Electronic theory of valency

To date there is no evidence that sodium forms any chloride other than NaCl indeed the electronic theory of valency predicts that Na" and CU, with their noble gas configurations, are likely to be the most stable ionic species. However, since some noble gas atoms can lose electrons to form cations (p. 354) we cannot rely fully on this theory. We therefore need to examine the evidence provided by energetic data. Let us consider the formation of a number of possible ionic compounds and first, the formation of sodium dichloride , NaCl2. The energy diagram for the formation of this hypothetical compound follows the pattern of that for NaCl but an additional endothermic step is added for the second ionisation energy of sodium. The lattice energy is calculated on the assumption that the compound is ionic and that Na is comparable in size with Mg ". The data are summarised below (standard enthalpies in kJ) ... 

Chemists were quick to appreciate Bohr s model because it provided an extremely clear and simple interpretation of chemistry. It explained the reason behind Mendeleev s table, that the position of each element in the table is nothing other than the number of electrons in the atom of the element, which, of course, represents an equal number of periodic changes in the nucleus. Each subsequent atom has one more electron, and the periodic valence changes reflect the successive filling of the orbital. Bohr s model also provided a simple basis for the electronic theory of valence. 

It is also noteworthy that Alfred Stock, who is universally acclaimed as the discoverer of the boron hydrides (1912). " was also the first to propose the use of the term "ligand (in a lecture in Berlin on 27 November 1916). Both events essentially predate the formulation by G. N. Lewis of the electronic theory of valency (1916). It is therefore felicitous that, albeit some 20 years after Stock s death in 1946, two such apparently disparate aspects of his work should be connected in the emerging concept of boranes as ligands . 

The apparent inertness of the noble gases gave them a key position in the electronic theories of valency as developed by G. N. Lewis (1916) and W. Kossel (1916) and the attainment of a stable octet was regarded as a prime criterion for bond formation between atoms (p. 21). Their monatomic, non-polar nature makes them the most nearly perfect gases known, and has led to continuous interest in their physical properties. 

Why do we want to model molecules and chemical reactions Chemists are interested in the distribution of electrons around the nuclei, and how these electrons rearrange in a chemical reaction this is what chemistry is all about. Thomson tried to develop an electronic theory of valence in 1897. He was quickly followed by Lewis, Langmuir and Kossel, but their models all suffered from the same defect in that they tried to treat the electrons as classical point electric charges at rest. 

This method was applied mainly by N. V. Sidgwick (The Electronic Theory of Valency, Clarendon Press, Oxford, 1927), who used it in the discussion of compounds such as the enolized (J-diketones see also Lassettre, loc. cit. (1). 

It was G. N. Lewis who extended the definitions of acids and bases still further, the underlying concept being derived from the electronic theory of valence. It provided a much broader definition of acids and bases than that provided by the Lowry-Bronsted concept, as it furnished explanations not in terms of ionic reactions but in terms of bond formation. According to this theory, an acid is any species that is capable of accepting a pair of electrons to establish a coordinate bond, whilst a base is any species capable of donating a pair of electrons to form such a coordinate bond. A Lewis acid is an electron pair acceptor, while a Lewis base is an electron pair donor. These definitions of acids and bases fit the Lowry-Bronsted and Arrhenius theories, and cover many other substances which could not be classified as acids or bases in terms of proton transfer. 

Combination with oxygen. On the basis of the electronic theory of valency the meaning of the term has been extended to include all reactions in which there occurs an increase in the ratio of the electronegative to the electropositive atoms or groups of a substance. The controlled oxidation of natural rubber produces resinous substances called Rubbones. 

G. N. Lewis, J. Am. Chem. Soc. 38 (1916), 762 G. N. Lewis, Valence and the Structure of Atoms and Molecules (New York, The Chemical Catalog Co., 1923). As observed by Pauling (in note 51, p. 5), this remarkable work forms the basis of the modern electronic theory of valence. ... 

N. V. Sidgwick, The Electronic Theory of Valency (London, Oxford University Press, 1929), p. 116. 

Lowry, T.M. The electronic theory of valency. Part IV. The origin of acidity. Trans. Faraday Soc. 1924, 13-15. 

