Question

The oxidation number of N and Cl in $\text{NOCl}{{\text{O}}_{\text{4}}}$ respectively are:
A. +2 and +7
B. +3 and +7
C. -3 and +5
D. +2 and -7

Answer 1

Hint: Think about the fixed oxidation numbers of certain atoms. Divide the given compounds into its constituent ions and then equate the charge with the oxidation states.

Complete answer:
First of all, we need to write the compound $\text{NOCl}{{\text{O}}_{\text{4}}}$ into its constituents in which they exist in the reactions. That is:
$NOCl{{O}_{4}}\to N{{O}^{+}}+Cl{{O}_{4}}^{-}$
Let’s find out the oxidation number of N in$N{{O}^{+}}$.
We know that the fixed oxidation number of oxygen is -2. Since, the charge present on the ion is +1, the addition of the oxidation states of the atoms in the ion should result in +1.
Let, $x$ be the oxidation state of $N$. Thus, the equation will be:
\[(1\times x)+(1\times \text{ oxidation state of }{{O}_{2}})=1\]
Putting the values that we know:
\[\begin{align}
  & x+(1\times -2)=1 \\
 & x=+3 \\
\end{align}\]
Let’s find out the oxidation number of Cl in $Cl{{O}_{4}}^{-}$.
We know that the fixed oxidation number of oxygen is -2. Since, the charge present on the ion is -1, the addition of the oxidation states of the atoms in the ion should result in -1.
Let, $y$ be the oxidation state of $Cl$. Thus, the equation will be:
\[(1\times y)+(4\times \text{ oxidation state of }{{O}_{2}})=-1\]
Putting the values that we know:
$\begin{align}
  & y+\left( 4\times -2 \right)=-1 \\
 & y-8=-1 \\
 & y=+7 \\
\end{align}$
So, the oxidation number of N and Cl in $\text{NOCl}{{\text{O}}_{\text{4}}}$ are +3 and +7 respectively.
Therefore, the correct option for this question is ‘B.+3 and +7’

Additional Information:
Oxidation number is a number given to an element in a chemical combination. It is basically the number of electrons that an atom can share, loss or gain during formation of a chemical bond with different elements.

Note: Don’t be confused with the oxidation number and valency of an atom. Here, the major difference is valency cannot be negative but oxidation number can be negative, zero and positive values. Oxidation number and oxidation state are almost the same thing but there is a difference while we are considering electronegativity of the atoms. Also remember that a chlorine atom usually has an oxidation state of -1, but it may change in the presence of electronegative compounds.