Chemistry for Everyone
Correspondence with Sir Lawrence Bragg Regarding Evidence for the Ionic Bond Norman C. Craig Department of Chemistry, Oberlin College, Oberlin, OH 44074;
[email protected] A cornerstone in the modern understanding of bonding is the ionic bond. Many chemists trace the key evidence for the ionic bond to W. H. and W. L. Braggs’ initial application of X-ray crystallography to the structure of sodium chloride and potassium chloride (1–5). The Braggs said as much in 1933 in the The Crystalline State (6 ), where we read The first crystal analysis had an effect on conceptions of chemical combination which was of the very highest importance, for it proved that in the typical inorganic salt NaCl there is no molecular grouping, and the same was found to be the case in the subsequent analysis of other simple inorganic salts. The inference that the structure consists of alternate ions of sodium and chlorine was an obvious one to make, though the evidence in its support was largely indirect.
In my first teaching of general chemistry in the late 1950s and through most of the 1960s, I attempted, in part, to use an intellectual history method. Thus, we considered how some of the important concepts in chemistry developed from the interplay of experiment and theory. For example, as part of stoichiometry, we traced the emergence of accepted atomic weights, which, after a convoluted history of chemical investigation and the newer complications of isotopy, were finally settled with mass spectrometry and the choice of carbon-12 as the defining standard. Another example was the beginning of X-ray crystallography through the application of the Bragg law and the emergence of the ionic bond. When I started teaching this early X-ray material, I accepted the idea expressed by the Braggs that the concept of ionic bonding was a direct consequence of the initial success in finding crystal structures by the X-ray method. After reading the early Bragg papers and weighing the evidence, I doubted that the concept of the ionic bond was a direct consequence of the initial X-ray investigations and told my students so. In the Braggs’ first paper they had the arrangement of atoms in sodium chloride correct but spoke of sodium chloride molecules. Furthermore, I knew that the ascendancy of organic chemistry in the second half of the 19th century had completely eclipsed the dualistic theory of Berzelius and Davy, which was, in modern terms, an ionic theory of bonding. That theory could not, however, explain bonding between like atoms as in the H2 molecule or between chains of carbon atoms in hydrocarbons and was discarded by organic chemists. By 1910 all bonding was regarded as covalent, as we would name it today. Thus, it was understandable that the Braggs looked for sodium chloride molecules in the crystal structure. In 1968 the Scientific American published an article by Sir Lawrence Bragg in which he reviewed the development of X-ray crystallography from the simple structures in sodium chloride, diamond, and zinc blende (ZnS) crystals to the structure of protein crystals (7 ). In this article, he wrote One of the first successes of X-ray analysis was to show that these compounds are not built of molecules. They are
ionic in character, with a regular alternation of positive and negative ions held together by electrical attraction. For instance, in the sodium chloride structure there are not sodium chloride groups but rather a chessboard pattern of positive sodium ions and negative chlorine ions. It was difficult in the early days to reconcile the new view of ionic compounds with classical chemical ideas, but once accepted the ionic view afforded a much fuller understanding of the construction of such compounds.
