality (that are preserved within transformations of matter) was raised quite early and crosses the whole history of scientific thought. The notion of element was first introduced by the Greek philosophers in the 5th century B.C.E., in the attempt to conciliate the uniqueness of being with the great heterogeneity of the physical reality in time and space. Their aim was to explain the multiplicity of things and events perceived by our senses through a limited number of general principles. Two main schools of thought identified such principles either in the elements (Aristotle) or in the atoms (Democritus). The concepts of atom and element are still found in modern science, although they have lost their metaphysical status to become scientific entities. In this paper we analyze the historical evolution of the concept of element, with the aim of pointing out its meaning within the context of current chemical knowledge.

“principle”, that is, a component of complex substances. The direct association of the elements, conceived as principles, with some observable propertieshot, cold, dry, humidsuggests that elements are to be intended as universal principles (or quality bearers or ways of being) rather than as material components; elements are thus abstract entities responsible for the main perceptible features of bodies, without being, in themselves, perceivable. The Element in the 18th Century

The Element in Ancient Times

The idea of the element as a potential able to generate all

The most structured doctrine of elements was proposed by Aristotle (4th century B.C.E.), who identified four principles or elements: air, fire, earth, and water. Elements convert into each other in an endless cycle, by interchanging qualities (from dry to humid, from hot to cold, etc.) and give rise to the objects of the physical world. Matter and shape are principle of the bodies, although they are not bodies, whereas the elements are simple bodies:1

perceptible properties of bodies survived until the chemical

© XXXX American Chemical Society and Division of Chemical Education, Inc.

revolution brought about by Lavoisier, who was extremely critical toward this idea. We provide an excerpt of Lavoisier’s thinking from the Introduction to the Traité Élementaire de Chimie:2

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dx.doi.org/10.1021/ed3004275 | J. Chem. Educ. XXXX, XXX, XXX−XXX

Journal of Chemical Education

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It will, no doubt, be a matter of surprise, that in a treatise upon the elements of chemistry, there should be no chapter on the constituent and elementary parts of matter [emphasis added]; but I shall take occasion, in this place, to remark, that the fondness for reducing all the bodies in nature to three or four elements [emphasis added], proceeds from a prejudice which has descended to us from the Greek Philosophers. The notion of four elements, which, by the variety of their proportions, compose all the known substances in nature, is a mere hypothesis, assumed long before the first principles of experimental philosophy or of chemistry had any existence. In those days, without possessing facts, they framed systems; while we, who have collected facts, seem determined to reject them, when they do not agree with our prejudices. The authority of these fathers of human philosophy still carry great weight, and there is reason to fear that it will even bear hard upon generations yet to come....All that can be said upon the number and nature of elements [emphasis added] is, in my opinion, confined to discussions entirely of a metaphysical nature. The subject only furnishes us with indefinite problems, which may be solved in a thousand different ways, not one of which, in all probability, is consistent with nature. Through a strictly empiricist viewpoint, Lavoisier tried to eradicate the philosophical conception of elements as abstract entities or principles, and replaced this conception with the idea of elements as simple substances that can be isolated and cannot be further decomposed. He modified the philosophical nature of the concept of element as primeval matter: according to him, the elements are no longer abstract causes of the behavior of bodies. They are rather tangible bodies, characterized by real and well-defined properties. Lavoisier identifies the elements on an experimental basis, as the end points of the chemical analysis of bodies, as witnessed by the following passage of the Traité Élementaire de Chimie:2 If, by the term elements, we mean to express those simple and indivisible atoms of which matter is composed, it is extremely probable we know nothing at all about them; but, if we apply the term elements, or principles of bodies, to express our idea of the last point which analysis is capable of reaching [emphasis added], we must admit, as elements, all the substances into which we are capable, by any means, to reduce bodies by decomposition. Not that we are entitled to affirm, that these substances we consider as simple may not be compounded of two, or even of a greater number of principles. Thus, Lavoisier abolishes the distinction between simple substance (a pure body that cannot be further decomposed by chemical methods) and element, which is no longer conceived as the abstract counterpart of any simple substance.

