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 PO₄^3-?

In PO₄^3- , 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.