Oxidation Numbers
المؤلف:
Jerome L. Rosenberg and Lawrence M. Epstein
المصدر:
College Chemistry
الجزء والصفحة:
p 84
12-7-2017
1885
Oxidation Numbers
The degree of oxidation of an element is specified by the atom's oxidation number (ON), rules for which are as follows:
1. The oxidation number of a monatomic ion is the charge on the ion, e.g., for Fe2+, ON(Fe) = +2, for Cl-, ON(Cl) = -1. The alkali metals (Group IA) and alkaline earth metals (Group IIA) are always ionic in compounds like NaCl or Mg(NO3)2.
2. The ON of an uncombined element is 0, e.g., the ON is 0 for H in H2(g), S in S8(s).
3. The following conventions apply to covalently bound atoms: Except for the elements, (a) ON(F) = -1 (always); (b) ON(O) = -2 (except in peroxides like HOOH, where ON(O) = -1); (c) ON (H) = +1 (except in metal hydrides like NaH, where ON(H) = -1).
4. In molecules or ions containing two or more atoms, the sum of the ONs of all the atoms equals the charge on the species, e.g., H2O: ON(O) + 2 ON(H) = -2 + 2(+1) = 0; to find ON(N) in NO3-: ON(N) + 3
ON(O) = -1, thus ON(N) = -1 -3(-2) = +5.
Example 1
What is the oxidation number of P in PO43-?
In PO43- , rule (4) gives
Example 2
What is the oxidation number of Si in H2SiO3?
In H2SiO3, rule (4) gives:
With rules (3b) and (3c), we have
It is important to recognize that oxidation numbers are a book-keeping device and should not be takento indicate true charges or even formal charges. In H2SiO3—SiO(OH)2—the Si-O and O-H bonds are covalent, i.e., electrons are shared between atoms; although we might expect the Si and H atoms to be somewhat positive and the O atoms somewhat negative, the charge separation is nowhere near as great as suggested by the ONs.
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