Oxidation number, definition

The concept of oxidation number leads directly to a working definition of the terms oxidation and reduction. Oxidation is defined as an increase in oxidation number and reduction as a decrease in oxidation number. Consider once again the reaction of zinc with a strong acid ... 

These definitions are of course compatible with the interpretation of oxidation and reduction in terms of loss and gain of electrons. An element that loses electrons must increase in oxidation number. The gain of electrons always results in a decrease in oxidation number. 

The modem theory of valency is not simple—it is not possible to assign in an unambiguous way definite valencies to the various atoms in a molecule or crystal. It is instead necessary to dissociate the concept of valency into several new concepts—ionic valency, covalency, metallic valency, oxidation number—that are capable of more precise treatment and even these more precise concepts in general involve an approximation, the complete description of the bonds between the atoms in a molecule or crystal being given only by a detailed discussion of its electronic structure. Nevertheless, these concepts, of ionic valency, covalency, etc., have been found to be so useful as to justify our considering them as constituting the modern theory of valency. 

The concept of the oxidation state of an element in a chemical compound has a long and confusing history. In the most pretentious form of the concept the oxidation state or oxidation number is the electrical charge localised on the concerned atom in the compound. Confusion arises when we realise that the definition of the atomic domain is arbitrary and the experimental determination of the electrical charge in the once chosen domain is often problematic. 

On trying to assign an oxidation number to the copper atom one finds that the atomic charges, as calculated with the MuUiken definition 183), are remarkably small ... 

The concepts of formal oxidation state, spectroscopic oxidation state, oxidation number are often subject to controversial discussion. Regarding the definitions, the reader is referred to Appendix F and [22, 23]. 

It should be clear by the definition given so far that the carbene-analogous state is limited to molecular species. The oligomer of EX2 (EX2)n is, of course, much more stable than EX2 in every respect. It should nevertheless be noted that also the oxidation number does not change in going from the monomer to the polymer the chemical, structural, and electronic properties of these species are completely different. 

Whether zinc is a main-group or transition metal depends, of course, on one s definition of transition metal and main-group metal. Those who classify zinc as a main-group metal cite its (almost) exclusive oxidation number of +2 in compounds (but see Section 2.06.15.2) and the absence of a partially filled r/ shell in the metal and its compounds. Those who classify zinc as a transition metal usually note its much greater effective nuclear charge, polarizing power and its limited, but well defined, coordination chemistry. 

Unfortunately, many compounds contain bonds that are a mixture of ionic and covalent. In such a case, a formal charge as written is unlikely to represent the actual number of charges gained or lost. For example, the complex ferrocyanide anion [Fe(CN)6]4- is prepared from aqueous Fe2+, but the central iron atom in the complex definitely does not bear a +2 charge (in fact, the charge is likely to be nearer +1.5). Therefore, we employ the concept of oxidation number. Oxidation numbers are cited with Roman numbers, so the oxidation number of the iron atom in the ferrocyanide complex is +11. The IUPAC name for the complex requires the oxidation number we call it hexacyanoferrate (II). 

Hydrogen is capable of forming compounds with all elements except the noble gases. In compounds with nonmetals, hydrogen usually behaves like a metal instead of a nonmetal. Therefore, when hydrogen combines with a nonmetal, it usually has a +1 oxidation number. When hydrogen combines with a metal, it usually has a —1 oxidation number. Hydrogen compounds with the transition metals are usually nonstoichiometric. Nonstoichiometric compounds have no definite formula. 

Today, many reactions in aqueous solutions can be described as oxidation-reduction reactions (redox reactions). Oxidation is the process in which the oxidation number of atoms increases. Reduction is the process in which the oxidation number of atoms is decreased or made more negative. In another definition, oxidation is the loss of electrons by an atom, and reduction is the gain of electrons. Let us look at the following reaction ... 

The Ni3S2 fragments in the two complexes (263) and (265) have a trigonal bipyramidal geometry with the two S atoms in the apical positions and the three nickel atoms in the equatorial plane, as found in other organometaUic compounds which contain the same Ni unit.1939 The formation of the enneanuclear complex (264), on the contrary, is exceptional and no other complex of this stoichiometry was isolated with analogous tertiary phosphines, with selenium, or with metals other than nickel. It is not possible to assign any definite oxidation number to nickel in complex (264). The lack of two electrons with respect to the situation of nine nickel(II) ions was reported to be essential for the existence of complex (264) because the oxidation is spontaneous and its reduction invariably leads to the decomposition of the cluster compound.19 ... 

Ans. (1) is Fe(OH)3 and (2) is Fe(OH)2- The percentages tell us that there are definitely two compounds. The reason the condition exists is that Fe can have two oxidation numbers, +3 and +2, producing two compounds with a small number ratio of the components. 

With such a definition in mind, one envisions that an electron will be transferred as a unit and thus reaches the conclusion that the resultant charge must be an integer. Alternately, Shriver, Atkins and Langford [12] seem to have no problem with fractional oxidation numbers and define the term as ... 

The new definition of formal oxidation number given by Pauling (154) defined the new class of coordination compounds as having a zero oxidation number. 

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