This seemed to contradict what I had been telling my students. The appearance of the article in Scientific American encouraged me to write to Sir Lawrence Bragg and ask for his comments on my reconstruction of the early work and for help in understanding the emergence of the ionic bond. What follows is the text of my letter and the text of Sir Lawrence Bragg’s response. He wrote by hand on two sides of a blue airmail letter folder. Letter from Norman Craig to Sir Lawrence Bragg Oberlin, OH July 15, 1968 Sir Lawrence Bragg The Royal Institute of Chemistry 30 Russell Square London, W. C. 1, England Dear Professor Bragg: Having recently read your article on “X-ray Crystallography” in the Scientific American, I am emboldened to write to you for further information concerning an aspect of your early X-ray work which has puzzled me for some time. It has to do with the evolution of the ionic model for bonding in crystals which, after the advent of X-ray crystallography, displaced the molecular model for substances like sodium chloride. From your early papers in the Proc. Roy. Soc., 1913–14 and the first volume of the Crystalline State I have tried with limited success to reconstruct the development of your thinking concerning the ionic model. My interest in this reconstruction arises from a desire to describe to undergraduate chemistry students the development of some of the important ideas in physical science. Accounts of the development of ideas are, I believe, urgently needed to complement the current emphasis on codified science. In the first paragraph on p. 65 of the Scientific American article you made direct reference to the importance of the early X-ray work in establishing the ionic model. You expressed a similar view in Chapter XII (Historical) in the first volume of the Crystalline State, and other writers have associated the proposal of ions in sodium chloride crystals with your earliest work. Yet, in the papers by you and your father published in 1913–14 I find only references to “molecules” in these crystals. Although your view of the structure of the sodium chloride crystal, which became the accepted structure, was made quite
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clear in the Proc. Roy. Soc. 89A, 248 (1913), you wrote of the “association of one molecule … with each diffraction center” and also referred to the possibility that “molecules grouped together in fours” as proposed by Pope and Barlow might have been associated with each lattice point. In the summary of that same article you wrote “in sodium chloride the sodium atom has six neighboring chlorine atoms equally close with which it might pair off to form a molecule of NaCl.” Furthermore, the tone of the description of the ionic bond on pp. 112-3 of the Crystalline State suggests that the acceptance of the ionic model was not immediate.1 Perhaps I can best outline my interest in the problem by posing some specific questions. With what ideas about bonding in sodium chloride and similar substances were you acquainted when you undertook the X-ray study of these crystals? Specifically, how did you first picture “molecules” of sodium chloride in the crystal lattice? Were you acquainted with Barlow’s diagrams, which you cite on p. 270 of the Crystalline State, before the X-ray investigation?2 If so, how was your view of NaCl “molecules” in crystals related to these models? Did the Pope and Barlow proposal of molecules “grouped together in fours” underlie the somewhat sketchy description of the sodium chloride structure toward the end of the article which appeared in the Proc. Roy. Soc. 88A, 428 (1913) and was revised in Proc. Roy. Soc. 89A, 246 (1914)? I am unable to produce a satisfactory sketch of the structure from the first paper because I cannot reconcile the description of the structure, which seems to be the accepted face-centered one, with the 1/2 which appears in the equation and with the statement that “the group 4NaCl is the smallest complete unit of the crystal pattern.” When did the ionic model or its equivalent become a working hypothesis for you in the X-ray work? Did the ionic model arise for you out the X-ray work alone or were you influenced by the work of others as suggested on p. 281 in the Crystalline State? Perhaps the answers to my questions are to be found in a published account of which I am unaware. If not, I would greatly appreciate receiving a reply from you concerning the manner in which the ionic model evolved. Any associated references to published material would be helpful to me in developing notes for my students. Sincerely yours, Norman C. Craig Professor of Chemistry Letter from Sir Lawrence Bragg to Norman Craig In the following transcription of Sir Lawrence Bragg’s response, [?] follows words for which the handwriting is unclear. Quietways Waldringfield in Woodbridge Suffolk, England July 31st 68 Dear Professor Craig, Your letter of July 15th raises very interesting problems and your comments are very shrewd. I will try to answer your questions as best I can, though it was all so long ago that my recollection is far from perfect. Before World War I, when I worked on the structure of NaCl, the only point I felt sure 954