Third, the electron theory of valence, cultivated mainly by Anglo-American physicists and physical chemists in the first two decades of the twentieth century, offered mechanical models for chemical affinity on the molecular level. These models combined data from structural chemistry with insights about physical mechanisms involving ions and electrons from the rapidly developing work of radiation physicists and spectroscopists. The further application of this third approach is the subject of chapters 6, 7, and 8, as it was developed by specific research schools in different national traditions and became part of the fundamental framework of the new subdiscipline of physical organic chemistry. 

See W. A. Noyes s resume of the development of the electron theory of valence from the chemical point of view in discussion at the 1923 Faraday Society Symposium, Trans.Far.Soc. 19 (1923) 476478. Also, Laidler, "Chemical Kinetics," 58. 

Lowry, the holder of the new chair in physical chemistry at Cambridge University, spoke in Paris in March 1924 on aspects of the theory of valence, including the electronic theory of valence, and in December 1925 on optical methods of verifying structural chemistry and his own hypothesis of semipolar double bonds in organic compounds. The occasions were meetings of the Societe de Chimique de France and the Societe de Chimie Physique. 62... 

In 1923, the Faraday Society arranged a conference on the electronic theory of valence, hosted by Lowry at Cambridge University. The temperature was 86 degrees in the shade, and the lecture theater was even hotter. Thomson, who chaired the first session, invited his colleagues to take off their coats but... 

Thomas Lowry, "Introductory Address to Part II Applications in Organic Chemistry of the Electronic Theory of Valency," 485487, Trans.Far.Soc. 19 (1923) 487. "Intramolecular Ionisation in Organic Compounds," ibid., 487496 and "The Transmission of Chemical Affinity by Single Bonds," ibid., 497502. 

C. K. Ingold, "The Principles of Aromatic Substitution from the Standpoint of the Electronic Theory of Valence," Recueil des Travaux Chimiques 48 (1929) 797812 "Significance of Tautomerism and of the Reactions of Aromatic Compounds in the Electronic Theory of Organic Reactions," JCS 136 (1933) ... 

Thomas Lowry, "Applications in Organic Chemistry of the Electronic Theory of Valence," Trans.Far.Soc. 

The "principle of microscopic reversibility", which indicates that the forward and the reverse reactions must proceed through the same pathway, assures us that we can use the same reaction mechanism for generating the intermediate precursors of the "synthesis tree", that we use for the synthesis in the laboratory. In other words, according to the "principle of microscopic reversibility", [26] two reciprocal reactions from the point of view of stoichiometry are also such from the point of view of their mechanism, provided that the reaction conditions are the same or at least very similar. A corollary is that the knowledge of synthetic methods and reaction mechanisms itself -according to the electronic theory of valence and the theory of frontier molecular orbitals- must be applied in order to generate the intermediate precursors of the "synthesis tree" and which will determine the correctness of a synthesis design and, ultimately, the success of it. 

Sidgwich, N. V. Electronic theory of valency, Chapt. 11. Oxford Univ. Press 1927. 

All of these early studies, however, contained, in addition to suggestions that have since been incorporated into the present theory, many others that have been discarded. The refinement of the electronic theory of valence into its present form has been due almost entirely to the development of the theory of quantum mechanics, which has not only provided a method for the calculation of the properties of simple molecules, leading to the complete elucidation of the phenomena involved in the formation of a covalent bond between two atoms and dispersing the veil of mystery that had shrouded the bond during the decades since its existence was first assumed, but has also introduced into chemical theory a new concept, that of resonance, which, if not entirely unanticipated in its applications to chemistry, nevertheless had not before been clearly recognized and understood. 

I have felt that in writing on this complex subject my primary duty hould be to present the theory of the chemical bond (from my point of view) in as straightforward a way as possible, relegating the historical development of the subject to a secondary place Many references are included to early work in this field the papers on the electronic theory of valence published during the last twenty years are so numerous, however, and often represent such small differences of opinion as to make the discussion of all of them unnecessary and even undesirable. 

Bennett to produce an elementary book on the electronic theory of valency, which, in various editions, continued to be used for more than a third of a century. 

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