Should Atoms be X-Rated? Henry A. Bent North Carolina State University, Raleigh. NC 27695
The structure of compounds is of secondary importance in the study of the elements. Mendeleeu X-rate atoms? That's crazy. What's chemistry about? Chemistry, according to a dictionary, is the science of the comoosition. structure.. orooerties. and reactions of matter. & inteipreted in terms of atoms and kolecules. Chemists. hrieflv out. manioulate atoms-in the lahoratory and in the m i n d Theidea ojthe atom permeates chemical thought. The atom is the central science's central concevt. chemists couldn't think chemically without atoms. Chemistry without atoms would be chemistry without atomic weights, the Periodic Table, chemical formulas, balanced equations, and molecular structures. Atomless chemistry would be, also, chemistry without kinetic theory, Boltzmann's factor, Einstein's interpretation of Brownian motion, Bohr's concept of stationary states, and quantum physics. Without atoms we wouldn't have modern molecular science. Had chemists to choose one event in the history of chemistrv that advanced chemical thoneht most. i t would prohabl; be Dalton's introduction of &mic thkory. The concept of the atom is the formative idea of chemistry. imagine organic chemistry without structural forkulas, and P Chem without atomic models of heat and order and disorder in solids, liquids, and gases. It strains the imagination. I t wasn't a coincidence that van't Hoff, creator of the thermodynamic theory of solutions, was one of the founding fathers of stereochemistry; that Clausius, creator of the energy and entropy functions, was one of the founding fathers of kinetic theorv: and that Gihhs. creator of the Gihhs function, was on^ ofihe founding filthirs of statistical mechanics. It helos to think ahout atoms when thinkine about chemistry. without atoms it's difficult to imagineVwhychemists have solutions. Yet things mix. irreversiblv. And there's a science of mixing, called chemistry. It's difficult to imagine, also, modern chemical analysis without atomic or molecular interpretations of NMR, IR, visible, UV, X-ray, and mass spectra. Analytical chemists wouldn't see the (invisible) light (of the elements) without
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spectroscopy. And spectroscopy wouldn't make an atom of sense without atoms. So, X-rate atoms? How crazy can one be? Hopefully, crazy enough t o be useful. As Einstein was fond of saying, an idea, to be extraordinarilv useful. must seem cram a t first. Surely it is profolnd, if cornrnonplace, tosay that of all the concents in chemistrv the conceot of the atom should be toorated: Yet, as Bohr-liked t o say, often the opposite of'a profound statement is a profound statement. Every thesis has an antithesis. There may be times when atoms should be X-rated. In Chapter IV of the first volume of his two-volume classic text "The Principles of Chemistry", Mendeleev discusses oxygen and ozone, as an example of allotropy-or, as he liked to call it, the "isomerism of the elements". He writes, It is easy to understand the difference between substances containine differentelements or the same elements in differentoroporrronr . . . Rut when the rompwition-i.~. ihe quality and quantity-of thaelemenrs in two substances is thesame yet their pr~pertte~ are different, it is dear that the conceptionsoldiverxe elements and of the varying composition of compounds alone are insufficientfor the expression of all the diversity of properties of matter in nature. Something else, still more profound and internal than the composition by weight of subst&es, must, judging from isomerism, determine the properties and transformation of substances. Mendeleev acknowledges immediately what that chemically determining, internal "something else" must be. In a noteworthy note (footnote 8a, Volume I, page 202) Mendeleev writes, Under that atomic concention of matter which is recoenized hv modern science (Chap,v ~ I )isomerism , cannot be regarxed otheiwise than as a modification in the distribution of similar atoms in space. Even in an introduction t o chemical science, notes Mendeleev, i t is im~ossiblet o avoid mentionina atoms and the conceptions o$ structure. But ~ e n d e l e e v i after s a more pro-
found thought. He is after the central oraanizina idea of the chemistrieiof the elements, the 1'eriodi;l.a~. Noting that in organicrhemistry,"where polvmeridesand . . other isomi~ridesare frequent, it is eiipecially important to have in view not only the composition hut also thc structure of compounds," Mendeleev continues (emphasis added), But the structure of compounds is a matter of secondary importance in the study of the elements, which forms the main object of the present work.