No matter how properties of simple bodies may change in the free state, something remains constant, and when the element forms compounds, this something is material existence [emphasis added] and establishes the characteristics of the compounds, which include the given element. In this respect we know only one constant peculiar to an element, namely, the atomic weight. The size [magnitude] of the atomic weight, by the very essence [emphasis added] of matter, is common to the simple body and all its compounds. Atomic weight belongs not to coal or diamond but to carbon. According to Mendeleev, the term “element” is abstract, but strictly associated with the empirical experience: a material, although invisible, part of a compound. As such, it has to be clearly distinguished from simple substances.4 It is necessary to distinguish the concept of a simple body from that of an element. A simple body, as we already know, is a substance, which, taken individually, cannot be altered chemically by any means produced up until now or be formed through the transformation of any other kinds of bodies. An element [emphasis added], on the other hand, is an abstract concept [emphasis added]; it is the material that is contained in a simple body and that can, without any change in weight, be converted into all the bodies that can be obtained from this simple body. The abstract nature of the element, in Mendeleev’s thought, is essential to explain the nature of chemical transformations: the entities ordered in the periodic table have to maintain their identity within a chemical combination. The term “essence”in the absence of an atomic model that Mendeleev could not yet rely onfulfilled the need for designating what, in these cases, keeps unchanged besides the empirical experience. The other basic remark by Mendeleev, often repeated in his writings, is the invitation to distinguish between the concepts of simple substance and elements:5 “The term simple substance and element are often confounded [emphasis added] with one another”. Simple substance is, in analogy with Lavoisier’s definition, a portion of matter than can be isolated and not further decomposed, the final product of the chemical analysis “endowed with physical properties and capable of chemical reactions”.5 Conversely, Mendeleev remarks that elements are not subjected to reciprocal transformations, as they represent the immutable essence of matter; this is why the element has to be regarded as a formal entity. Adopting this abstract conception of element allowed Mendeleev to overcome the apparent paradox of the conservation of elements within their compounds. In what sense are mercury and oxygen found in mercury oxide? Mendeleev says that simple substances do not keep their identity within a compound; only the (abstract) elements, which are more basic than simple substances and are identified by the atomic weight, preserve their identity:6 It is useful in this sense to make a clear distinction between the conception of an element as a separate homogeneous substance, and as a material but invisible part of a compound. Mercury oxide does not contain two simple bodies, a gas and a metal, but two elements, mercury and oxygen, that, when free, are a gas and a metal. Neither mercury as a metal nor oxygen as a gas is contained in mercury oxide; it only contains the substance of the elements [emphasis added], just as steam only contains the substance of ice, but not ice itself, or as corn contains the substance of the seed but not the seed itself.

The Element in the 19th Century

Nevertheless, the antimetaphysical attitude by Lavoisier did not eradicate the philosophical root of the term element. This is found again in the writings by Mendeleev, who refers to elements as to the “very essence” of matter that maintain their own identity within a chemical transformation and are identified by the atomic weight, a measurable parameter:3 B



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Dumon, the main obstacle in teaching the concept of element is the need for switching from the macroscopic to the formal level, “The element must undergo a change of existence if it is to become an abstract concept in the written chemical formulae: a heavy task for pupils!”16,17 A similar argument is sustained by Kruse and Roehrig:18 Many fundamental concepts in chemistry involve microscopic and symbolic representations, which are especially difficult for students to learn. Students’ understandings rely primarily on sensory experiences that provide information about tangible, macroscopic phenomena....Understanding the microscopic and symbolic representations of matter requires some abstraction, often through the development and use of mental models and images....Most students are unable to visualize and interpret these representations. Even teachers may have alternative conceptions of the concept of element and this inevitably reflects in their teaching choices. A study by Papageorgiou and Sakka reports19 With respect to the concept of element..., a high percentage of teachers (47%) gave a variety of definitions which cannot be put in specific categories. Some of these definitions did not have any meaning whereas others showed that elements are confused with atoms. A further source of difficulties that sums up with those previously discussed stems from the variety of definitions of chemical elements found in textbooks; many mix up the macroscopic, the microscopic and the formal levels. A survey of the definitions of chemical element found in precollege and university-level textbooks highlights that most of them are similar to Lavoisier’s element: “A substance that cannot be broken down into simpler substances.”

