Question

What is the oxidation number of iron in $F{e}_2{O}_3$ ?

Answer 1

Hint: The oxidation number of any atom of the molecule depends upon its combining capacity and/or valency; but is calculated by simple mathematical calculations. The calculations depend upon the overall charge on the molecule completely.

Complete answer:
Let us learn about the calculations based on the oxidation number;
When we say that the calculations are based on the overall charge on the molecule; we have three possibilities-
Charge is zero i.e. neutral molecule.
Charge is positive i.e. cationic molecule.
Charge is negative i.e. anionic molecule.
Now, let us consider any molecule i.e. $X_{2}Y$ (a neutral molecule) and we have given the charge on Y is +2.
To calculate the charge on X;
$\begin{array}{rl}& (2\times x)+(+2)=0\\ & 2x=-2\\ & x=-1\end{array}$
Thus, the charge on X in the molecule $X_{2}Y$ is -1.
Illustration-
Given that,
A neutral molecule i.e. $F{e}_2{O}_3$ and we know the charge on Oxygen is -2. Thus,
$\begin{array}{rl}& (2\times x)+(3\times (-2))=0\\ & 2x-6=0\\ & 2x=6\\ & x=+3\end{array}$
Thus, the oxidation number of iron in $F{e}_2{O}_3$ is +3.

Note:
Do note that the atom will have unique valency but this is not true when it comes for oxidation number. The oxidation number will be different for the same atom in different molecules. For example;
Fe has oxidation number +3 in $F{e}_2{O}_3$ (as stated above) but the same atom has oxidation number of +2 in FeO.