In Mendeleev's view, "the study of the e1ements"is what a beginning course in chemistry should be about. Concluding his footnote in the spirit of operationalism (the doctrine that the meaning of concepts are derived from specific operations), Mendeleev notes that [A]sthe chemical elements can only be conceived from the simple and compound substances they form, they [theelementsand their compounds] compose the chief subject of our work. It does not aim at embracing the whole province of chemistry, hut only at acquainting the reader with that portion which concerns the doetrine of the chemical elements [emphasis added]. In that respect it forms an introduction to chemistry as a science. Mendeleev's guiding pedagogical principle may have been noted: Restraint, Restraint, Restraint. Mendeleev doesn't introduce atoms until Chapter Seven, the Periodic Law until Volume 11.The mole concept he doesn't introduce a t all. Some things Mendeleev leaves for analytical, organic, and physical chemistry. What Mendeleev does do, however, he does once and for all. Ozone decomposes to oxygen, nothing else. That's pure chemistry. I t won't change. Reactions are forever. In Mendeleev's introduction to chemistry, Atomic Theory receives second billing. Top billing goes to The Doctrine of the Chemical Elements. First Lavoisier, then Dalton. Today the opposite is often the case. Atoms are not on t a ~ . ~hey'r; on top: Top billing goes to atomic structure: electrons, ionization energies, the Bohr model, orbitals, angular momentum, electron spin, quantum numhers, the aufbau principle, even Schrodinger's equation. That's fine physics. So is classical mechanics and electricity and magnetism. But it's not, in itself, chemistry. Atoms in chemistry are means to an end, not an end in themselves. They're a means t o understanding the chemistries of the elements. Atoms help us understand the law of multiple proportions, for example, and the isomerization of the elements. Often, however, means and ends are inverted. A course in chemistry easily becomes a watered-down course in atomicphysics, not acourse about sodium in water, liquid ammonia, and chlorine; magnesium burning in air, steam, and carhon dioxide: aluminum reactine with rust. iodine, and tarnished si1ver;and so forth, forward and back: ward and up and down in the Periodic Table.
Of course, without theatom andatomic weights, Mendeleev wouldn't have discovered the Periodic Table of the elements. Atomic Theory greatly illuminates The Doctrine of the Chemical Elements. But for every gain there is a loss. The brilliance of Atomic Theorv often blinds us to the simple fact that The Doctrine of the Chemical Elements doesn't rewire atoms. Indeed, preocmoation with Atomic Theorv may divert attention from the deeper concept. First things first. First Lavoisier, oxygen, and the chemical elements; then the elementary composition of compounds, Dalton, and the atom. Lavoisier accomplished his great work on the chemical elements without once writing a chemical formula-for, being the father of the father of Atomic Theory, he didn't have atoms. But, to paraphrase Mendeleev, The Atomic Theory is of secondaryimportance in the study of the elements. One can study the elements without atoms. iust as one can study thermodynamics without partition iinctions. And sometimes one should. A child can eniov . . the chemistries of the elements, if not Atomic Theory. It's a good exercise, says the physicist Fevnman, to see how simple one's methodiean he. F& if we mike uncomplicated things look complicated, complicated things will look
impossible^ Atoms are to the study of the elements as statistical mechanics is to the study of thermodynamics: illuminating, if a t times distracting; and not essential. The teachings of two areat chemists illustrate that point. - In writing the first moderuiext on the "Elements of Chemistry", Lavoisier describes in timeless fashion, essentially entirely in words, working directly from The Doctrine of the Elements, without atoms, formulas. or eauations, the central facts of the chemistries of the elements. Similarly Faraday, in his classic lectures on "The Chemical History of a Candle", describes simply and dramatically the central facts of the chemistries of carbon, hydrogen, and oxygen, water and carbon dioxide, without once mentioning atoms, a chemical formula, or a chemical equation. Moral: Two cheers for atoms. Three cheers for the elements Atoms are optional. Elements are essential. Elements without atoms, formulas, and equations are o.k. But atoms, formulas, and equations without elements? No way!
Faraday, F. "The Chemical
Yark, 1960.
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