The periodic table is a classification of elements, not of simple substances as conceived by Lavoisier:7 The central idea that aided me in undertaking the study of the periodic table consists precisely in this absolute distinction between an atom and a simple body. The Element in the 20th Century

The problem of unambiguously defining what is conserved in a chemical transformation has been a matter of discussion for a long time within the chemistry community, even after the appearance of the first atomic models. In 1931, Paneth, a pioneer of radiochemistry, remarked8 that the term element was associated with two distinct meanings: the first one was by Lavoisier, who identified the element with the simple substance; the other one was by Mendeleev, who intended the element as a material component of simple substances and compounds, imperceptible by the senses. According to Paneth, these two meanings have to be kept apart because it is not correct to assign to the element (conceived as a portion of matter that keeps unchanged within a chemical transformation) the properties of the corresponding simple substance. For example, it is not correct to describe the element sulfur as a yellow, odorless and flavorless substance, insoluble in water, and so on, because these properties belong to the simple substance sulfur. In fact, although these properties are typical of sulfur, they are not found in sulfur compounds. Paneth suggests separating the two meanings by using the distinct terms basic substance and simple substance, the former being “the indestructible substance present in compounds and simple substances”,8 whereas the latter is “that form of occurrence in which an isolated basic substance, not combined with any other, appears to our senses”.8 The basic substance has distinct properties from the simple substance and it contributes to the generation of the infinitely diverse properties exhibited by simple substances and compounds where it is contained. According to Paneth, “the fundamental principle of chemistry that elements persist in their compounds refers to the quality-less basic substances only”;8 this principle would be incomprehensible by assigning the element any other meaning. Paneth’s basic substance belongs to the same formal reality as Mendeleev’s element. As Mendeleev’s element is identified by the atomic weight, Paneth’s basic substance is identified by the atomic number. The changeover from atomic weight to atomic number was a consequence of the discovery of isotopes. Paneth thinks of the element as a “transcendental principle underlying the phenomena”.8 Thus, he criticizes Lavoisier’s definition of element and the fact that it is still adopted in chemistry teaching. He also rejects the following expression, proposed after the discovery of isotopes, “A chemical element is a substance all of whose atoms have the same nuclear charge”,8 because the term “substance” cannot designate a mixture of distinct components, not even a mixture of isotopes. The formal character of the element (according to Paneth’s thought) clearly emerges from the following definition: “A chemical element is the class of all atoms of equal nuclear charge”;8 this definitely rejects the identification of the element with a simple substance.

Reference 20

“Pure substances may fall into two categories: elements, or simple substances, that cannot be decomposed into simpler ones; compounds, substances that can be further decomposed as they are formed by simpler substances.”

Reference 21

“A pure substance containing only one type of matter, which cannot be broken down by chemical methods.”

Reference 22

In some cases, the definition puts the element in relationship with the atom: “An elementary substance (or element) is an homogeneous portion of matter made by a single type of atom.” Reference 23

“Element is a substance made by a single type of atoms.”

Reference 24

“Substances made by a single type of atoms are classified as elements.”

