How can an oxidation number be determined?

1 Answer
Oct 8, 2016

Answer:

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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