Q:

Stability order of Cr2+ ,Mn2+ , Fe2+ ,Co2+

Stability order is given as $\ce{Mn^2+ > Fe^2+ > Cr^2+> Co^2+}$
Of course I know that $\ce{Mn^2+}$ is the most stable form as it has half filled degenerate orbitals. But my query is regarding the stability of $\ce{Fe^2+}$ and $\ce{Cr^2+}$. 
It is given in my book that $\ce{Fe^2+}$ has greater stability than $\ce{Cr^2+}$ through exchange energy. I want to know how exchange energy stabilizes $\ce{Fe^2+}$ more than $\ce{Cr^2+}$ . 

A:

Formation of ion takes place by removing electron from outermost shell. 
Electronic configuration for Fe2+ is [Ar]3d6 4s0
Electronic configuration of Cr2+ is [Ar]3d4 4s0
Cr2+ is a stronger reducing agent that Fe2+ in aqueous solution(d3 configuration more stabls than d5 in aqueous solution). 
So Cr2+ has higher tendency on losing electron compared to Fe2+ so Fe2+ is more stable than Cr2+.
Mn2+ has most stable and Co2+ is least stable(least exchange energy due to only 3 unpaired electrons).

