Question

Explain why: $M{{n}^{+2}}$ is more stable than $F{{e}^{+2}}$ towards oxidation state to +3 state.(At. No. of Mn=25, Fe=26)

Answer 1

Hint: Fully filled and Half filled electronic configurations are more stable. Oxidation states with stable configurations are more easy to obtain than oxidation states with relatively less stable electron configurations.

Complete answer:
Manganese has atomic number 25, and electronic configuration of manganese is $1{{s}^{2}},2{{s}^{2}},2{{p}^{6}},3{{s}^{2}},3{{p}^{6}},4{{s}^{2}},3{{d}^{5}}$.
On removal of two electrons,it becomes $M{{n}^{+2}}$, whose configuration is $[Ar]4{{s}^{0}},3{{d}^{5}}$.
As we can see that on removal of two electrons, manganese have a configuration where d-block is half filled. We know that half filled and fully filled orbitals are more stable than any other configuration. Therefore, $M{{n}^{+2}}$ is stable at this oxidation state.
Iron has atomic number 26, and electronic configuration of Iron is $1{{s}^{2}},2{{s}^{2}},2{{p}^{6}},3{{s}^{2}},3{{p}^{6}},4{{s}^{2}},3{{d}^{6}}$.
On removal of two electrons, it becomes $F{{e}^{+2}}$, whose configuration is $[Ar]4{{s}^{0}},3{{d}^{6}}$.
Now Iron has six electrons in d-orbital, making the d-orbital unsymmetrically filled. So to obtain symmetry and more stability it will lose one more electron to go on to +3 oxidation state.
After losing one more electron from $F{{e}^{+2}}$, it becomes, $F{{e}^{+3}}$, whose configuration is $[Ar]4{{s}^{0}},3{{d}^{5}}$.
Now we see, in $F{{e}^{+3}}$, d-orbital is half filled with ${{d}^{5}}$configuration. It is more stable than ${{d}^{6}}$ configuration in $F{{e}^{+2}}$.
Now, when we compare, manganese and iron, we see that manganese has half filled d-orbital in +2 oxidation state which is more stable than any other oxidation state of manganese, and iron have half filled d-orbital in +3 oxidation state, therefore, iron tends to move toward +3 oxidation state and Manganese is stable at +2 oxidation state.

Note: The reason for the stability of half filled and fully filled orbitals or configurations is because of symmetry and exchange energy. The half filled and fully-filled orbitals are more symmetrical than any other configurations and symmetry leads to more stability. Also electrons present in different subshells exchange their positions, when electrons are filled symmetrically, exchange energy is less, hence greater stability.