Question

why is mn 2+ is more stable than mn 3+

Answer 1

The reason why Mn+2 ion is more stable than Mn+ ion is actually related to the electronic arrangement of the two ions. We know that the electronic configuration of Manganese is [Ar] 4s23d5. So, the electronic configuration of Mn+ ion will be [Ar] 4s13d5 while that of Mn+2ion will be [Ar] 3d5. Thus, it is the Mn+2 ion which has half filled d-orbitals. The half filled orbitals have additional stability associated with them due to the following reasons

1) Symmetrical arrangement of electrons

2) Stability due to the exchange energy (Half filled electronic configuration have large exchange energy and consequently have greater stability).

On the other hand hand, the Mn+ ion has only one electron on the 4s orbitals, which is readily lost in order to achieve stable half filled electronic configuration. 

Answer 2

If we look at the electronic configuration of Mn2+ , it is 1s2 2s2 2p6 3s2 3p6 4s0 3d5 . The 3d orbital is half-filled, which is relatively stable. in Mn3+ , there are only 4 electrons in the 3d orbital, which is less stable.