Valency and oxidation number

In conclusion we can say that the valency of an element is a number which expresses how many atoms of hydrogen or other atoms equivalent to hydrogen can unite with one atom of the element in question. If necessary the valency of the element is denoted by a roman numeral following the symbol like C1(I), Br(I), N(III) or as a superscript, like Cl1, Br1, N111, etc. 

Some elements, like hydrogen, oxygen, or the alkali metals, seem always to have the same valency in all of their compounds. Other elements however show different valencies thus, for example, chlorine can be mono-, tri-, penta- or heptavalent in its compounds. It is true that compounds of the same element with different valencies show different physical and chemical characteristics. 

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Valency and oxidation numbers a historical sketch of bonding theory prior to quantum mechanics... 

Complexation studies (continued) polymer backbone, 1, 258 polymerization monitoring, 1, 259 pulse radiolysis initiation, 1, 535 in thin film and wire depositions, 1, 259 valence and oxidation number, 1, 18 t/-Complexes with niobium, 5, 66, 5, 68 with tantalum, 5, 108... 

Up to now we have used the terms valence and oxidation number without defining them explicitly in a quantitative sense. In the discussion to follow we shall understand valence in the quantitative sense to mean the number of bonds that link an atom to its neighbors. By oxidation number we shall mean the number of electrons that must be removed from each atom of the free element to form the compound xmder consideration, subject to the convention that combined oxygen shall have an oxidation number of —2 (except in peroxycompounds) and that covalently bound hydrogen shall have an oxidation number of +1. In the ion [Co(NH3)6]", for example, cobalt is 6-covalent and has an oxidation number of +3. 

You w3l nodoe that die compounds of iron are named ironfil) sulphate and irondll) sulphate to show which of its valencies iron is using in the compound. This is always done with tbe compounds of elements of variable valency. For valency and oxidation number, see Chapter 8, p. 72. 

In each case where the d" value determined by consideration of valence and oxidation number differ, that determined by employing the valence shows a better correspondence to the molecular orbital diagram of the compound under consideration. 

Established Practice in the English Language. The nearly literal translation of the French terms in English, Russian and other languages resulted in the system whose use has become standard practice in English-speaking as well as other countries. The system has been molded by the fact that elemental composition and valence (or oxidation number) are the principal variables for most inorganic compounds other than the most complex, whereas connectivity and the possibility foi isomers have been of little concern. 

It should be apparent that valency number and oxidation number are two quite different concepts, not to be confused. Where it is possible to assign either a valency or an oxidation number to an atom, the two are often the same, but this is not invariably so. For example, in the N2... 

The Racah parameters listed in table 11.1 indicate that the values of B increase with increasing oxidation state and number of 3d electrons in the first transition metal series. These features are also demonstrated by the free-ion Racah B values for cations with the 3cP and 3d5 configurations plotted in fig. 11.1. Similar factors, namely high valences and large number of 3d electrons,... 

Many chemical molecules that are not stable under normal conditions at room temperature are found in high-temperature vapors. These include gaseous molecular forms of compounds such as metal oxides or chlorides that are normally encountered as solids at room temperature. Other species found in high-temperature vapors have unusual valencies and coordination numbers. Examples are the molecules OPCl and O2PCI, which contain unusual two-coordinate trivalent and three-coordinate pentavalent phosphorus atoms, respectively. It is of interest to study such species to extend our knowledge of the structure and bonding of small molecules to new regions of the periodic table. Moreover, many such species are proposed chemical reaction intermediates. 

Figure 4.4 gives a summary of the variable valence of the first-row transition metals as a function of their d configuration and oxidation number. 

Two properties that are characteristic of second-row atoms in the Periodic Table, compared to the corresponding valence isoelectronic first-row atoms, are hypervalency (increased coordination) and the relative importance of d-type orbitals to their molecular electronic structure description. Hypervalency in sulphur compounds is represented by trivalent, tetravalent and hexavalent sulphur where a central sulphur atom is bonded to more than two ligand atoms or groups, compared to the oxygen atom which is almost exclusively divalent. Sulphur-containing compounds are typically classified in this manner9. Here, we have not differentiated between coordination number, valency and oxidation state. This point will be addressed later. 

Comment on the magnesium oxidation state, valence, and coordination number in Jones s reagent. 

Each of the elements of group IV has four valence electrons, which occupy s and p orbitals of the outermost shell. The maximum oxidation number of these elements is +4. All of the compounds of silicon correspond to this oxidation number. Germanium, tin, and lead form two series of compounds, representing oxidation number +4 and oxidation number 4-2, the latter being more important than the former for lead. 

In order to deal with oxidation-reduction reactions that are more complex than the simple ones discussed so far, we must introduce the concept of oxidation numbers (sometimes called oxidation states or valence states). Oxidation numbers permit us to identify and balance redox reactions and to determine the oxidant and reductant. 

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