An ion Mn^a+ has the magnetic moment equal to 4.9 BM. The value of a is : (atomic no. of Mn = 25)

Magnetic moment =n(n+1)​; where n is no. of unpaired electron
here,4.9= $\sqrt{\mathrm{n}(\mathrm{n}+2)}$

→n=4 means this ion has 4 unpaired electrons.

electronic configuration of Mn=1s²2s²2p⁶3s²3p⁶3d⁵4s² have 5 unpaired electron so to have a ion with 4 unpaired electron, it has to loose 3 electron and ion forms is

25​Mn³⁺:1s²2s²2p⁶3s²3p63d⁴; 

∴a=3