Chemical Education Today
Logic Lessons Lost by Roy W. Clark Department of Chemistry and Biochemistry, Middle Tennessee State University, Murfreesboro, Tennessee 37132
[email protected] William B. Jensen's three-part series, which appeared in this Journal in 1998 (1-3), is a long, and very important, logic lesson for all chemistry teachers. It should be required reading for all textbook authors. Jensen reminds us of many possible logical improvements in our terminology and word choices while teaching chemistry. Two words that Jensen insisted should be changed were the definition of element and the use of the word atom. Defining and Redefining Some Chemical Terms Element The correct definition of element is based on the nucleus only and not on purity. We no longer say “elements are substances that cannot be decomposed into simpler substances by chemical or physical means.” Very early in the 20th century an element was properly defined as a class of nuclei, all of which have the same atomic number. I checked a sample of textbooks published in the period 1921-2005 and found that most of the 1921-1990 texts had it right. The erroneous ones were surprisingly some of the newer texts (1990-2005). It seems that several authors had simply copied the older definition of element, a definition that has been wrong since isotopes were discovered. Perhaps that is not so serious an example as is Jensen's other bombshell. Can readers of this Journal really have ignored this? Atom Jensen reminds us that bonded atoms aren't atoms any more. He puts this more elegantly, but I wish to rephrase this problem more bluntly: The free and lonesome atom of electron configuration fame is no longer the same atom once it bonds with other atoms. A bonded atom should not be called an atom any more. Energy has changed the electron structure of bonded atoms during the process of bond formation. Although we use the same symbol, that same symbol now means an altered atom, a bonded atom. The bonded atom is a different species. The implication is that either the free atom or the bound atom needs a different name. Jensen's solution is to continue to call the free atom ...atom, and to recognize that molecules do not contain atoms, because neutral atoms cannot serve as structural units within polyatomic molecules and ions. Consequently, carbon atoms are no longer “in” carbon dioxide molecules. The carbon nucleus is in there surrounded by some electrons. If CO2 were ionic, we could call these entities ions, I suppose, but in the case of CO2, we cannot do that. What then do we call the C in CO2? Jensen does not say. What Jensen does tell us is that we should call this CO2 unit a polynuclear molecule. I agree. I shall call it that, but what now is
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the C? Surely we call it something. OK, molecules do not contain atoms but what do they contain? Distorted atoms (datoms)? Atome Chemists called these datoms ...atoms long before isolated atoms made an appearance. Isolated atoms are rare. Were it not for the inert gases we would rarely meet them in our laboratories. I say the solution to this name game is as follows: Call the rare species, exemplified by the inert gases and physics' energy level diagrams, atomes. This leaves those marvelous things in our molecules to be called atoms. This is what we have always called them and likely always will. Thus, I propose that atoms bonded to other atoms continue to be called atoms as they have been for centuries. Those rare atoms that exist isolated and alone shall instead be called atomes. This new word, atome, is phonetically similar to atom. It should not be a difficult concept for students. It follows from this definition that atome shall be the new name for inert gas atoms, and also the name for all isolated monatomic species that we formerly called free neutral atoms. Thus He, Ne, Ar, Kr, Xe, and Rn are atomes, be they multitudes at STP or single atoms in an electron configuration chart. The consequences will be that all combined atoms may remain atoms in our textbooks. That is both logical and good. The only renaming that must go on (and it must), are the noble gases and those atomes postulated as a theoretical intermediate in some hypothetical reaction. True, if temperatures were allowed sufficiently high, all the elements might become atomes. However, the two cases we deal with in beginning chemistry are the atome table (usually called the electron configuration table), and the table of elements at STP, all of which (with the exceptions mentioned) are atoms, despite their atomelike symbols. If one created sufficiently low temperature periodic tables, these inert gas atomes would become atoms too. In the case of helium, near absolute zero they become weird atoms. F2, Cl2, N2, O2 are diatomic molecules. They are not diatomeic molecules because of the energy difference between each molecule and its ingredients. Each consists of two attached atoms. These atoms are no longer atomes because the energy difference has altered their structure. H2O is a still a triatomic molecule. It has been created from three atomes and appropriate energy changes and now is a molecule of H2O. Carbon dioxide molecules contain three atoms: one carbon atom and two oxygen atoms. These oxygen atoms do not have the electron configuration of oxygen atomes, being energy-altered compared to the free state. Incidentally, H2O is not a molecule present in the liquid we call water. Water at STP is a much more complex combination of atoms, though its vapor can be
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r 2010 American Chemical Society and Division of Chemical Education, Inc. pubs.acs.org/jchemeduc Vol. 87 No. 9 September 2010 10.1021/ed1002916 Published on Web 07/13/2010
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Chemical Education Today
imagined as H2O molecules. Although water contains many atoms, it is very unlikely to contain any atomes at all. In my new convention, a majority of things we have called atoms in the past are still to be called atoms, as long as they are bonded to something. Only when atom symbols appear unconnected will they be called atomes. Reaction intermediates that are symbolized as single symbol fragments will be atomes, as long as they retain a neutralizing supply of electrons. Because I am addressing chemists, there is no need to continue with more examples. The word choices of atome and atom that I propose will also serve to untangle the century-old and verbally confused periodic table that graces most chemistry classrooms. For my entire teaching career, students have asked me whether it is a periodic table of elements at STP or a periodic table of lone atoms. Had I an opportunity to teach all those enquiring minds again I would tell them that there are two periodic tables, not one (4).
• There is a periodic table of elements at STP based on properties of the elements (as they are often pictured in photographs on some charts). The great majority of these elements at STP consist of atoms (not atomes). That is to say, they are a condensed phase. In fact, they are all atoms with the exception of one column, the noble gases. Because these do not form a condensed phase at STP, they consist of Avogadro's number of atomes. • There is also a periodic table of atomes, largely reflecting discoveries in physics and theoretical chemistry. Each symbol on this chart represents not the room temperature and pressure element, but one atome of the symbolized species. This table should not be superimposed on the elemental periodic table. Chemists have wasted enormous amounts of their time trying to make these two charts into one chart, an obvious impossibility. One should never plot properties of atomes in the same
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table as one plots the properties of elements at STP, yet it is common practice. Why should this not be undertaken? Because condensed forms do not have the same properties as the atomes from which they were formed. The chart of atomes has traditionally been used to obfuscate students about the arrangement of electrons in atomes. By using the same chart for atomes and atoms, we imply that this “electron configuration” is the starting point for all bonding possibilities. The truth is that most bond making goes on via atome with atom.
This periodic table confusion began in the 19th century. Then, it seemed convenient to use the same symbol to represent both the “simple substances” (5), and the imaginary species that was behind it all, the atome. This convention may have been convenient, but it was wrong. A beach does not have the properties of a grain of sand (6). Atoms were in our labs first. Atomes showed up later. Should chemists continue to ignore William B. Jensen's excellent advice (and this addendum thereto), I fear that none of the above will ever be straightened out. Perhaps JCE could reprint Jensen's lecture series and catalyze some complacent readers into action. Literature Cited Jensen, W. B. J. Chem. Educ. 1998, 75, 679. Jensen, W. B. J. Chem. Educ. 1998, 75, 817. Jensen, W. B. J. Chem. Educ. 1998, 75, 961. Moore, J. W. J. Chem. Educ. 2003, 80, 847. See Jensen, W. B. Mendeleev on the Periodic Law; Dover: Mineola, NY, 2002; p 3. 6. Clark, R. W. J. Chem. Educ. 2007, 84, 588.
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r 2010 American Chemical Society and Division of Chemical Education, Inc.
