Answer the question on the basis of passage given below:

In the periodic table, the d-block contains the elements of group 3 to 12. The d-orbitals are progressively filled in each of the four long periods. The elements of d-block referred as transition metals have partially filled d-orbitals and exhibit certain characteristic properties such as variety of oxidation states, formation of coloured ions, act as catalyst and slow paramagnetic behavior.

The two inner transition metals series 4f and 5f are known as Lanthanoids and Actinoids respectively. The lanthanoids resemble one another more closely as compared to ordinary transition in any series.

For the four successive transition elements given below, the stability of +2 oxidation state will be

(A)    Cr (Atomic number, Z = 24)

(B)    Mn (Z = 25)

(C)    Fe (Z = 26)

(D)   Co (Z = 27)

Choose the correct answer from the options given below:

Mn > Fe > Cr > Co

The electronic configurations of Cr is:

\(_{24}Cr = [_{18}Ar]3d^54s^1\)

In the first excited state the \(4s\) electron is excited and \(3d^5\) configuration is attained which is a highly stable state due to the presence of half-filled d-orbitals.

In the second excited state, i.e., in \(+2\) oxidation state, one electron from the d-orbital is lost and 3d4

configuration is achieved which is less stable than half \(_{26}Fe = [_{18}Ar]3d^64s^2\) -filled orbital. Therefore, the +2 oxidation state is not stable in Cr.

The electronic configuration of Mn is:

\(_{25}Mn = [_{18}Ar]3d^54s^2\)

In the second excited state, that is on losing 2 electrons from its \(4s\) subshell, manganese acquires \(3d^5\) valence shell configuration which is very stable due to the half-filled valence shell. Therefore, in Mn, \(+2\) oxidation state is very stable.

The electronic configuration of Fe is

\(_{26}Fe = [_{18}Ar]3d^64s^2\)

The removal of 2 electrons results in \(3d^6\) configuration which is less stable than \(d^5\) but more stable than \(d^4\) configuration, because the addition of one more electron to the half-filled state makes the atom quite unstable but more energy is required to excite an electron from the half-filled state. Hence, the latter brings more instability.

The electronic configuration of Co is:

\(_{27}Co = [Ar]3d^74s^2\)

In the \(+2\) state, the valence shell configuration is \(3d^7\), which is not stable because the \(d^6\) configuration is already unstable due to the loss of the half-filled state, and the addition of one more electron makes the atom highly unstable. So, \(+2\) state in Co is the least stable among these elements.

Therefore, the order of stability is (2)  Mn>Fe>Cr>Co.