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

Explanation

Both the species, `Cr^(3+)(3d^(3))` and `Co^(2+)(3d^(7))`, contain the same number of unpaired electrons (i.e., 3), but total number of electrons present in them are different. Due to symmetrical electronic configuration there is no orbital contribution in `Cr^(3+)` ion. But appreciable orbital contribution is involved in `Co^(3+)` ion.