Question

How can an oxidation number be determined?

Answer 1

Oxidation state/number is the charge left on the central atom, when all the bonding pairs of electrons are broken with the charge assigned to the more electronegative atom.

Explanation:

Redox chemistry is conceived to occur on the basis of addition OR subtraction of electrons. Typically, an element in a compound or complex (or the element itself) is assigned an oxidation number. The sum of the oxidation numbers always equals the charge on the ion.

I will consider a few simple examples: `MnO_4^-`, and `Cr_2O_7^(2-)`.

TO begin, elemental oxygen, `O_2`, has a ZERO oxidation number. Typically it reacts to give the oxide, `O^(2-)`. We conceive that the oxygen molecule has picked up 4 electrons to give `2xxO^(2-)`. From where does it get the electrons? Well, of course from the metal, the which formally LOSES electrons upon oxidation.

Thus for `MnO_4^-`, the oxidation numbers of `Mn` and of `O` must sum to `-1`, the charge on the ion. Thus `Mn_"ON"+4xx(-2)` `=` `-1`. So `Mn_"ON"`, the oxidation number of the metal is clearly `+VII` (typically we use Roman numerals to specify the oxidation number).

And likewise for `Cr_2O_7^(2-)`, `2xxCr_"ON"+7xx(-2)=-2`. It does not take too much arithmetic to work out that `Cr_"ON"=+VI`.

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