Reference 25

None of these formulations is exempt from criticism. Those that define the element as a simple substance leave the problem of the persistence of the elements in compounds unresolved. Those that identify the element with the atom drive toward the wrong idea that the atoms are the true elements within substances. In fact, isolated atoms (intended as individual entities defined by a nucleus and a cloud of electrons) do not survive as such within simple substances and compounds, as their

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TEACHING THE CONCEPT OF CHEMICAL ELEMENT Teaching the concept of chemical element implies taking a stand between the above-discussed instances, bearing in mind the need for a definition that considers the progress of chemistry knowledge9 as well as the number of studies showing that students have problems in learning this concept.10−15 According to Laugier and C



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This conflation does not help highlight the formal nature of the concept of element. Other definitions of element were proposed, in view of their use in chemistry teaching. Roundy’s definition clearly stems from the development of the atomic structure models, “Each element is defined by its atomic number (or number of protons in the nuclei)”,30 whereas Luft writes:31,32 Element is an immaterial entity without physical or chemical properties, root of a specific chemical species and common feature to his atoms, molecules, ions and isotopes. It is characterized by two data: a symbol and an order number, the atomic number, that indicates the position of the chemical species within the Periodic Table. In both statements, the notion of chemical element is finally put in relationship with the atomic nucleus by mentioning the atomic number. The nucleus is found as such in simple substances, compounds, and ions, and is conserved in chemical transformations. It is thus correct to state that an element is identified by a name, a symbol, a position in the periodic table and an atomic number:33 four essential features that clearly place the concept of element within the field of formal concepts. Relating the identity of an element to the atomic number recalls the fact that elements occur as isotopic mixtures. On the basis of these speculations, Jensen34 claims that the term element refers to a specific type of nuclei or, more accurately, to a class of nuclei with the same atomic number. In our opinion, Jensen’s formulation is advantageous for several reasons: it is simple, unambiguous, and it clearly highlights the formal and systematic character of the concept of element. This last aspect is central, as it was the necessary and essential premise for the systematic classification found in the periodic table. The problem of defining an element is especially crucial for those who are involved in chemistry teaching. Although it is true (as Hammond says in a discussion in Chemistry International35) that “chemists generally understand the difference between elements and elementary substances and use the term ‘element’ as a shorthand descriptor”, the same is not true for students, for whom a lack of distinction between formal and material level may lead to misconceptions of the concept of element.10 For these reasons, the formulation proposed by Jensen looks particularly useful for underlying the concept and overcoming the conceptual obstacle inherent in the term element.

electronic clouds interact with each other. We agree with G. Villani26 who notes that the status of an atom within a molecule is so specific that chemists talk about atoms in situ. Villani remarks that the properties of hydrogen atoms within methane, benzene, or water are clearly different, although they keep some features that allow identification of them as hydrogen and to designate them with the same symbol, H. The symbol expresses the formal character of the element already underlined by Mendeleev. As for those definitions of element that recall the concept of atom, it is interesting to read the indication by IUPAC (the International Union of Pure and Applied Chemistry), according to which an element is:27 1. A species of atoms; all atoms with the same number of protons in the atomic nucleus. 2. A pure chemical substance composed of atoms with the same number of protons in the atomic nucleus. Sometimes this concept is called elementary substance as distinct from the chemical element as defined under 1, but mostly the term chemical element is used for both concepts. These definitions suggest two distinct meanings for the same term and this is clearly stated: “the term chemical element is used for both concepts”. So, they do not abolish the conceptual confusion between simple substance, atom and element; nor do they acknowledge the formal value of the concept of element, in spite of the fact that it was already invoked by Mendeleev. In fact, according to definition 1, the word “atom” means either free atom, atom within a molecule, ions, etc. Thus, the free hydrogen atom (H), the hydrogen ion (H+), the isotopes of hydrogen (D and T), the hydrogen atoms within compounds such as CH4, D2O, CH3D, all belong to the class chemical element hydrogen. Nelson remarks28 that a drawback of these formulations is that they compel students to approach chemistry from the viewpoint of the microscopic world of atoms and atomic structures. This is against the psychology of learning because a correct understanding of chemistry requires a progression from the macroscopic level toward the concepts and models of the microscopic level. Based on these premises, he first proposed a definition of element close to Lavoisier’s conception: “a substance that does not undergo chemical decomposition into, and cannot be made by chemical composition of, other substances”. Following some criticisms, Nelson revised this definition by introducing the distinction between elementary substance and element. Elementary substance is “a substance that does not undergo chemical decomposition into, and cannot be made by chemical composition of, other substances”.29 Therefore elementary substance is a simple substance. Nelson’s element is then “a basic type of matter existing as elementary substances that can be interconverted without change in mass”.29 This definition of element stems from the need to take into account the phenomenon of allotropy: elementary substances like graphite and diamond, says Nelson, are variants of a same element, carbon. In distinguishing between elementary substance and element, Nelson implicitly admits that the two concepts must not be confused. Nelson’s basic type of matter recalls Paneth’s basic substance: Nelson’s basic type of matter exists as elementary substance just like Paneth’s simple substance is the way a basic substance appears. In spite of the distinction between the two concepts that are indicated by distinct terms, Nelson’s terminological choice (basic type of matter versus elementary substance) has the disadvantage of mixing up the abstract and the material levels of thought, which should instead be kept apart.

