# Exceptional Cases of Evaluation of Oxidation Numbers

The rules described earlier are usually helpful in determination of the oxidation number of a specific atom in simple molecules but these rules fail in following cases. In these cases, the oxidation numbers are evaluated using the concepts of chemical bonding involved.

**Type I. **In molecules containing peroxide linkage in addition to element-oxygen bonds. For example,

(i) **Oxidation number of S in H _{2}SO_{5}**

(Permonosulphuric acid or Caro's acid)

By usual method; _{}

_{}or _{}

But this cannot be true as maximum oxidation number for S cannot exceed + 6. Since S has only 6 electrons in its valence shell. This exceptional value is due to the fact that two oxygen atoms in _{}shows peroxide linkage as shown below,

_{}

Therefore the evaluation of o.n. of sulphur here should be made as follows,

2 × (+1) + x + 3 × (–2) + 2 × (–1)

(for H) (for S) (for O) (for O–O)

or 2 + x – 6 – 2 = 0 or x = + 6.

(ii) **Oxidation number of S in H _{2}S_{2}O_{8}**

(Peroxidisulphuric acid or Marshall's acid)

By usual method; _{}

1 × 2 + 2x + 8 (–2) = 0

2x = + 16 – 2 = 14 or x = + 7

Similarly Caro's acid, Marshall's acid also has a peroxide linkage so that in which S shows +6 oxidation state.

_{}

Therefore the evaluation of oxidation state of sulphur should be made as follow,

2 × (+1) + 2 × (x) + 6 × (–2) + 2 × (–1) = 0

(for H) (for S) (for O) (for O–O)

or 2 + 2x – 12 – 2 = 0 or x = + 6.

(iii) **Oxidation number of Cr in CrO _{5}**

(Blue perchromate)

By usual method _{}; x – 10 = 0 or x = + 10

This cannot be true as maximum O. N. of Cr cannot be more than + 6. Since Cr has only five electrons in 3d orbitals and one electron in 4s orbital. This exceptional value is due to the fact that four oxygen atoms in CrO_{5}are in peroxide linkage.

The chemical structure of CrO_{5} is

_{}

Therefore, the evaluation of o.n. of Cr should be made as follows

x + 1 × (– 2) + 4 (–1) = 0

(for Cr) (for O) (for O–O)

or x – 2 – 4 = 0 or x = + 6.