Question

Oxidation number of P in ${{P}_{2}}O_{7}^{4-}$?
A. +3
B. +4
C. +5
D. +6

Answer 1

Hint: Oxidation number also called as oxidation state and it is defined as the number of electrons gained or lost by an atom when it is going from its free state to the combined state with the atoms of other elements. Here the oxidation number of oxygen will be -2.

Complete Solution :
- From your chemistry lessons you have learned about the oxidation state. Oxidation number also called an oxidation state is defined as the number of electrons gained or lost by an atom when it is going from its free state to the combined state with the atoms of other elements. The oxidation state of any atom can be negative, positive as well as zero. The substance which is present in its elemental state will have the oxidation state as zero.
- If the oxidation number of an atom is increasing then it refers to the oxidation of the atom whereas if the oxidation number is decreasing then it refers to reduction of that atom. And the oxidation number of any compound will be equal to the overall charge that is present at the atom.
- As in the question we have asked to find the oxidation state of P in compound ${{P}_{2}}O_{7}^{4-}$, so the oxidation number of the compound will be -4 because the total charge on the compound is -4.
- Now to find the oxidation state of P we should know the Oxidation state of oxygen. There are variable oxidation states shown by the oxygen because the oxidation states of the p-block element vary. The oxidation state shown by oxygen in most of the cases are -2. Here the oxidation number of oxygen will be -2. Let us take the oxidation number of both the p atom as 'x'.

 \[Oxidation\,state\,of\,P+Oxidation\,state\,of\,O\times 7=-4\]
\[\therefore 2x+-2\times 7=-4\]
\[2x-14=-4\]
\[\therefore x=+5\]
Therefore the oxidation state of P in ${{P}_{2}}O_{7}^{4-}$ is +5.
So, the correct answer is “Option C”.

Note: The other oxidation states shown by oxygen is -2, -1, +1 and +2. In case oxygen is bonded to more electronegative atoms then the oxidation state shown by that oxygen will be positive because the atom with more electronegativity will tend to pull the electron towards themselves and thus acquires negative charge on them. The more electronegative atom other than oxygen is only fluorine.