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PROBLEMS OF LANGUAGE In conclusion to this survey, we believe that it is useful to stress the importance of an accurate use of terminology while talking about elements: inappropriate (or even poor) choices of terms have raised ambiguities that have been transmitted and amplified in teaching. The first source of ambiguity lies in the use of a same word (“element”) for indicating either a specific “class of nuclei” or a “simple substance”. This double meaning may be admitted in discussions among experts who are able to understand the correct sense in relationship with the context, but it is dangerous in a chemistry teaching context, because students are beginners who need a language devoid of ambiguities. It is thus most advisable to reserve the word element to a “class of nuclei”, whereas the term simple substance needs to refer to the material body whereof the element is found in concrete terms. Only rarely is the difference between element and simple substance reflected explicitly in the language: this is the case of allotropy (where, for example, carbon indicates the element, whereas the terms coal, graphite, and diamond designate simple substances). This language distinction is also found in words D



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such as dihydrogen, dinitrogen, and so on, for simple substances, whereas hydrogen, nitrogen, and so forth indicate the elements of which they are made. The use of such distinct nomenclature for simple substances and elements is recommended by IUPAC. Another source of ambiguity rises from the habit of assigning a status of concreteness to the concept of element, instead of specifying its formal and categorical character (e.g., “The chemical element sodium, belonging to the first group of the periodic table, exhibits chemical properties similar to those of the other elements in the same column”). Stating that the simple substance potassium exhibits chemical properties similar to those of the simple substance sodium is correct, but it is wrong to compare the chemical properties of elements within a column of the periodic table. An element is a name, a symbol, an atomic number, and a position within the table; a chemical element does not exhibit macroscopic properties. To avoid the translation of language ambiguities into conceptual ambiguities, the conceptual structure shown in Table 1 may be of some help to students.

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Marco Ghirardi and Fabio Popolla for fruitful discussions.

