Valency of an atom

The two reaction schemes of Figures 3-13 and 3-15 encompass a large proportion of all organic reactions. However, these reactions do not involve a change in the number of bonds at the atoms participating in them. Therefore, when oxidation and reduction reactions that also change the valency of an atom ate to be considered, an additional reaction scheme must be introduced in which free electron pairs are involved. Figure 3-16 shows such a scheme and some specific reaction types. 

The discussion of interatomic distances is less simple for intermetallic compounds than for pure metals among the complicating factors are the partial ionic character of bonds, the transfer of electrons, with consequent changes in valency, and the preferential use of the valencies of an atom in the formation of strong bonds rather than weaker ones. These factors, which of course participate in minimizing the energy of the system, usually operate to decrease the interatomic distances. Then-effects may be illustrated by some examples. 

According to this principle, the valence of an atom is considered a constant When more atoms coordinate to the same atom, the valence has to be distributed over more bonds, and hence the strength per bond decreases. When the chemical bonds are equivalent, the bond strength of an individual bond s n) depends in the following way on the corresponding bond strength of a complex with a single... 

Valency, as the principle is called today, is one of the fundamental concepts in chemistry. In more modern terms, the valence of an atom equals the number of bonds that an atom has for combining with other elements hydrogen, for example, has one, while other elements have more. 

Unless the bonding about A is of extreme ionic or covalent type, only the total VA is expected to match the empirical valency of an atom as reflected, e.g., in its position in the periodic table (Appendix B). 

The successive synthons and retrons correspond correctly, but one of the disconnections in the sequence was not done for a reason such as violation of the valence of an atom, the existence of restrictions on the application of a disconnection, or special attributes not fulfilled by an atom. 

These terms are frequently used and may lead to confusion, if used in the wrong context. As a reminder, the valency of an atom is strictly the number of bonds (including o, n and 8) in which it participates in any particular compound. In IF , the iodine atom participates in seven a bonds to the fluorine atoms. The fluorine atoms are individually monovalent. Oxidation states are more useful than valency in describing ionic compounds. In the crystalline solid CaF0, the calcium is best thought of as composed of calcium dications and fluoride anions, Ca2+(F ),. The calcium is in its +2 oxidation state, having lost its valency electrons, and the fluorine atoms are in the -1 oxidation state, both having accepted an elec-... 

There are certain existence requirements for an atom set of an R-category. We will refer to valency of an atom, as the number of its designated bonds participating in the R-matrix. Other bonds associated with the atom but not taking part in the reaction are of no concern to us. 

R224 We have 4 atoms, A, B, C and D, all monovalent. We will list their bonds in linear order such that 1 represents a single bond between the 2 neighboring atoms, 2 represents a double bond, 0 for no bond and so on. The valency of an atom is thus the simple addition of the bonds adjacent to it... 

Similar to SMILES, InChI does not store atom coordinates. In contrast to SMILES, which by default omits hydrogen atoms that are then added implicitly to match the most common valency of an atom, InChI stores hydrogen atoms but does not store bond orders. These two techniques are just different approaches to the same problem for a given molecular skeleton, the bond orders and number of hydrogen atoms... 

The change in valence of an atom obviously involves the gain or loss of electrons. For example,... 

Knowledge of the interatomic distances allows the experimental valence of an atom to be calculated using tables of bond-valence parameters. If the apparent valence of an atom is higher than that expected for an ionic bonding model, say... 

The actual charge or valence of an atom in combination is of importance in the reactions to be considered, and when, under certain conditions or by various treatments, the valence is changed, the mechanism of the reaction involving such a change is of interest. These questions are fundamental and will be considered in detail as occasion arises. 

A hypervalent compound is typically considered to be one in which the central atom has in its valence shell more than eight valence electrons, as in NF5 or the known PF5 and SFe, which appear to have 10, 10, and 12 electrons, respectively, in their valence shells [10]. The valence of an atom may reasonably be taken as the number of electrons it uses in bonding, as expressed in a Lewis stmcture. Thus carbon in CH2, CH3 and CH4 is taken, nncontroversially, to be di-, tri- and tetrava-lent, respectively. Nitrogen in NF3 is trivalent, and in NF5 would be pentavalent, according to simple Lewis stmctures ... 

Should dative bonds be counted when we define the valency of the boron and nitrogen atoms There does not seem to be general agreement between chemists at this point. Some (including the author of this book) do not include dative bonds in the valency, others count both normal and dative bonds and describe both the B and the N atom as tetravalent. Some define the valency of an atom in terms of the number of electrons used for bond formation. This means that the N atom in amine borane is pentavalent while the B atom remains trivalent [2]. 

Over the last decades, the concept of bond-valence calculations has been widely applied in solid state chemistry for prediction of bond lengths, as well as for determination of coordination numbers in corresponding compoimds. Besides, this concept serves as a criterion to verify the reliability of structure determinations (Brown, 1978 Brown and Altermatt, 1985 Altermatt and Brown, 1987). According to their approach, the atomic valence of an atom, Vi, is defined as the sum of bond valences z>ij... 

The valence of an atom is also derived from the rules of QM, which set a limit on the number of electron pairs that an atom can have in its valence shell (the outermost electronic shell). The chemists have discovered these rules and have given them other names, which we shall be using during the course. For example, the Octet Rule (developed by the chemist G.N. Lewis) limits many atoms to have up to eight electrons in their valence shell, namely foirr pairs of electrons.This detemfines the connectivity of the atom in the bond construction game. 

The valency of an atom is equal to the number of atoms in direct union with it. In C2H2 there are two bivalent carbon atoms, in benzene six tervalent carbon atoms, each linked to an atom of hydrogen. In formulae Lossen uses only single bonds between atoms. The valency of a radical is a number expressing how many atoms, not belonging to the radical, are directly bound to the atoms which compose the radical. A radical is an atomic complex contained in a molecule, all the constituents of which are linked either directly or indirectly . Each part of a molecule may be called a radical, but usually only those atomic aggregates are called radicals which have the character just described. Lossen s... 

Werner s most important contribution, his theory of valency and the structure of coordination compounds, was first presented in 1891 to qualify for a post in the Zurich Polytechnic. He assumed that the valency of an atom, including the carbon atom, is an attractive force emanating from the centre and acting uniformly towards all parts of the surface, rather than directed valency bonds. Although he claimed that this would lead to van t Hoff s configurational formulae, it is on the basis of the latter and directed bonds that Werner s own theory of coordination compounds has been most successfully explained. 

As a final example of a hypervalent species, let us consider SiFg , whose Lewis structure is shown in Figure 1.6. As shown, silicon uses all four of its valence electrons in bonding in SiFs " and is therefore tetravalent. Indeed, the species may be represented by a no-bond resonance form consisting of SiF4 and two closed-shell F anions the latter do not contribute to the valence of the silicon. This is a general point coordination of closed-shell Lewis bases has no effect on the valence of an atom. For SiFg , we can also determine the valence of silicon more mechanically ... 

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