Oxidation numbers fractional

Rather than quantifying a complex total petroleum hydrocarbon mixture as a single number, petroleum hydrocarbon fraction methods break the mixture into discrete hydrocarbon fractions, thus providing data that can be used in a risk assessment and in characterizing product type and compositional changes such as may occur during weathering (oxidation). The fractionation methods can be used to measure both volatile and extractable hydrocarbons. 

The oxidation number of S in S4Og (tetrathionate) is +2.5. The fractional oxidation state comes about because six O atoms contribute —12. Because the charge is —2, the four S atoms must contribute +10. The average oxidation number of S must be +x = 2.5. 

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 ... 

Occasionally, oxidation numbers are fractional. For example, the oxidation number of iron in Fe304 is + 2f. This does not mean that electrons have been split the oxidation number simply gives the average of the numbers of electrons still controlled by the three iron atoms. 

The rules discussed so far allow us to make educated guesses about possible compounds formed by elements. However, not all compounds deduced by application of the rules actually exist. Moreover, some elements have more oxidation numbers than the rules identify. Nitrogen, for example, exhibits every integral oxidation number from -3 to +5, as well as a fractional oxidation number, -5, in HN3 and its salts. Except for fluorine, the halogens also exhibit most of the integral oxidation numbers from -1 to +7. Detailed study of the chemistry of the elements and their compounds is necessary to know which compounds actually exist. 

Convention 1 is fundamental because it guarantees charge conservation The total number of electrons must remain constant in chemical reactions. This rule also makes the oxidation numbers of the neutral atoms of all elements zero. Conventions 2 to 5 are based on the principle that in ionic compounds the oxidation number should equal the charge on the ion. Note that fractional oxidation numbers, although uncommon, are allowed and, in fact, are necessary to be consistent with this set of conventions. 

This results in one oxygen having two lone pairs, one homoatomic bond and one unpaired electron, which means that it has a share in the six electrons in the valence shell, exactly as it would have had in the elemental state, and so its oxidation number is zero. The other oxygen has three lone pairs and one homoatomic bond. Thus, it has a share in seven electrons, which is one more than the elemental state, and so it has an oxidation state of-1. This analysis resolves the problem of fractional oxidation numbers that was encountered above. 

Thirdly, and on a slightly different note, one can hypothesise that there is a reorganisation of the electrons in the n molecular orbitals to give another canonical structure, where the central nitrogen still has an oxidation number of+1, but now each of the terminal nitrogen atoms has an oxidation number of-1. Again, this analysis resolves the problem of fractional oxidation numbers encountered above. 

Note that oxidation numbers are no longer recommended when naming homopolyatomic ions. This is to avoid ambiguity. Oxidation numbers refer to the individual atoms of the element in question, even if they are appended to a name containing a multiplicative prefix, cf. Example 12 above. To conform to this practice, dimercury(2+) (see Section IR-5.3.2.3) would have to be named dimercury(I) dioxide(2—) (see Section IR-5.3.3.3) would be dioxide(—I) and ions such as pentabismuth(4+) (see Section IR-5.3.2.3) and dioxide(l—) (see Section IR-5.3.3.3), with fractional formal oxidation numbers, could not be named at all. 

The fraction n/m is equal to the exponent j in equation 6. It corresponds approximately to the oxidation number of the surface metal center = 2.1 for BeO (17), j = 3.25 for a-FeOOH (6), andj = 3.95 for Si02 (quartz) (19, 20). The following question arises What is the value ofj in reductive, proton-catalyzed dissolution of oxide minerals ... 

Oxidation number is a formal concept adopted for our convenience. The numbers are determined by relying on rules. These rules can result in a fractional oxidation number, as shown here. This does not mean that electronic charges are spht. 

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