Microscopic Sublevel

Formal Level

Atom, ion Atom, ion

Element Element

REFERENCES

(1) Aristotle. Metaphysics, Book V, Part 3 (translated by W. D. Ross). Available at http://classics.mit.edu/Aristotle/metaphysics.5.v.html (accessed Sep 2013). (2) Lavoisier, A. L. Traité Élémentaire de Chimie, I, Discours Préliminaire; pp. XIV-XVIII; Cuchet: Paris, 1789; pp 6−7 (ed. 1864). http://www. chem.yale.edu/∼chem125/125/history99/2Pre1800/Lavoisier/ Preface/discours.html (accessed Sep 2013). (3) Mendeleev, D. Sootnoshenie Svoistv s Atomnym Vesom Elementov. Zh. Russ. Khim. O-va. 1869, 1 (2/3), 60−77. Cited in Kaji, M. Mendeleev’s Discovery of the Periodic Law: The Origin and the Reception. Found. Chem. 2003, 5, 189−214. (4) Mendeleev, D. Lektsii po Obshchei Khimii 1867/68 g., Lecture V, St. Petersburg, reported in Works, XV, pp 381−382. Cited in Kaji, M. Mendeleev’s Discovery of the Periodic Law: The Origin and the Reception. Found. Chem. 2003, 5, 189−214. (5) Mendeleev, D. Die Periodische Gesetzmässigkeit der Chemischen Elements. Ann. Chem. Pharm. 1871, 8, 133−229. Cited in Mendeleev, D. On the Periodic Law: Selected Writings; Jensen, W. B., ed.; Dover Publications: Mineola, NY, 2005; pp 1869−1905. (6) Mendeleev, D. The Principles of Chemistry; Longmans, Green and Co.: London, 1891. Cited by Scerri, E. J. Chem. Educ. 2008, 85, 585− 589. (7) Mendeleev, D. Rev. Gen. Chim. Pure Appl. 1899, 1, 211−214. Cited in Mendeleev, D. On the Periodic Law: Selected Writings; Jensen, W. B., ed.; Dover Publications: Mineola, NY, 2005; pp 1869−1905. (8) Paneth, F. Found. Chem. 2003, 5, 113−145. (9) Myers, R. J. J. Chem. Educ. 2012, 89, 832−833. Myers writes, “It is time to consider the old definitions as purely historical. Chemistry has gotten very complex over the last 50 years...the classical definitions of elements and compounds are too out-of-date for what we now know about chemistry.” (10) Stains, M.; Talanquer, V. J. Chem. Educ. 2007, 84, 880−883. The authors report that “Research in chemical education over the past 20 years has shown that a large proportion of students in introductory chemistry courses have problems understanding the concepts of element, compound, and mixture.” (11) Taber, K. Chemical Misconceptions: Prevention, Diagnosis, and Cure: Classroom Resources; Royal Society of Chemistry: London, 2002. (12) Barker, V. Beyond Appearances: Students’ Misconceptions about Basic Chemical Ideas; Royal Society of Chemistry: London, 2000. Available at http://www.rsc.org/images/Misconceptions_update_ tcm18-188603.pdf (accessed Sep 2013). (13) Barker, V.; Millar, R. Int. J. Sci. Educ. 1999, 21, 645−665. (14) Ben-Zvi, R.; Eylon, B.-S.; Silberstein, J. J. Chem. Educ. 1986, 63, 64−66. (15) Briggs, H.; Holding, B. Aspects of Secondary Students’ Understanding of Elementary Ideas in Chemistry: Full Report; Children’s Learning in Science Project Leeds, University of Leeds: Leeds, U.K., 1986. (16) Laugier, A.; Dumon, A. Chem. Ed. Res. Pract. 2004, 5, 327−342. (17) Laugier, A.; Dumon, A. Didaskalia 2003, 22, 69−97. (18) Kruse, R. A.; Roehrig, G. H. J. Chem. Educ. 2005, 82, 1246−1250. (19) Papageorgiou, G.; Sakka, D. Chem. Ed. Res. Pract. 2000, 1, 237− 247. (20) Tro, N. J. Essentials Introductory Chemistry, 4th ed.; Prentice Hall: Glenview, IL, 2011.

Physical Level Simple substances (a single element) Compounds (more elements)

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Table 1. The Distinction between Physical and Formal Levels To Help Discriminate between the Concepts of Simple Substances, Atom, and Element Macroscopic Sublevel