about and boldly [?] stuck to was that Na was surrounded equally by six Cl’s, and vice versa. I remember well being begged by the Professor of Chemistry at Leeds to find that an Na was just a little closer to one of the chlorines than to the others, and resisting stoutly [?] because I rightly said that diffraction was so sensitive to small movements of the atoms that I would have spotted a small displacement. But I had no idea then of ‘ions’. The structure of the atom was still a complete mystery, and we had been firmly taught that NaCl was a molecule. To bracket it the other way, just after the war I brought out some ideas about atomic sizes, and they made the metals too large and the anions too small, though the sums of the radii were right. I was corrected almost at once by Wasastjerna, who pointed out that metals were in the form of small ions, and the non-metals large ions. He deduced their relative sizes from their refractivity. So by 1920 or so the conception of ions was established. If I remember rightly, it was the ideas of Lewis and Langmuir which made the idea of an electron passing from cation to anion familiar—and then of course the Bohr atom replaced the box-like Lewis– Langmuir atom. To try to answer some of your questions. Yes, I did know Barlow’s diagrams. It was a scrap of knowledge which helped me to interpret the Laue photographs, and to arrive at the NaCl structure. I was puzzled in trying to picture the molecule of NaCl, and when in 1920 the idea of an electron passing from Na to Cl, to make Na+ and Cl᎑, was mooted [?] I at once realized that of course that was the answer. I am in the country and cannot check the papers that you mention. The 1/2 you refer to is probably a reference to the fact that each cube corner in the NaCl structure has 1/2 NaCl associated with it (i.e. alternatively Na and Cl). My ideas, at any rate, were so naive in those days. It was hard to picture charged atoms in contact, why didn’t they discharge to each other? The idea that energy was released when the electron passed from Na to Cl came as a glorious revelation. I have never claimed the original work on NaCl established the ionic idea—I have always said ‘As we realized later, it was in accord with an ionic structure’. I hope this answers some of your questions. Yours very sincerely, W. L. Bragg Reflections Bragg’s response confirms that the ionic model did not arise directly from the first good experimental evidence for the arrangement of the atoms in crystals. The alternating arrangement of sodium atoms and chlorine atoms in the sodium chloride crystal is consistent with ionic bonding but does not prove ionic bonding. Some crystals with alternating arrangements of different atoms, such as zinc blende with the diamond structure, are largely covalent. Prior ideas about bonding were also an impediment. New theory had to develop, which, of course, occurred in the period from the mid-1910s to 1925. Even Bragg’s statement that energy is released when an electron moves from a sodium atom to a chlorine atom is not correct unless this statement is interpreted as referring to a two-step process. The electron transfer itself requires net energy because the ionization energy of the sodium atom is greater than the electron affinity of the chlorine atom. It is only after the sodium and chloride ions come close together
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that enough energy is released to offset the difference between the ionization energy and the electron affinity.3 In addition, as Bragg reports, properly partitioning the interatomic distance between two types of ions in a crystal was also not obvious and was first achieved by Wasastjerna in 1923 (10). The development and confirmation of the ionic model for bonding was a product of new theory and further experiments in the late 1910s and the 1920s (11).4 The ionic model is consistent with the Bragg structure for crystals such as sodium chloride; the ionic model is not, however, a simple consequence of this structure. Endnote I had often wondered if this exchange of letters with Sir Lawrence Bragg had been lost in my files forever. Fortunately, while weeding my files in preparation for moving to a new building, I found this priceless correspondence and copies of pertinent papers in a