The oxidation number or state of an atom/ion is the number of electrons an atom/ion that the molecule has either gained or lost compared to the neutral atom.
Oxidation number or oxidation state of an atom or ion in a molecule/ion is assigned by:
- Summing up the constant oxidation state of other atoms/molecules/ions that are bonded to it and
- Equating the total oxidation state of a molecule or ion to the total charge of the molecule or ion.
The oxidation number of Fe in K4Fe(CN)6 can be calculated as follows,
4(+1) + x + 6(-1) = 0
Hence, x = +2