Question

Oxidation number of $Cr$ in ${K_3}Cr{O_8}$ is
$A.$ $+ 6$
$B.$ $+ 5$
$C.$ $+ 3$
$D.$ $+ 2$

Answer 1

Oxidation number is referred to as oxidation state. Oxidation number is defined as the charge that atoms appear to have on forming ionic bonds with other heteroatoms. It is also defined as the total number of electrons present that an atom either gains or loses in order to form a chemical bond.

Complete solution:
Before calculating the oxidation number of $Cr$ , we have to look at the structure of ${K_3}Cr{O_8}$ . In the structure of ${K_3}Cr{O_8}$ the oxygen atom has linked by peroxide linkage, so each oxygen has oxidation number of $- 1$ and the oxidation number of potassium is $1.$ The structure of ${K_3}Cr{O_8}$ is given below.
Let $x$ be the oxidation number of $Cr$ in ${K_3}Cr{O_8}.$ Here the overall charge on the complex is $0$ so, the sum of oxidation states of all element in it should be equal to $0.$
Now, we know the complex has peroxide linkage, so each oxygen atom has oxidation number of $- 1$ and the oxidation number of potassium is $1.$
Therefore, oxidation number of $Cr$ in ${K_3}Cr{O_8}$ can be calculated as:
$3 \times \left( 1 \right) + x + 8\left( { - 1} \right) = 0$
On solving the above expression we will get the oxidation number of $Cr.$
$x = 5$
Hence, we get the value of $x$ is $5$ . We know the value of $x$ is oxidation number of $Cr.$
So, the correct option is $+ 5.$

Hence the correct option is (B).

Note: There are certain rules for finding the oxidation number of an atom. These rules are stated below;
-The oxidation number of a free element is always zero, the oxidation number of a monatomic ion equal to the charge of that ion, the oxidation number of hydrogen is $+ 1$ , but when it combine with more electropositive element the oxidation number of hydrogen is $- 1$ , the oxidation number of oxygen in compound is usually $2$ but it is $1$ when there is a peroxide linkage and the sum of the oxidation number of all the atom in a neutral compound is $0$ whereas in case of charged compound the sum is equal to that charge.