folder associated with lecture notes from my teaching of general chemistry in the 1960s. Acknowledgment I am grateful to William B. Jensen for a sympathetic reading of the original manuscript of this paper and for drawing my attention to his 1992–93 paper on the history of recognizing three types of bonding: ionic, covalent, and metallic (11). Notes 1. “These ions, originally postulated to explain the phenomena of electrolytic conduction in solution, exist also in the solid state. Instead of supposing that the atoms acquire their charges as the salt is dissolved in water, we must now picture the process as a breaking up of the crystal lattice which already consists of negative and positive ions, as was surmised many years ago by Berzelius. Such a structure explains the regular alternation of the atoms, and the non-existence of grouping into pairs corresponding to ‘molecules of sodium chloride’” (6, pp 112–113). 2. In Figure 8 of his paper entitled “A Mechanical Cause of Homogeneity of Structure and Symmetry Geometrically Investigated; with Special Application to Crystals and to Chemical Combination”, W. Barlow anticipates the crystal structure of sodium chloride with a sketch of a face-centered structure complete with spheres of different size (8). 3. Of course, quantum restrictions for orbitals help make the electron affinity of the chlorine atom a substantial fraction of the ionization energy of sodium, a factor which assists ionic bonding. A deeper description of the reason for ionic bonding was supplied by Pimentel and Spratley. They noted that the average distance between the electron in the outer s orbital and the nucleus in an alkali atom is greater than the interatomic distance in the ion dimer, thereby giving a greater attraction of the sodium ion for the negative charge
of the anion than for the 2s electron (9). Without quantized energy levels, however, Na+ and Cl᎑ could not coexist as neighbors. In terms of classical physics, the ions would discharge each other just as Bragg muses in his response. 4. In the light of my correspondence with Sir Lawrence Bragg, my reading of G. N. Lewis’s famous papers of 1913 and 1916 does not conform to the accepted interpretation that he distinguished ionic bonding at this early time (12, 13). Lewis clearly speaks about polar bonding of various degrees of charge separation, and he knows that distinct ions are produced in solvents of high dielectric constant in accord with conductivity studies. However, Lewis stops short of proposing the occurrence of distinct ions in crystals. Rather, he seems to be still thinking in terms of molecules in crystals. In contrast, Born and coworkers in Germany made substantial progress during the 1915-20 period in developing an ionic model for crystals such as sodium chloride. This work was described by Born in The Constitution of Matter, which was published in 1923 in translation as a revision of the 1920 version (14). Strangely enough, the second lecture, entitled “From Mechanical Ether to Electrical Matter”, which deals with the structure of ionic crystals, makes no mention of the Braggs’ work and contains, with the exception of a 1909 paper by Richards on compressibilities of crystals, only references to the German literature. Similarly, in his letter Bragg makes no mention of the influence of the developments in Germany during this period. Very likely these silences reflect the absence of communication of scientific developments between warring nations during World War I, which is consistent with Born’s biographical account of this period (15).
Literature Cited 1. Bragg, W. H. Proc. R. Soc. London 1913, 89A, 246–248. 2. Bragg, W. L. Proc. R. Soc. London 1913, 89A, 248–277. 3. Bragg, W. H.; Bragg, L. W. Proc. R. Soc. London 1913, 89A, 277–291. 4. Bragg, W. H. Proc. R. Soc. London 1914, 90A, 430–438. 5. Bragg, W. L. Proc. R. Soc. London 1914, 90A, 468–489. 6. Bragg, W. H.; Bragg, W. L. The Crystalline State, Vol. I; Bell: London, 1933 (reprinted 1955); pp 280–281. 7. Bragg, Sir Lawrence. Sci. Am. 1968, 219, 58–70; p 65. 8. Barlow, W. Sci. Proc. R. Dublin Soc. 1897, VII (Part VI), 527–690. 9. Pimentel, G. C.; Spratley, R. D. Understanding Chemistry; Holden-Day: San Francisco, 1971; p 623. 10. Wasastjerna, J. K. Soc. Sci. Fenn. Commun. Phys. Math. 1923, 38, 1. 11. Jensen, W. B. Bull. Hist. Chem. 1992–1993, 13–14, 47. 12. Lewis, G. N. J. Am. Chem. Soc. 1913, 35, 1448. 13. Lewis, G. N. J. Am. Chem. Soc. 1916, 38, 762. 14. Born, M. The Constitution of Matter: Modern Atomic and Electron Theories; Methuen: London, 1923; translated by E. W. Blair and T. S. Wheeler. 15. Born, M. My Life: Recollections of a Nobel Laureate, Charles Scribner’s Sons: New York, 1975.
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