Question

: The oxidation number of manganese in $KMn{O_4}$ is ____.
A.$ + 3$
B.$ + 5$
C.$ + 7$
D.$ + 9$

Answer 1

Hint: Oxidation number of an element: It is defined as the total number of electrons that an atom accepts or loses in order to make a chemical bond which results in the formation of a chemical compound.

Complete step by step answer:
First of all we will read about transition elements.
Transition elements: Those elements which are in groups from three to eleven. They are called a transition because they are in between the s- block elements and p-block elements. For example: Scandium, iron, zinc, etc. They have fully or at least one electron in their d-orbits. For example: scandium has one d-electron, zinc has ten d-electrons. Due to the different number of electrons present in their d-shells they show different valencies. For example: Scandium having one electron can show three valencies as one, two and three. Because the atomic number of scandium is $21$and its electronic configuration is $Ar3{d^1}4{s^2}$. By losing only one s-electron it will attain one valency and by losing both the s-electrons it will attain valency two. And by losing two s-electrons and one d-electron it will achieve three valencies which are stable states of scandium. Because after losing three electrons it will attain electronic configuration of noble gas argon.
 Here we are given with the transition metal i.e. manganese. The atomic number of manganese is $25$. Hence its electronic configuration is as: $Ar3{d^5}4{s^2}$. So it can show a number of valencies by losing five d-electrons and two s-electrons. Here we are given with the compound $KMn{O_4}$ and we know that the oxidation number of $K$ is $ + 1$ and oxygen is as $ - 2$. And this is a neutral compound. So the sum of all charges will be equal to $0$. Now suppose the oxidation number of manganese in this compound as $x$. So the equation of sum of charges is as:
$
  1 + x + 4 \times ( - 2) = 0 \\
  x = + 7 \\
 $.
So the oxidation number of manganese in this compound will be $ + 7$.

Hence option C is correct.

Note:
If a compound has net charge either positive or negative then the sum of all charges in the compound will be equal to that net charge on the compound. By using this concept we can calculate the oxidation number of elements in charged species.