Chemical valence

The subject of chemical valence is a vast one, and is not covered in the present book. Excellent introductions to it exist in other monographs [4] to which the interested reader may refer. A few brief comments are made here for completeness, since the connection with the periodic table has been mentioned above. 

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Lennard-Jones, J., Proc. Roy. Soc. London) A198, 1, The molecular orbital theory of chemical valency. I. The determination of molecular orbitals."... 

The application of the quantum mechanics to the interaction of more complicated atoms, and to the non-polar chemical bond in general, is now being made (45). A discussion of this work can not be given here it is, however, worthy of mention that qualitative conclusions have been drawn which are completely equivalent to G. N. Lewis s theory of the shared electron pair. The further results which have so far been obtained are promising and we may look forward with some confidence to the future explanation of chemical valence in general in terms of the Pauli exclusion principle and the Heisenberg-Dirac resonance phenomenon. 

This graduate-level text presents the first comprehensive overview of modern chemical valency and bonding theory, written by internationally recognized experts in the held. The authors build on the foundation of Lewis- and Pauling-like localized structural and hybridization concepts to present a book that is directly based on current ab initio computational technology. 

Impurities can carry a charge relative to the host structure, as, for example, with a Ca2+ ion substituted on a Na+ site in NaCl or F substituted for O2- in CaO. In essence, this means that the impurity carries a different chemical valence, that is,... 

Jain, S. R., "Energetics of Propellants, Fuels, and Explosives A Chemical Valence Approach," Propellants Explos. Pyrotech., 12 (1987). 

G. N. Lewis, W. C. Bray, and K. G. Falk developed theories of chemical valence addressing the problem of polar and nonpolar bonds. All were members of Noyes s MIT research group in the first decade of the 1900s, at a time when enthusiasm for the study of physics along with physical chemistry was at high tide in Noyes s laboratory. 

But it was not really until 1931, when Slater and Pauling independently developed methods to explain directed chemical valence by orbital orientation that it can truly be said that a chemical quantum mechanics, rather than an application of quantum mechanics to chemistry, had been created. In a study of Slater, S. S. Schweber notes the distinction between the Heitler-London-Pauling-Slater theory and the Heitler-London theory. Heitler and London successfully explained the electron-valence pair on the basis of the Goudsmit-Uhlenbeck theory of spin. Slater and Pauling explained the carbon tetrahedron. This second explanation distinguishes quantum chemistry from quantum physics.2... 

Lewis and many other chemists saw in the Pauli exclusion principle and the Uhlenbeck-Goudsmit spin hypothesis firm physical support for the chemical valence theory of the electron pair. In fact, the Pauli exclusion principle has to be postulated within the physics of the quantum theory.23 Accepting the Nobel Prize in physics for 1945, Pauli expressed regret that the principle cannot be derived ab initio. 

The interests and approaches of chemists and physicists reconverged toward the end of the nineteenth century, in a period when physicists began developing an energy-physics as an alternative to force-physics and chemists became interested in the material electron as a binding agent in a chemistry of space that included mobile atoms and the chemical valence bond. 

There can be, however, no doubt that in catalytic processes, purely physical factors play an important role, in addition to the chemical valence forces. This is particularly true for the solid catalysts of heterogeneous reactions for which the properties of surfaces, as the seats of catalytic action are of prime importance. The total surface areas, the fine structure of the surfaces, the transport of reactants to and from surfaces, and the adsorption of the reactants on the surfaces, can all be considered as processes of a predominantly physical nature which contribute to the catalytic overall effect. Any attempt, however, to draw too sharp a line between chemical and physical processes would be futile. This is illustrated clearly by the fact that the adsorption of gases on surfaces can be described either as a mere physical condensation of the gas molecules on top of the solid surface, as well as the result of chemical affinities between adsorbate and adsorbent. Every single case of adsorption may lie closer to either one of the hypothetical extremes of a purely physcial or of a purely chemical adsorption, and it would be misleading to maintain an artificial differentiation between physical and chemical factors. 

In the mathematical theory of networks valence is defined as the number of links terminating at a node, and it was in this sense that the term was introduced into chemistry. However, chemists were later forced to distinguish between a chemical valence (bonding power) and a coordinative valence (number of bonds). They chose to keep the term valence for the chemical valence and introduced the term coordination number for the coordinative valence. This book follows the chemical convention. The term valence is always used in the sense of bonding power unless otherwise stated, and coordination number is used to indicate the number of bonds. 

The periodic system developed from Bohr s atomic theory is of the greatest importance in chemical science because it demonstrates that the properties of the elements depend on their positions in the system. It is immediately apparent that chemical valency depends on the number of loosely-bound electrons in the atom. Thus, the alkali metals have one such electron while the divalent alkaline-earth metals have two, etc. Valency is therefore closely connected with electronic structure and provides the foundation for the modern theory of the chemical bond, the basis of which is to be found in the coupling or transfer of the valency electrons. 

In most of the cases, one is interested in having a relatively high DNA coverage of the particle surface. However, very recently some interest arose as to whether DNA functionalization could be used to achieve a completely different aim the realization of colloidal particles with a limited number of active spots, so to break the spherical symmetry of the interaction potential. Within this limit a strong directionality of the bonds is introduced, which mimics the chemical valence of molecules at much longer length-scales [166, 167]. 

Shortly after quantum mechanics evolved Heitler and London[l] applied the then new ideas to the problem of molecule formation and chemical valence. Their treatment of the H2 molecule was qualitatively very successful, and this led to numerous studies by various workers applying the same ideas to other substances. Many of these involved refinements of the original Heitler-London procedure, and within three or four years, a group of ideas and procedures had become reasonably well codified in what was called the valence bond (VB) method for molecular structure. 

If in the structure single bonds only occur, then the chemical valency is equal to the coordination number. For fractional bonds (as occur e.g. in PbS) with bond number n the relations are... 

The questions concerning the bonding in boranes posed by what were perceived to be their unusual structures are still being discussed today and the resulting research has had major consequences. In 1970, Linus Pauling151 stated that the formulas of the boranes do not conform in any simple way to chemical valence theory . 

R.F. Nalewajski, A.M. Koster, K. Jug, Chemical valence from the two-particle density matrix, Theoret. Chim. Acta (Berl.) 85 (1993) 463. 

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