Question

The oxidation number of underlined element in $K[\underline {Co} {({C_2}{O_4})_2}.{(N{H_3})_2}]$ compound is:
A.$ + 3$
B.$ + 2$
C.$ + 1$
D.$0$

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.
Here we have to find the oxidation number of cobalt. We know that the atomic number of cobalt is $27$. So its electronic configuration will be $Ar3{d^7}4{s^2}$. So it can show a number of valencies by losing seven d-electrons and two s-electrons. In the compound given in the question the ligands use are oxalate ions whose valency is $ - 2$ and amino groups whose valency is $0$ and the oxidation number of potassium is $ + 1$ . Now the compound is neutral so the sum of all charges will be equal to $0$.
Now suppose the oxidation number of cobalt in this compound is $x$. So the equation of sum of charges is as:
$
  1 + x + 2 \times ( - 2) + 0 = 0 \\
  x = + 3 \\
 $.
So the oxidation number of cobalt in this compound will be $ + 3$.

Hence option A is correct.

Note:
Ligands: These are the molecules which form bonds with the central metal atom or ions. They can be of two types based on electron pair donor or acceptor. If they accept electron pair i.e. ligand is electron pair acceptor then it is Lewis acid and if it is electron pair donor then they will be Lewis base.