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CONCLUSIONS From antiquity to present, the notion of element has undergone a constant evolution from a strictly philosophical to the scientific domain. Such conceptual progress has not always been paralleled by a terminological evolution: as a result, some ambiguities may still be found in the definition of element available in several precollege and university-level textbooks, as well as in the indications by IUPAC. For Greek philosophers, elements were principles, unobservable entities working as quality bearers. This viewpoint is definitely abandoned by Lavoisier, who identifies the elements with the simple substances that may be isolated through chemical procedures. The abstract conception of element reappears with Mendeleev, whose elements, intended as “essence” of simple substances and compounds, remain invisible but are characterized by a physical property: the atomic weight. Although Mendeleev employed a term borrowed from metaphysics, he modified substantially the nature of the concept by assigning to the elements a measurable property; Paneth resumed the concept suggested by Mendeleev and proposed the element as immaterial entity characterized by the atomic number. Nevertheless, his choice of using the expression basic substance for indicating the element does not sufficiently emphasize the formal character of the concept of element, and turns out to feed the linguistic confusion. Even the more recent proposal by Nelson, who defined the element as a basic type of matter, does not solve such ambiguity, whereas Luft made an important advancement in stressing the relationship between the concept of element and the atomic nucleus. At the teaching level, we strongly support the solution proposed by Jensen, who retrieves the positions of Paneth and Luft and gets to a simple and unambiguous definition that clearly underlines the formal nature of such concept. E



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(21) Rodato, S. Il Libro di Chimica; CLITT: Rome, 2008. (22) Timberlake, K.; Timberlake, W. Basic Chemistry, 2nd ed.; Pearson Prentice Hall: Upper Saddle River, NJ, 2008. (23) Lisi, G. Chimica e Laboratorio, Loffredo: Napoli, 2008. (24) Atkins, P.; Jones, L. Chemical Principles, 3rd ed.; Freeman & Co: New York, 2004. (25) Kotz, J. C.; Treichel, P. M.; Weaver, G. C. Chemistry and Chemical Reactivity, 6th ed.; Thomas Learning, Inc.: Philadelphia, PA, 2006. (26) Villani, G. La Chimica: Una Scienza della Complessità ante Litteram. In Strutture di Mondo, Urbani Ulivi, L. , Ed.; Il Mulino: Bologna, Italy, 2010; p 80. Villani writes: “It is evident that atoms within a molecule have a specific status: they are not the same as the isolated atoms and, in fact, we talk about atoms in situ. Let’s consider four molecules that contain hydrogen atoms: water, methane, ethyl alcohol, and benzene. Any chemist knows well that the hydrogen in water is different (e.g., is more acidic) from hydrogen in methane; and ethyl alcohol contains two distinct types of hydrogen atoms..., that are both different from those found in water and methane: the same is true for benzene....Obviously, the choice of designating these atoms with the very same symbol (H) indicates that these atoms share some features, although they are not identical.” [Translated by the authors.] According to Villani, this problem pertains to the debate between a reductionist versus a complex view of chemistry. In fact, chemistry is an intrinsically complex science as it is a “science of relationships”. (27) IUPAC. Compendium of Chemical Terminology, 2nd ed., (the “Gold Book”), McNaught, A., Wilkinson, A., compilers; Blackwell Scientific Publications: Oxford, U.K., 1997. Available http://goldbook. iupac.org/ (accessed Sep 2013). (28) Nelson, P. Chem. Educ. Res. Pract. 2003, 4, 19−24. (29) Nelson, P. Chem. Educ. Res. Pract. 2006, 7, 288−289. (30) Roundy, W. H. J. Chem. Educ. 1989, 66, 729−730. (31) Luft, R. Dictionnaire des Corps Purs Simples de la Chimie; Cultures et Techniques: Nantes, France, 1997. (32) Translated by the authors. (33) Regis, A. Ma Cosa è un Elemento? In Proceedings of the XII National Conference of the Didactic Division of the Italian Chemical Society, Trieste, Italy, November 7−10, 2001; pp 133−138. (34) Jensen, W. J. Chem. Educ. 1998, 75, 817−828. (35) Hammond, J. E., Chem. Int. 2005, 27 (3); http://www.iupac.org/ publications/ci/2005/2703/ud2_scerri.html (accessed Sep 2013).

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