Question

The oxidation number of nitrogen in $N{{O}_{3}}^{-}$ is?
(A) +1
(B) +2
(C) +3
(D) +5

Answer 1

Hint: $N{{O}_{3}}^{-}$ is a polyatomic ion. For polyatomic ions, the sum of oxidation numbers of all the atoms of the ion will be equal to the net charge of the ion. This can be used to calculate the oxidation number of nitrogen in $N{{O}_{3}}^{-}$.

Complete-step- by- step answer:
Let us first understand what is meant by oxidation number.
The oxidation number of an element is also called an oxidation state. Oxidation number can be defined as the number that is assigned to elements in a chemical substance. It is basically the number of electrons that an atom is capable of sharing, gaining or losing in a chemical reaction while forming bonds with atoms of another element. In a different way, it can be defined as the hypothetical charge that an atom would possess if all its bonds to atoms of different elements were completely ionic, with no covalent component in it. The value of oxidation number can be positive, negative or even zero.

Now that we have understood what is meant by oxidation number, let us see the rules we need to follow to calculate the oxidation number of an element in a chemical substance.
i) Hydrogen and oxygen exhibit the oxidation states of +1 and -2 respectively most of the time. H shows a -1 state only when bonded with an element having less electronegativity than H and O shows -1 state only in peroxides.
ii) Any free element will have an oxidation number of 0, and the sum of all oxidation numbers of the atoms of a neutral compound will be 0.
iii) We have to find the oxidation number of Nitrogen in $N{{O}_{3}}^{-}$. $N{{O}_{3}}^{-}$ is a polyatomic ion.
iv) In the case of polyatomic ions, the sum of oxidation numbers of all the atoms of the ion will be equal to the net charge of the ion.

We can use this idea to find the oxidation number of N in $N{{O}_{3}}^{-}$. Let us take the oxidation number of N to be y. We know that the oxidation number of 1 oxygen atom is -2 (it is not a peroxide) and the net charge of the ion is -1.

So, (Oxidation number of nitrogen x number of nitrogen atoms) + (Oxidation number of oxygen x number of oxygen atoms) = Net charge of nitrate ion.
Substituting the values,
(y x 1) + (-2 x 3) = -1
y + (-6) = -1
y = 5

Therefore, we get the value of the oxidation number of N in $N{{O}_{3}}^{-}$ as 5.

So, the correct answer is option (D).

Note: The oxidation number of oxygen should not be blindly taken as -2 each and every time. Oxygen has the oxidation state of -1 in peroxides. It is also important to remember that all atoms of an element need not have the same oxidation number in a compound.