Although `Cr^(3+)` and `Co^(2+)` ions have same number of unpaired electrons but the magnetic moment of `Cr^(3+)` is 3.87 B.M. and that of `Co^(2+)` is 4.87 B.M. Why ?

Explanation

`Cr^(3+) = [Ar]3d^(3) ` and `Co^(2+) = [Ar] 3d^(7)` . Though both have same number of unpaired electrons but total number of electrons present in them are different . Total moment depends upon total spin quantum number (S) as well as the resultant orbital angular momentum of all the electrons (L) , .e.g. , `mu = sqrt( 4S ( S+1 ) + L ( L + 1))` . Orbital contribution is negligible in case of `Cr^(3+)` but not in case of `Co^(2+)`.