Question

Why is Manganese more stable in + 2 state than the + 3 state and the reverse is true for Iron?

Answer 1

To solve this problem we have to know the electronic configuration of Iron and Manganese which are $\left( {\left[ {Ar} \right]3{d^6}4{s^2}} \right)and{\text{ }}\left( {\left[ {Ar} \right]3{d^5}4{s^2}} \right)$ respectively.
All atoms gain extra stability when their valence orbital remains fulfilled or half-filled by an electron.
An atom or element in which a number of loosely bound valence electrons are removed to get stable configuration, is called its oxidation state.

Complete answer:
According to the periodic table Iron and Manganese are d block elements. Their respective electronic configurations are $\left[ {Ar} \right]\;3{d^6}4{s^2}\; and {\text{ }}\left[ {Ar} \right]{\text{ }}3{d^5}4{s^2}$ ,where Ar is a noble gas element.
We know that an atom gains extra stability when its valence orbital remains fulfilled or half-filled by an electron ( For d orbital it is $d^{10}$ or $d^5$).
Here from above electronic configurations we see that 3d and 4s valence orbitals electrons number of Iron and Manganese are (6 ,2) and ( 5, 2) respectively.
If Iron loses its three electrons (two from s orbital and one from d orbital) then five electrons will remain in its ‘d’ orbital. So it become half-filled $\left( {{d^5}} \right).$ If it loses two electron then extra one electron will remain in ‘d’ orbital after becoming ${d^5}.$
For Manganese if it loses two electrons from ‘s’ orbital then the orbital becomes half-filled for having five electrons. If it loses three electron then one electron remain less to become half filled $\left( {{d^5}} \right).$
So iron is more stable in +3 oxidation state due to the fact that of half filled electronic configuration and Manganese is more stable in +2 oxidation state.

Therefore, from the above fact we see that Manganese is more stable in + 2 oxidation state than in + 3 and Iron is more stable in + 3 than in + 2.

Note: 1. Here from the above fact we get an idea about how to find out the stable oxidation state of an atom.
2.It should be noted that for block elements, electrons start to lose from orbital then from orbital.